# Learning Dart as a Swift developer

> Leverage your Swift knowledge when learning Dart.



This guide aims to leverage your Swift programming
knowledge when learning Dart. It showcases key
similarities and differences in both languages,
and introduces Dart concepts that aren't present in Swift.
As a Swift developer, Dart might feel familiar,
as both languages share many concepts. 

Both Swift and Dart support sound null safety. 
Neither language allows variables to be null by default.
 
Like Swift, Dart has similar support for
[collections](#collections), [generics](#generics),
[concurrency](#concurrency) (using async/await),
and [extensions](#extension-methods).

[Mixins](#mixins) are another concept in Dart
that might be new for Swift developers.
Like Swift, Dart offers AOT (ahead-of-time) compilation.
However, Dart also supports a JIT (just-in-time)
compilation mode to aid with various development aspects,
such as incremental recompilation or debugging.
For more information, check out the [Dart overview][].

[Dart overview]: /overview#native-platform

:::note
Flutter uses the Dart language,
so if you are coding Flutter, you might find
[Flutter for iOS developers][] to be useful.
:::

[Flutter for iOS developers]: https://docs.flutter.dev/get-started/flutter-for/ios-devs

## Conventions and linting

Swift and Dart both have linting tools to enforce
standard conventions. However, while Swift has
`SwiftLint` as a standalone tool,
Dart has official layout conventions and includes a
linter to make compliance effortless.
To customize the lint rules for your project,
follow the [Customizing static analysis][] instructions.
(Note that the IDE plugins for Dart and Flutter also
provide this functionality.)

[Customizing static analysis]: /tools/analysis

:::tip
Dart provides [`dart fix`][],
which finds and fixes errors found by the analyzer.
:::

[`dart fix`]: /tools/dart-fix

Dart also provides a code formatter,
which can automatically format any Dart project when
running `dart format` from the command line
or through the IDE.

:::tip
Dart supports optional trailing
commas for any comma-separated values,
such as function parameters or list items.
This forces the formatter to place each item
onto its own line, which helps with readability,
especially when you have a lot of nested code
(as can happen in Flutter layout code).

For more information on using commas to make
your code more readable, check out
[Using trailing commas][] on docs.flutter.dev.
:::

[Using trailing commas]: https://docs.flutter.dev/development/tools/formatting#using-trailing-commas

For more information about Dart conventions and linting,
check out [Effective Dart][] and [Linter rules][]. 

[Effective Dart]: /effective-dart
[Linter rules]: /tools/linter-rules

## Variables

Declaring and initializing variables in Dart
is a bit different when compared to Swift.
A variable declaration always begins with the
variable's type, the `var` keyword, or the `final` keyword.
As in Swift, Dart supports type inference where the
compiler infers the type based on the value assigned
to the variable:

```dart
// String-typed variable.
String name = 'Bob';

// Immutable String-typed variable.
final String name = 'Bob';

// This is the same as `String name = 'Bob';`
// since Dart infers the type to be String.
var name = 'Bob';

// And this is the same as `final String name = 'Bob';`.
final name = 'Bob';
```

Each Dart statement ends with a semicolon to
indicate the end of the statement.
You can replace `var` in Dart with an explicit type.
However, by convention, [`var` is recommended
when the analyzer can implicitly infer the type][inference].

[inference]: /effective-dart/design#types

```dart
// Declare a variable first:
String name; 
// Initialize the variable later:
name = 'bob';
// Declare and initialize a variable at once with inference:
var name = 'bob';
```

The Swift equivalent of the above Dart code
would look as follows:

```swift
// Declare a variable first: 
var name: String
// Initialize the variable later
name = "bob"

// Declare and initialize a variable at once with inference:
var name = "bob"
```

In Dart, when a variable without an explicit type
is initialized after its declaration,
its type is inferred as the catch-all `dynamic` type.
Likewise, when a type cannot be automatically inferred,
it defaults to the `dynamic` type,
**which removes all type-safety**.
Therefore, the Dart linter discourages this by
generating a warning. If you _intend_ to allow a
variable to have any type, it's preferred to
assign it to `Object?` rather than `dynamic`.

For more information, check out the
[Variables section][] in the Dart language tour.

[Variables section]: /language/variables

### Final

The `final` keyword in Dart indicates that a
variable can be set only once. This is similar
to the `let` keyword in Swift. 

In both Dart and Swift, you can only initialize a
`final` variable once, either in the declaration
statement or in the initializer list.
Any attempt to assign a value a second time results
in a compile-time error. Both of the following code
snippets are valid, but subsequently setting `name`
results in compilation errors.

```dart
final String name;
if (b1) {
  name = 'John';
} else {
  name = 'Jane';
}
```

```swift
let name: String
if (b1) {
  name = "John"
} else {
  name = "Jane"
}
```

### Const

Besides `final`, Dart also has the `const` keyword.
One benefit of `const` is that it's fully evaluated
at compile-time and can't be modified during the
lifetime of the application.

```dart
const bar = 1000000; // Unit of pressure (dynes/cm2)
const double atm = 1.01325 * bar; // Standard atmosphere
```

A `const` variable defined at a class level
needs to be marked as `static const`.

```dart
class StandardAtmosphere {
  static const bar = 1000000; // Unit of pressure (dynes/cm2)
  static const double atm = 1.01325 * bar; // Standard atmosphere
}
```

The `const` keyword isn't just for declaring
constant variables; it can also be used to create
constant values:

```dart
var foo = const ['one', 'two', 'three'];
foo.add('four'); // Error: foo contains a constant value.
foo = ['apple', 'pear']; // This is allowed as foo itself isn't constant.
foo.add('orange'); // Allowed as foo no longer contains a constant value.
```

In the above example, you can't change the `const` value
(add, update, or remove the elements in the given list),
but you can assign a new value to `foo`.
After `foo` is assigned a new (non-constant) list,
you _can_ add, update, or remove the contents of the list. 

You can also assign a constant value to a `final` field.
You can't use the `final` field in a constant context,
but you can use the constant. For example:

```dart
final foo1 = const [1, 2, 3];
const foo2 = [1, 2, 3]; // Equivalent to `const [1, 2, 3]`
const bar2 = foo2; // OK
const bar1 = foo1; // Compile-time error, `foo1` isn't constant
```

You can also define `const` constructors,
making those classes immutable (unchanging)
and making it possible to create instances of
those classes as compile-time constants.
For more information, check out
[const constructors](#const-constructors).

## Built-in types
Dart includes a number of types in the platform
libraries, such as:

* Basic value types like
  * Numbers (`num`, `int`, `double`)
  * Strings (`String`)
  * Booleans (`bool`)
  * The value null (`Null`)
* Collections
  * Lists/arrays (`List`)
  * Sets (`Set`)
  * Maps/dictionaries (`Map`)

For more information, check out [Built-in types][]
in the Dart language tour.

[Built-in types]: /language/built-in-types

### Numbers

Dart defines three numeric types for holding numbers:

`num`
: A generic 64-bit number type.

`int`
: A platform-dependent integer number.
  In native code, it's a 64-bit two's complement integer.
  On the web, it's a non-fractional 64-bit,
  floating-point number.

`double`
: A 64-bit floating point number.

Unlike Swift, there are no specific types for
unsigned integers.

All these types are also classes in the Dart API.
Both the `int` and `double` types share `num`
as their parent class:

<img
  src="/assets/img/number-class-hierarchy.svg"
  alt="Object is the parent of num, which is the parent of int and double">

As number values are technically class instances,
they have the convenience of exposing their own
utility functions. Because of this, a `int` can,
for example, be turned into a `double` as follows:



```dart
int intVariable = 3;
double doubleVariable = intVariable.toDouble();
```

The same is accomplished in Swift using the
specialized initializer:

```swift
var intVariable: Int = 3
var doubleVariable: Double = Double(intVariable)
```

In the case of literal values,
Dart automatically converts the integer literal
into a `double` value. The following code
is perfectly fine:

```dart
double doubleValue = 3;
```

Unlike Swift, in Dart you can compare an integer
value to a double using the equality (`==`) operator,
as shown below:

```dart
int intVariable = 3;
double doubleVariable = 3.0;
print(intVariable == doubleVariable); // true
```
 
This code prints `true`. However, in Dart the
underlying implementation numbers is different between
the web and native platforms. The [Numbers in Dart][]
page details these differences and shows how to
write code so the differences don't matter.

[Numbers in Dart]: /resources/language/number-representation

### Strings

As with Swift, Dart represents a series of characters
using the `String` type, though Dart doesn't support
a `Character` type representing one character.
A `String` can be defined with either single or
double quotes, however, _single quotes are preferred_.

```dart
String c = 'a'; // There isn't a specialized "Character" type
String s1 = 'This is a String';
String s2 = "This is also a String";
```

```swift
let c: Character = "a"
let s1: String = "This is a String"
let s2: String = "This is also a String"
```

#### Escaping special characters

Escaping special characters in Dart is similar
to Swift (and most other languages).
To include special characters escape them
using the backslash character.

The following code shows some examples:

```dart
final singleQuotes = 'I\'m learning Dart'; // I'm learning Dart
final doubleQuotes = "Escaping the \" character"; // Escaping the " character
final unicode = '\u{1F60E}'; // 😎,  Unicode scalar U+1F60E
```

Note that 4-digit hexadecimal values can also
be used directly (for example, `\u2665`),
however, curly braces also work.
For more information on working with
unicode characters, check out
[Runes and grapheme clusters][]
in the Dart language tour.

[Runes and grapheme clusters]: /language/built-in-types#runes-and-grapheme-clusters

#### String concatenation and multiline declaration

In both Dart and Swift,
you can escape the line breaks in a multiline string,
which allows you to keep your source code easier
to read but still output the `String` in a single line.
Dart has several ways to define multiline strings:

1. Using implicit string concatenation:
   Any neighboring string literals are automatically
   concatenated, even when spread over multiple lines:


   ```dart
   final s1 = 'String '
     'concatenation'
     " even works over line breaks.";
   ```
 
2. Using a multiline string literal:
   When using three quotation marks
   (either single or double) on either
   side of the string,
   the literal is allowed to span multiple lines:


   ```dart
   final s2 = '''You can create
   multiline strings like this one.''';

   final s3 = """This is also a
   multiline string.""";
   ```

3. Dart also supports concatenating strings using
   the `+` operator. This works with both string
   literals and string variables:


   ```dart
   final name = 'John';
   final greeting = 'Hello ' + name + '!';
   ```

#### String interpolation

Insert expressions into the string literal using the
`${<expression>}` syntax. Dart expands on this by
allowing the curly braces to be omitted when the
expression is a single identifier:

```dart
var food = 'bread';
var str = 'I eat $food'; // I eat bread
var str = 'I eat ${bakery.bestSeller}'; // I eat bread
```

In Swift, you can achieve the same result by
surrounding the variable or expression with
parentheses and prefixing with a backslash: 

```swift
let s = "string interpolation"
let c = "Swift has \(s), which is very handy."
```

#### Raw strings

As in Swift, you can define raw strings in Dart.
A raw string ignores the escape character and
includes any special characters present in the string.
You can do this in Dart by prefixing the string literal
with the letter `r`, as shown in the following example.

```dart
// Include the \n characters.
final s1 = r'Includes the \n characters.';
// Also includes the \n characters.
final s2 = r"Also includes the \n characters.";

final s3 = r'''
  The \n characters are also included
  when using raw multiline strings.
  ''';
final s4 = r"""
  The \n characters are also included
  when using raw multiline strings.
  """;
```

```swift
let s1 = #"Includes the \n characters."#
let s2 = #"""
  The \n characters are also included
  when using raw multiline strings.
  """#
```

#### Equality

As in Swift, Dart's equality operator (`==`)
compares whether two strings are equal.
Two strings are equal if they contain the same
sequence of code units.

```dart
final s1 = 'String '
  'concatenation'
  " works even over line breaks.";
assert(s1 ==
  'String concatenation works even over '
  'line breaks.');
```

#### Commonly used API

Dart offers several common API for strings.
For example, both Dart and Swift allow you
to check if a string is empty with `isEmpty`.
There are other convenience methods,
such as `toUpperCase` and `toLowerCase`.
For more information,
check out [Strings][] in the Dart language tour. 

[Strings]: /language/built-in-types#strings

### Booleans

Booleans represent a binary value in both Dart
(`bool`) and Swift (`Bool`).

### Null safety

Dart enforces sound null safety.
By default, types don't allow a null value unless marked as nullable.
Dart indicates this with a question mark (`?`) at the end of the type.
This works like Swift's _optionals_.

### Null-aware operators

Dart supports several operators to deal with nullability.
The null coalescing operator (`??`),
and optional chaining operator (`?.`)
are available in Dart and operate the same as in Swift:

```dart
a = a ?? b;
```

```swift
let str: String? = nil
let count = str?.count ?? 0
```

In addition, Dart offers a
null safe version of the cascade operator (`?..`).
This operator ignores any operations when
the target expression resolves to `null`.
Dart also offers the null assignment operator (`??=`),
which Swift doesn't.
If a variable with a nullable type has a current value of `null`,
this operator assigns a value to that variable.
Expressed as `a ??= b;`, it serves as shorthand for the following:

```dart
a = a ?? b;

// Assign b to a if a is null; otherwise, a stays the same
a ??= b; 
```

```swift
a = a ?? b
```

#### ! Operator (also called "force unwrap")

In cases where it's safe to assume that a
nullable variable or expression is, in fact, non-null,
it's possible to tell the compiler to repress
any compile-time errors.
This is done using the suffix `!` operator,
by placing it as a suffix to the expression.
(Don't confuse this with Dart's "not" operator,
which uses the same symbol):

```dart
int? a = 5;

int b = a; // Not allowed.
int b = a!; // Allowed.
```

At runtime, if `a` turns out to be null,
a runtime error occurs.

Like the `?.` operator,
use the `!` operator when accessing properties
or methods on an object:

```dart
myObject!.someProperty;
myObject!.someMethod();
```

If `myObject` is `null` at runtime,
a runtime error occurs.

### Late fields 

The `late` keyword can be assigned to class fields
to indicate they are initialized at a later point,
while remaining non-nullable. This is similar to Swift's
"implicitly unwrapped optionals."
This is useful for cases where a variable is
never observed before being initialized,
allowing it to be initialized later.
A non-nullable `late` field can't have null
assigned at a later point. Also,
a non-nullable `late` field throws a runtime error
when observed before being initialized,
a scenario you want to avoid in a well-behaved app.

```dart
// Using null safety:
class Coffee {
  late String _temperature;

  void heat() { _temperature = 'hot'; }
  void chill() { _temperature = 'iced'; }

  String serve() => _temperature + ' coffee';
}
```

In this case, `_temperature` is only initialized
after calling `heat()` or `chill(`).
If `serve(`) is called before the others,
a runtime exception occurs.
Note that `_temperature` can't ever be `null`.

You can also use the `late` keyword to make
initialization lazy when combining it with an initializer:

```dart
class Weather {
  late int _temperature = _readThermometer();
}
```

In this case, `_readThermometer()` is only run
when the field is first accessed,
rather than on initialization. 

Another advantage in Dart is using the `late`
keyword to delay initialization of `final` variables.
While you don't have to immediately initialize
the `final` variable when marking it as `late`,
it can still be initialized only once.
A second assignment results in a runtime error.

```dart
late final int a;
a = 1;
a = 2; // Throws a runtime exception because
       // "a" is already initialized.
```

## Functions

Swift uses the `main.swift` file as an app's
entry point. Dart uses the `main` function
as an app's entry point. Every program must
have a `main` function to be executable. For example:

```dart
void main() {
  // main function is the entry point
  print("hello world");
}
```

```swift
// main.swift file is the entry point
print("hello world")
```

Dart doesn't support `Tuples` (though there are
[several tuple packages][] available on pub.dev).
In the case that a function needs to return
multiple values, you can wrap them in a collection,
like a list, set, or map,
or you can write a wrapper class where an instance
can be returned that contains these values.
More on this can be found in the sections on
[collections](#collections) and [classes](#classes).

[several tuple packages]: https://pub.dev/packages?q=tuples

### Exception and error handling

As in Swift, Dart's functions and methods support
handling both [exceptions][] and [errors][].
Dart _errors_ typically represent programmer mistakes
or system failures like stack overflow.
Dart errors aren't supposed to be caught.
On the other hand, Dart _exceptions_ represent
a recoverable failure, and are intended to be caught.
For example, at runtime the code might try to access
a streaming feed, but instead receives an exception which,
if not caught, results in termination of the app.
You can manage exceptions in Dart by wrapping the
function call in a `try-catch` block.

[errors]: https://api.dart.dev/dart-core/Error-class.html
[exceptions]: https://api.dart.dev/dart-core/Exception-class.html

```dart
try {
  // Create audio player object
  audioPlayer = AVAudioPlayer(soundUrl);
            
  // Play the sound
  audioPlayer.play();
}
catch {
  // Couldn't create audio player object, log the exception
  print("Couldn't create the audio player for file $soundFilename");
}
```

Similarly, Swift uses a `do-try-catch` block.
For example:

```swift
do {
  // Create audio player object
  audioPlayer = try AVAudioPlayer(contentsOf: soundURL)
            
  // Play the sound
  audioPlayer?.play()
}
catch {
  // Couldn't create audio player object, log the error
  print("Couldn't create the audio player for file \(soundFilename)")
}
```

You can use the `try-catch` block in both
synchronous and asynchronous Dart code.
For more information,
see the docs for the [`Error`][errors] and
[`Exception`][exceptions] classes.

### Parameters

Similar to Swift, Dart supports named parameters
in its functions. However, unlike Swift,
these aren't the default in Dart.
The default parameter type in Dart is a
positional parameter.

```dart
int multiply(int a, int b) {
  return a * b;
}
```

The equivalent in Swift prefaces a parameter
with an underscore to remove the need
for an argument label.

```swift
func multiply(_ a: Int, _ b: Int) -> Int {
  return a * b
}
```

When creating named parameters in Dart,
define them in a separate block of curly braces,
after positional parameters:

```dart
int multiply(int a, int b, {int c = 1, int d = 1}) {
  return a * b * c * d;
}

// Calling a function with both required and named parameters
multiply(3, 5); // 15
multiply(3, 5, c: 2); // 30
multiply(3, 5, d: 3); // 45
multiply(3, 5, c: 2, d: 3); // 90
```

```swift
// The Swift equivalent
func multiply(_ a: Int, _ b: Int, c: Int = 1, d: Int = 1) -> Int {
  return a * b * c * d
}
```

Named parameters must include one of the following:

* A default value
* A `?` at the end of the type to set the type as nullable
* The keyword `required` before the variable type

To learn more about nullable types, check out [null safety](#null-safety).

To mark a named parameter as required in Dart,
you must prefix it with the `required` keyword:

```dart
int multiply(int a, int b, { required int c }) {
  return a * b * c;
}
// When calling the function, c has to be provided
multiply(3, 5, c: 2);
```

A third parameter type is the _optional
positional parameter_. As the name suggests,
these are similar to the default positional parameters,
but they can be omitted when calling the function.
They must be listed after any required positional
parameters and can't be used in conjunction
with named parameters.

```dart
int multiply(int a, int b, [int c = 1, int d = 1]) {
  return a * b * c * d;
}
// Calling a function with both required and optional positioned parameters.
multiply(3, 5); // 15
multiply(3, 5, 2); // 30
multiply(3, 5, 2, 3); // 90
```

```swift
// The Swift equivalent
func multiply(_ a: Int, _ b: Int, _ c: Int = 1, _ d: Int = 1) -> Int {
  return a * b * c * d
}
```

Like named parameters, optional positional parameters
must have either a default value or a nullable type.

### First-class functions

As in Swift, Dart functions are also
[first class citizens][],
which means they're treated as any other object.
For example, the following code shows how to
return a function from a function:

[first class citizens]: https://en.wikipedia.org/wiki/First-class_citizen

```dart
typedef int MultiplierFunction(int value);
// Define a function that returns another function
MultiplierFunction multiplyBy(int multiplier) {
  return (int value) {
    return value * multiplier;
  };
}
// Call function that returns new function
MultiplierFunction multiplyByTwo = multiplyBy(2);
// Call the new function
print(multiplyByTwo(3)); // 6
```

```swift
// The Swift equivalent of the Dart function below
// Define a function that returns a closure
typealias MultiplierFunction = (Int) -> (Int)

func multiplyBy(_ multiplier: Int) -> MultiplierFunction {
  return { $0 * multiplier} // Returns a closure
}

// Call function that returns a function
let multiplyByTwo = multiplyBy(2)
// Call the new function
print(multiplyByTwo(3)) // 6
```

### Anonymous functions

[_Anonymous functions_][] in Dart work almost
identically to closures in Swift,
save for a difference in syntax. As with named functions,
you can pass anonymous functions like any other value.
For example, you can store anonymous functions in a variable,
pass them as an argument to another function,
or return them from another function.

[_Anonymous functions_]: https://en.wikipedia.org/wiki/Anonymous_function

Dart has two ways to declare anonymous functions.
The first, with curly braces,
works like any other function.
It allows you to use multiple lines, and it
needs a return statement for any value to be returned.

```dart
// Multi line anonymous function
[1,2,3].map((element) { 
  return element * 2; 
}).toList(); // [2, 4, 6]
```

:::note
As the map function used in the example returns
an `Iterable<T`> rather than a `List<T>`,
the `toList` function needs to be called on
the returned `Iterator` to turn it back into a `List`. 
:::

```swift
  // Swift equivalent anonymous function
  [1, 2, 3].map { $0 * 2 }
```

The other method uses an arrow function,
named after the arrow-like symbol used in its syntax.
You can use this shorthand syntax when your
function body contains only a single expression
and where the value is returned.
This omits the need for any braces or
a return statement, as these are implied.

```dart
// Single-line anonymous function
[1,2,3].map((element) => element * 2).toList(); // [2, 4, 6]
```

The choice between the arrow syntax or
curly braces is available for any function,
not just anonymous functions.

```dart
multiply(int a, int b) => a * b;

multiply(int a, int b) {
  return a * b;
}
```

### Generator functions

Dart supports [_generator functions_][]
that return an iterable collection of
items that are built lazily. Add items
to the final iterable using the `yield` keyword,
or add whole collections of items using `yield*`.

[_generator functions_]: /language/functions#generators

The following example shows how to write a basic
generator function:

```dart
Iterable<int> listNumbers(int n) sync* {
  int k = 0;
  while (k < n) yield k++;
}

// Returns an `Iterable<int>` that iterates
// through 0, 1, 2, 3, and 4.
print(listNumbers(5));

Iterable<int> doubleNumbersTo(int n) sync* {
  int k = 0;
  while (k < n) { 
    yield* [k, k]; 
    k++;
  }
}

print(doubleNumbersTo(3)); // Returns an iterable with [0, 0], [1, 1], and [2, 2].
```

This is an example of a **synchronous** generator function.
You can also define **asynchronous** generator functions,
which return streams instead of iterables.
Learn more in the [Concurrency](#concurrency) section.

## Statements 

This section covers the similarities and
differences in statements between Dart and Swift.

### Control flow (if/else, for, while, switch)

All control flow statements in Dart work
similarly to their Swift counterparts,
save for a few differences in syntax.

#### if

Unlike Swift, the `if` statement in Dart
requires parentheses around the condition.
While the Dart style guide recommends using
curly braces around flow control statements
(as shown below), when you have an `if` statement
with no else clause and the whole if statement
fits on one line, you can omit the braces if you prefer.

```dart
var a = 1;
// Parentheses for conditions are required in Dart.
if (a == 1) {
  print('a == 1');
} else if (a == 2) {
  print('a == 2');
} else {
  print('a != 1 && a != 2');
}

// Curly braces are optional for single line `if` statements.
if (a == 1) print('a == 1');
```

```swift
let a = 1;
if a == 1 {
  print("a == 1")
} else if a == 2 {
  print("a == 2")
} else {
  print("a != 1 && a != 2")
}
```

#### for(-in)

In Swift, the `for` loop is only used to loop
over collections. To loop over a chunk of code
multiple times, Swift allows you to loop over a range.
Dart doesn't support syntax for defining ranges,
but includes a standard for loop,
in addition to `for-in` that loops over collections.

Dart's `for-in` loop works like its Swift counterpart,
and it can loop over any value that is an `Iterable`,
as in the `List` example below:

```dart
var list = [0, 1, 2, 3, 4];
for (var i in list) {
  print(i);
}
```

```swift
let array = [0, 1, 2, 3, 4]
for i in array {
  print(i)
}
```

Dart doesn't have any special syntax with
`for-in` loops that allow you to loop over maps,
like Swift has for dictionaries.
To achieve a similar effect, you can
extract the map's entries as an `Iterable` type.
Alternatively, you can use `Map.forEach`:

```dart
Map<String, int> dict = {
  'Foo': 1,
  'Bar': 2
};
for (var e in dict.entries) {
  print('${e.key}, ${e.value}');
}
dict.forEach((key, value) {
  print('$key, $value');
});
```

```swift
var dict:[String:Int] = [
  "Foo":1,
  "Bar":2
]
for (key, value) in dict {
   print("\(key),\(value)")
}
```

### Operators

Unlike Swift, Dart doesn't allow the addition
of new operators, but it does allow you to overload
existing operators with the operator keyword.
For example:

```dart
class Vector {
  final double x;
  final double y;
  final double z;

  Vector operator +(Vector v) {
    return Vector(x: x + v.x, y: y + v.y, z: z+v.z);
  }
}
```

```swift
struct Vector {
  let x: Double
  let y: Double
  let z: Double
}

func +(lhs: Vector, rhs: Vector) -> Vector {
  return Vector(x: lhs.x + rhs.x, y: lhs.y + rhs.y, z: lhs.z + rhs.z)
}

...
```

### Arithmetic operators

For the most part, arithmetic operators behave
the same in Swift and Dart, with the notable
exception of the divide operator (`/`).
In Swift (and many other programming languages),
the result of `let x = 5/2` is `2` (an integer).
In Dart, `int x = 5/2,` results in a value of
`2.5` (a floating point value). To get an integer result,
use Dart's truncating division operator (`~/`). 


:::secondary Difference between web and VM
On the web, an `integer` is also a `double`
(because all numbers are),
but on the VM it's a pure floored `int 2`.
(_Floored_ meaning that the result is truncated
and not rounded.) For example:

```dart
assert(25 == 50.4 ~/ 2);
assert(25 == 50.6 ~/ 2);
assert(25 == 51.6 ~/ 2);
```
:::

While the `++` and `–` operators existed in
earlier versions of Swift, they've been
[removed in Swift 3.0][].
The Dart equivalents operate the same way.
For example:

[removed in Swift 3.0]: https://www.appsloveworld.com/swift/100/9/-is-deprecated-it-will-be-removed-in-swift-3

```dart
assert(2 + 3 == 5);
assert(2 - 3 == -1);
assert(2 * 3 == 6);
assert(5 / 2 == 2.5); // Result is a double
assert(5 ~/ 2 == 2); // Result is an int
assert(5 % 2 == 1); // Remainder

a = 0;
b = ++a; // Increment a before b gets its value.
assert(a == b); // 1 == 1

a = 0;
b = a++; // Increment a AFTER b gets its value.
assert(a != b); // 1 != 0
```

### Type test operators

The implementation of test operators is a bit
different between the two languages. 





| **Meaning**                                        | **Dart operator** | **Swift equivalent**     |
|----------------------------------------------------|-------------------|--------------------------|
| Typecast (description below)                       | expr as T         | expr as! T<br>expr as? T |
| True if the object has the specified type          | expr is T         | expr is T                |
| True if the object doesn't have the specified type | expr is! T        | !(expr is T)             |

{:.table .table-striped .nowrap}

The result of `obj is T` is `true` if `obj`
is a subtype of the type specified by `T`.
For example, `obj is Object?` is always true.

Use the typecast operator to cast an object
to a particular type&mdash;if and only if&mdash;you're
sure that the object is of that type. For example:

```dart
(person as Employee).employeeNumber = 4204583;
```

Dart has only the single-type casting operator,
which acts like Swift's `as!` operator.
There's no equivalent for Swift's `as?` operator.

```swift
(person as! Employee).employeeNumber = 4204583;
```

If you aren't sure that the object is of type `T`,
then use `is T` to check before using the object. 

In Dart, type promotion updates the types of
local variables within the scope of the `if` statement.
This also happens for null checks.
Promotion only applies to _local_ variables,
not to instance variables.

```dart
if (person is Employee) {
  person.employeeNumber = 4204583;
}
```

```swift
// Swift requires the variable to be cast.
if let person = person as? Employee {
  print(person.employeeNumber) 
}
```

#### Logical operators

The logical operators (such as AND (`&&`),
OR (`||`) and NOT (`!`)) are identical
in both languages. For example:

```dart
if (!done && (col == 0 || col == 3)) {
  // ...Do something...
}
```

#### Bitwise and shift operators

:::secondary Numbers on the web
On the web,
Dart uses JavaScript bitwise operations
for better performance, but this can cause
different behavior between native and web
applications. For more information,
see [Bitwise operations][] in the
[Numbers in Dart][] page.
:::

[Bitwise operations]: /resources/language/number-representation#bitwise-operations
[Numbers in Dart]: /resources/language/number-representation

The bitwise operators are mostly identical
in both languages.

For example: 

```dart
final value = 0x22;
final bitmask = 0x0f;

assert((value & bitmask) == 0x02); // AND
assert((value & ~bitmask) == 0x20); // AND NOT
assert((value | bitmask) == 0x2f); // OR
assert((value ^ bitmask) == 0x2d); // XOR
assert((value << 4) == 0x220); // Shift left
assert((value >> 4) == 0x02); // Shift right
assert((-value >> 4) == -0x03); // Shift right // Result may differ on the web
```

#### Conditional operator

Both Dart and Swift contain a conditional operator
(`?:`) for evaluating expressions that might otherwise
require `if-else` statements:

```dart
final displayLabel = canAfford ? 'Please pay below' : 'Insufficient funds';
```

```swift
let displayLabel = canAfford ?  "Please pay below" : "Insufficient funds"
```

#### Cascades (.. operator) 

Unlike Swift, Dart supports cascading with
the cascading operator. This allows you to
chain multiple method calls or property
assignments on a single object.

The following example shows setting the
value of multiple properties, then calling
multiple methods on a newly constructed object,
all within a single chain using the cascade operator:

```dart
Animal animal = Animal()
  ..name = 'Bob'
  ..age = 5
  ..feed()
  ..walk();

print(animal.name); // "Bob"
print(animal.age); // 5
```

```swift
var animal = Animal()
animal.name = "Bob"
animal.age = 5
animal.feed()
animal.walk()

print(animal.name)
print(animal.age)
```

## Collections

This section covers some collection types in
Swift and how they compare to their equivalents in Dart.

### Lists

`List` literals are defined the same way in Dart as
arrays are in Swift, using square brackets and
separated by commas. The syntax between the two
languages is very similar, however there are some
subtle differences, shown in the following example:

```dart
final List<String> list1 = <String>['one', 'two', 'three']; // Initialize list and specify full type
final list2 = <String>['one', 'two', 'three']; // Initialize list using shorthand type
final list3 = ['one', 'two', 'three']; // Dart can also infer the type
```

```swift
var list1: Array<String> = ["one", "two", "three"] // Initialize array and specify the full type
var list2: [String] = ["one", "two", "three"] // Initialize array using shorthand type
var list3 = ["one", "two", "three"] // Swift can also infer the type
```

The following code samples give an overview of the
basic actions that you can perform on a Dart `List`.
The first example shows how to retrieve a value
from a list using the `index` operator:

```dart
final fruits = ['apple', 'orange', 'pear'];
final fruit = fruits[1];
```

To add a value to the back of the list,
use the `add` method. To add another `List`
use the `addAll` method:

```dart
final fruits = ['apple', 'orange', 'pear'];
fruits.add('peach');
fruits.addAll(['kiwi', 'mango']);
```

For the complete List API,
refer to the [`List` class] documentation.

[`List` class]: https://api.dart.dev/dart-core/List-class.html

#### Unmodifiable

Assigning an array to a constant (`let` in Swift)
makes the array immutable, meaning its size and
content can't be changed. You also can't assign
a new array to a constant.

In Dart, this works a little differently and,
depending on your needs,
you have several options to choose from:

* If the list is a compile-time constant and shouldn't
  be modified, use the `const` keyword:<br>
  `const fruits = ['apple', 'orange', 'pear'];`
* Assign the list to a `final` field.
  This means that the list itself doesn't
  have to be a compile-time constant and
  ensures that the field can't be overridden
  with another list. However,
  it still allows the size or the contents
  of the list to be modified:<br>
  `final fruits = ['apple', 'orange', 'pear'];`
* Create a `final List` using the unmodifiable
  constructor (shown in the following example).
  This creates a `List` that can't change its size
  or content, making it behave just like a constant
  `Array` in Swift.

```dart
final fruits = List<String>.unmodifiable(['apple', 'orange', 'pear']);
```

```swift
let fruits = ["apple", "orange", "pear"]
```

#### Spread operators

Another useful feature in Dart is the **spread operator**
(`...`) and the **null-aware spread operator** (`...?`),
which provide a concise way to insert multiple values
into a collection.

For example, you can use the spread operator
(`...`) to insert all the values of a list
into another list, as shown below:

```dart
final list = [1, 2, 3];
final list2 = [0, ...list]; // [ 0, 1, 2, 3 ]
assert(list2.length == 4);
```

Though Swift has no spread operator,
the equivalent to line 2 above would
be the following:

```swift
let list2 = [0] + list
```

If the expression to the right of the spread operator
might be `null`, you can avoid exceptions by using
a null-aware spread operator (`...?`):

```dart
List<int>? list;
final list2 = [0, ...?list]; //[ 0 ]
assert(list2.length == 1);
```

```swift
let list2 = [0] + list ?? []
```

### Sets

Both Dart and Swift support defining `Set`s with literals.
Sets are defined in the same way as lists,
but using curly braces instead of square brackets.
Sets are unordered collections that only contain unique items.
The uniqueness of these items is implemented using
hash codes, meaning that objects need hash values
to be stored in a `Set`. Every Dart object contains
a hash code, while in Swift you need to explicitly
apply the `Hashable` protocol before the object
can be stored in a `Set`.

:::note
In Dart, the `hashCode` inherited from the
`Object` class is only based on the object's identity.
If the `==` operator can make non-identical objects equal,
the `hashCode` getter needs to be overridden to match
the equality. For more information,
check out the API page for the [`hashCode` property][].
:::

The following code snippets show the differences
between initializing a `Set` in Dart and Swift:

```dart
final abc = {'a', 'b', 'c'};
```

```swift
var abc: Set<String> = ["a", "b", "c"]
```

You don't create an empty set in Dart
by specifying empty curly braces (`{}`);
this results in creating an empty `Map`.
To create an empty `Set`,
precede the `{}` declaration with a type argument
or assign `{}` to a variable of type `Set`:

```dart
final names = <String>{};
Set<String> alsoNames = {}; // This works, too.
// final names = {}; // Creates an empty map, not a set.
```

#### Unmodifiable

Similar to `List`, `Set` also has an unmodifiable version.
For example: 

```dart
final abc = Set<String>.unmodifiable(['a', 'b', 'c']);
```

```swift
let abc: Set<String> = ["a", "b", "c"]
```

### Maps

The `Map` type in Dart can be compared with the
`Dictionary` type in Swift. Both types associate
keys and values. These keys and values can be any
type of object. Each key occurs only once,
but you can use the same value multiple times. 

In both languages, the dictionary is based on a hash table,
which means that keys need to be hashable.
In Dart, every object contains a hash while in Swift
you need to explicitly apply the `Hashable` protocol
before the object can be stored in a `Dictionary`.

:::note
In Dart, the `hashCode` inherited from the `Object`
class is only based on the object's identity.
If the `==` operator can make non-identical objects equal,
the `hashCode` getter needs to be overridden
to match the equality. For more information,
check out the API page for the [`hashCode` property][].
:::

[`hashCode` property]: https://api.dart.dev/dart-core/Object/hashCode.html

Here are a couple of simple `Map` and `Dictionary` examples,
created using literals:

```dart
final gifts = {
 'first': 'partridge',
 'second': 'turtle doves',
 'fifth': 'golden rings',
};

final nobleGases = {
 2: 'helium',
 10: 'neon',
 18: 'argon',
};
```

```swift
let gifts = [
   "first": "partridge",
   "second": "turtle doves",
   "fifth": "golden rings",
]

let nobleGases = [
   2: "helium",
   10: "neon",
   18: "argon",
]
```

The following code samples provide an overview
of the basic actions that you can perform on a
Dart `Map`. The first example shows how to
retrieve a value from a `Map` using the `key` operator:

```dart
final gifts = {'first': 'partridge'};
final gift = gifts['first']; // 'partridge'
```

Use the `containsKey` method to check whether a
key is already present in the `Map`:

```dart
final gifts = {'first': 'partridge'};
assert(gifts.containsKey('fifth')); // false
```

Use the index assignment operator (`[]=`) to add
or update an entry in the `Map`. If the `Map`
doesn't contain the key yet, the entry is added.
If the key is present, the entry's value is updated:

```dart
final gifts = {'first': 'partridge'};
gifts['second'] = 'turtle'; // Gets added
gifts['second'] = 'turtle doves'; // Gets updated
```

To remove an entry from the `Map` use the `remove` method,
and to remove all entries that satisfy a given test
use the `removeWhere` method:

```dart
final gifts = {'first': 'partridge'};
gifts.remove('first');
gifts.removeWhere((key, value) => value == 'partridge');
```

## Classes 

Dart doesn't define an interface type&mdash;_any_
class can be used as an interface.
If you want to introduce just an interface,
create an abstract class with no concrete members.
To get a more detailed understanding of these categories,
check out the documentation in the
[abstract classes](#abstract-classes),
[implicit interfaces](#implicit-interfaces),
and [extending a class](#extending-a-class) sections.

Dart doesn't provide support for value types.
As mentioned in the [Built-in types](#built-in-types) section,
all types in Dart are reference types (even primitives),
meaning that Dart doesn't provide a `struct` keyword.

### Enums

Enumerated types, often called enumerations or enums,
are a special kind of class used to represent
a fixed number of constant values. Enums have been
part of the Dart language for a long time,
but Dart 2.17 added enhanced enums support for members.
That means you can add fields holding state,
constructors that set that state,
methods with functionality,
and even override existing members.
For more information, check out
[Declaring enhanced enums][] in the Dart language tour.

[Declaring enhanced enums]: /language/enums#declaring-enhanced-enums

### Constructors 

Dart's class constructors work similarly to class
initializers in Swift. However, in Dart,
they offer more functionality for setting class properties.

#### Standard constructor

A standard class constructor looks very similar
to a Swift initializer, both in declaring it and calling it.
Rather than the `init` keyword, Dart uses the full class name.
The `new` keyword, once required for creating new
class instances, is now optional and no longer recommended. 

```dart
class Point {
  double x = 0;
  double y = 0;

  Point(double x, double y) {
    // There's a better way to do this in Dart, stay tuned.
    this.x = x;
    this.y = y;
  }
}

// Create a new instance of the Point class
Point p = Point(3, 5);
```

#### Constructor parameters

Since writing code to assign all class fields
in the constructor is often quite redundant,
Dart has some syntactic sugar to make this easier:

```dart
class Point {
  double x;
  double y;

  // Syntactic sugar for setting x and y
  // before the constructor body runs.
  Point(this.x, this.y);
}

// Create a new instance of the Point class
Point p = Point(3, 5);
```

Similar to functions, constructors can also take
optional positional or named parameters:

```dart
class Point {
  ...
  // With an optional positioned parameter
  Point(this.x, [this.y = 0]);
  // With named parameters
  Point({required this.y, this.x = 0});
  // With both positional and named parameters
  Point(int x, int y, {int scale = 1}) {
    ...
  }
  ...
}
```

#### Initializer lists

You can also use initializer lists,
which run after any fields that are directly set
using `this` in the constructor parameters,
but still before the constructor body:

```dart
class Point {
  ...
  Point(Map<String, double> json)
      : x = json['x']!,
        y = json['y']! {
    print('In Point.fromJson(): ($x, $y)');
  }
  ...
}
```

An initializer list is a good place to use an assert.

#### Named constructors

Unlike Swift, Dart allows classes to have multiple
constructors by allowing you to name them.
You have the option of using one unnamed constructor,
but any additional constructors must be named.
A class can also have only named constructors.

```dart
class Point {
  double x;
  double y;

  Point(this.x, this.y);

  // Named constructor
  Point.fromJson(Map<String, double> json)
      : x = json['x']!,
        y = json['y']!;
}
```

#### Const constructors

When your class instances are always immutable
(unchanging), you can enforce this by adding a
`const` constructor. Removing a `const` constructor
is a breaking change for those who use your class,
so employ this feature judiciously.
Defining a constructor as `const` makes the class
unmodifiable: all non-static fields in the class
must be flagged as `final`. 

```dart
class ImmutablePoint {
  final double x, y;

  const ImmutablePoint(this.x, this.y);
}
```

This also means you can use that class as a constant value,
making the object a compile-time constant:

```dart
const ImmutablePoint origin = ImmutablePoint(0, 0);
```

#### Constructor redirection

You can call constructors from other constructors,
for example, to prevent code duplication or to
add additional defaults for parameters:

```dart
class Point {
  double x, y;

  // The main constructor for this class.
  Point(this.x, this.y);

  // Delegates to the main constructor.
  Point.alongXAxis(double x) : this(x, 0);
}
```

#### Factory constructors

You can use factory constructors when you don't
need to create a new class instance.
One example is if a cached instance could be returned instead:

```dart
class Logger {
  static final Map<String, Logger> _cache =
    <String, Logger>{};
  
  final String name;
  
  // Factory constructor that returns a cached copy,
  // or creates a new one if it's not yet available.
  factory Logger(String name)=> _cache[name] ??= Logger._internal(name);
  // Private constructor used only in this library
  Logger._internal(this.name);
}
```

### Methods

In both Dart and Swift, methods are functions
that provide behavior for an object. 

```dart
void doSomething() { // This is a function
 // Implementation..
}

class Example {
 void doSomething() { // This is a method
   // Implementation..
 }
}
```

```swift
func doSomething() { // This is a function
  // Implementation..
}

class Example {
  func doSomething() { // This is a method
    // Implementation..
  }
}
```

### Getters and setters

You can define getters and setters by prefixing a field name
with a `get` or `set` keyword. You might recall
that each instance field has an implicit getter,
plus a setter if appropriate. In Swift,
the syntax is a little different,
as the `get` and `set` keywords need to be defined
inside of a property statement and can only be defined
as a statement, not as an expression:

```dart
class Rectangle {
  double left, top, width, height;

  Rectangle(this.left, this.top, this.width, this.height);

  // Define two calculated properties: right and bottom.
  double get right => left + width;
  set right(double value) => width = value - left;

  double get bottom => top + height;
  set bottom(double value) => height = value - top;
}
```

```swift
class Rectangle {
 var left, top, width, height: Double;

 init(left: Double, top: Double, width: Double, height: Double) {
   self.left = left
   self.top = top
   self.width = width
   self.height = height
 }

 // Define two calculated properties: right and bottom.
 var right: Double {
   get {
     return left + width
   }
   set { width = newValue - left }
 }

 var bottom: Double {
   get {
     return top + height
   }
   set { height = newValue - top }
 }
}
```

### Abstract classes

Dart has the concept of _abstract_ classes,
something that isn't supported by Swift.
Abstract classes can't be instantiated directly and
can only be subclassed. This makes abstract classes
useful to define interfaces (comparable to a protocol in Swift).
 
Abstract classes often contain _abstract_ methods,
which are method declarations that don't have an
implementation. Non-abstract subclasses are forced
to override these methods and provide an appropriate
implementation. An abstract class can also contain methods
with a default implementation.
Subclasses inherit this implementation
if they don't override these methods when
extending the abstract class.

To define an abstract class, use the `abstract` modifier.
The following example declares an abstract class
that has an abstract method and a method
containing a default implementation:

```dart
// This class is declared abstract and thus can't be instantiated.
abstract class AbstractContainer {
  void updateChildren(); // Abstract method.

  // Method with default implementation.
  String toString() => "AbstractContainer";
}
```

### Implicit interfaces

In the Dart language, every class implicitly
defines an interface containing all the
instance members of the class and of any
interface it implements. If you want to create
a class `A` that supports class `B`'s API
without inheriting `B`'s implementation,
class `A` should implement the `B` interface. 

Unlike Dart, Swift classes don't implicitly
define an interface. The interface needs to be explicitly
defined as a protocol and implemented by the developer.

A class can implement one or more interfaces
and then provide the APIs required by the interfaces.
Both Dart and Swift have a different ways of
implementing interfaces. For example:

```dart
abstract class Animal {
  int getLegs();
  void makeNoise();
}

class Dog implements Animal {
  @override
  int getLegs() => 4;

  @override
  void makeNoise() => print('Woof woof');
}
```

```swift
protocol Animal {
   func getLegs() -> Int;
   func makeNoise()
}

class Dog: Animal {
  func getLegs() -> Int {
    return 4;
  }

  func makeNoise() {
    print("Woof woof"); 
  }
}
```

### Extending a class

Class inheritance in Dart is very similar to Swift.
In Dart, you can use `extends` to create a subclass,
and `super` to refer to the superclass:

```dart
abstract class Animal {
  // Define constructors, fields, methods...
}

class Dog extends Animal {
  // Define constructors, fields, methods...
}
```

```swift
class Animal {
  // Define constructors, fields, methods...
}

class Dog: Animal {
  // Define constructors, fields, methods...
}
```

### Mixins 

Mixins allow your code to share functionality between classes.
You can use the mixin's fields and methods in a class,
using their functionality as if it were part of the class.
A class can use multiple mixins&mdash;which is useful
when multiple classes share the same functionality&mdash;without
needing to inherit from each other or share a common ancestor.

While Swift doesn't support mixins, it can approximate
this functionality if you write a protocol together
with an extension that provides default implementations
for the methods specified in the protocol.
The main issue with this approach is that,
unlike in Dart,
these protocol extensions don't maintain their own state. 

You can declare a mixin just like a regular class,
as long as it doesn't extend any class other than `Object`
and has no constructors. Use the `with` keyword to add
one or more comma-separated mixins to a class. 

The following example shows how this behavior is
achieved in Dart, and how similar behavior is
replicated in Swift:

```dart
abstract class Animal {}

// Defining the mixins
mixin Flyer {
  fly() => print('Flaps wings');
}
mixin Walker {
  walk() => print('Walks legs');
}
  
class Bat extends Animal with Flyer {}
class Goose extends Animal with Flyer, Walker {}
class Dog extends Animal with Walker {}

// Correct calls
Bat().fly();
Goose().fly();
Goose().walk(); 
Dog().walk();

// Incorrect calls
Bat().walk(); // Not using the Walker mixin
Dog().fly(); // Not using the Flyer mixin
class Animal {
}
```

```swift
// Defining the "mixins"
protocol Flyer {
  func fly()
}

extension Flyer {
  func fly() {
    print("Flaps wings")
  }
}

protocol Walker {
  func walk()
}

extension Walker {
  func walk() {
    print("Walks legs")
  }
}

class Bat: Animal, Flyer {}
class Goose: Animal, Flyer, Walker {}
class Dog: Animal, Walker {}

// Correct calls
Bat().fly();
Goose().fly();
Goose().walk();
Dog().walk();

// Incorrect calls
Bat().walk(); // `bat` doesn't have the `walk` method
Dog().fly(); // "dog" doesn't have the `fly` method
```

Replacing the `class` keyword with `mixin`
prevents the mixin from being used as a regular class.

```dart
mixin Walker {
  walk() => print('Walks legs');
}

// Impossible, as Walker is no longer a class.
class Bat extends Walker {}
```

:::note
The `extends` keyword is covered further in
[Extending a class][] in the Dart language tour.
:::

[Extending a class]: /language/generics#restricting-the-parameterized-type

Since you can use multiple mixins,
their methods or fields can overlap with
each other when used on the same class.
They can even overlap with the class that uses them,
or that class's superclass. To work around this,
Dart stacks them on top of each other,
so the order in which they are added to a class matters. 

To give an example:

```dart
class Bird extends Animal with Consumer, Flyer {
```

When a method is called on an instance of `Bird`,
Dart starts at the bottom of the stack with its own class,
`Bird`, which takes precedence over other implementations.
If `Bird` has no implementation,
then Dart keeps moving up the stack,
with `Flyer` next, followed by `Consumer`,
until an implementation is found.
If no implementation is found,
the parent class, `Animal`, is checked last. 

### Extension methods 

Like Swift, Dart offers extension methods that allow you
to add functionality&mdash;specifically, methods,
getters, setters, and operators&mdash;to existing types. 
The syntax in both Dart and Swift for creating an
extension looks very similar:

```dart
extension <name> on <type> {
  (<member definition>)*
}
```

```swift
extension <type> {
  (<member definition>)*
}
```

As an example, the following extension on
the `String` class from the Dart SDK
allows parsing of integers:

```dart
extension NumberParsing on String {
  int parseInt() {
    return int.parse(this);
  }
}

print('21'.parseInt() * 2); // 42
```

```swift
extension String {
  func parseInt() -> Int {
    return Int(self) ?? 0
  }
}

print("21".parseInt() * 2) // 42
```

Although extensions are similar in Dart and Swift,
there are some key differences. The following sections
cover the most important differences,
but check out [Extension methods][] for a complete
overview.

[Extension methods]: /language/extension-methods

:::note
Dart doesn't support extending an existing type
to make it adopt one or more protocols.
:::

#### Named extensions

Although not mandatory, you can name an extension in Dart.
Naming an extension allows you to control its
scope&mdash;meaning it's possible to either
hide or show the extension in case it conflicts
with another library. If the name begins with an underscore,
the extension is only available within the library
in which it's defined.

```dart
// Hide "MyExtension" when importing types from
// "path/to/file.dart".
import 'path/to/file.dart' hide MyExtension; 
// Only show "MyExtension" when importing types
// from "path/to/file.dart".
import 'path/to/file.dart' show MyExtension;

// The `shout()` method is only available within this library.
extension _Private on String {
  String shout() => this.toUpperCase();
}
```

#### Initializers

In Swift, you can use extensions to add new
convenience initializers to a type. In Dart,
you can't use extensions to add additional
constructors to a class, but you can add a
static extension method that creates an instance
of the type. Consider the following example: 

```dart
class Person {
  Person(this.fullName);

  final String fullName;
}

extension ExtendedPerson on Person {
  static Person create(String firstName, String lastName) {
    return Person("$firstName $lastName");
  }
}

// To use the factory method, use the name of
// the extension, not the type.
final person = ExtendedPerson.create('John', 'Doe');
```

### Overriding members

Overriding instance methods (including operators,
getters, and setters), is also very similar between
the two languages. In Dart, you can use the
`@override` annotation to indicate that you are
intentionally overriding a member:

```dart
class Animal {
  void makeNoise => print('Noise');
}

class Dog implements Animal {
  @override
  void makeNoise() => print('Woof woof');
}
```

In Swift, you add the `override` keyword
to the method definition:

```swift
class Animal {
  func makeNoise() {
    print("Noise")
  }
}

class Dog: Animal {
  override func makeNoise() {
    print("Woof woof"); 
  }
}
```

## Generics 

As in Swift, Dart supports the use of generics
to improve type safety or reduce code duplication. 

### Generic methods 

You can apply generics to methods.
To define a generic type, place it between
`< >` symbols after the method name.
This type can then be used within the method
(as the return type), or in the method's parameters:

```dart
// Defining a method that uses generics.
T transform<T>(T param) {
  // For example,  doing some transformation on `param`...
  return param;
}

// Calling the method. Variable "str" will be
// of type String.
var str = transform('string value'); 
```

In this case, passing `String` to the `transform`
method ensures that it returns a `String`.
Likewise, if an `int` is provided,
the return value is an `int`.

Define multiple generics by separating them
with a comma:

```dart
// Defining a method with multiple generics.
T transform<T, Q>(T param1, Q param2) {
  // ...
}
// Calling the method with explicitly-defined types.
transform<int, String>(5, 'string value');
// Types are optional when they can be inferred.
transform(5, 'string value');
```

### Generic classes

Generics can also be applied to classes.
You can specify the type when calling a constructor,
which allows you to tailor reusable classes to specific types.

In the following example,
the `Cache` class is for caching specific types:

```dart
class Cache<T> {
  T getByKey(String key) {}
  void setByKey(String key, T value) {}
}
// Creating a cache for strings.
// stringCache has type Cache<String>
var stringCache = Cache<String>();
// Valid, setting a string value.
stringCache.setByKey('Foo', 'Bar')
// Invalid, int type doesn't match generic.
stringCache.setByKey('Baz', 5)
```

If the type declaration is omitted,
the runtime type is `Cache<dynamic>`
and both calls to `setByKey` are valid.

### Restricting generics
You can use generics to restrict your code to a
family of types using `extends`. This ensures
that your class is instantiated with a generic type
that extends a specific type (and is similar to Swift):

```dart
class NumberManager<T extends num> {
  // ...
}
// Valid
var manager = NumberManager<int>(); 
var manager = NumberManager<double>(); 
// Invalid, neither String nor its parent classes extend num.
var manager = NumberManager<String>(); 
```

### Generics in literals

`Map-`, `Set-`, and `List-` literals can explicitly
declare generic types, which is useful when the
type isn't inferred or is incorrectly inferred. 

For example, the `List` class has a generic definition:
`class List<E>`. Generic type `E` refers to the type
of the list's contents. Normally,
this type is automatically inferred,
which is used in some membery types of the `List` class's.
(For example, its first getter returns a value of type `E`).
When defining a `List` literal,
you can explicitly define the generic type as follows:

```dart
var objList = [5, 2.0]; // Type: List<num> // Automatic type inference
var objList = <Object>[5, 2.0]; // Type: List<Object> // Explicit type definition
var objSet = <Object>{5, 2.0}; // Sets work identically
```

This is also true for a `Map`,
which also define its `key` and `value` types
using generics (`class Map<K, V>)`:

```dart
// Automatic type inference
var map = {
  'foo': 'bar'
}; // Type: Map<String, String>
// Explicit type definition:
var map = <String, Object>{
  'foo': 'bar'
}; // Type: Map<String, Object>
```

## Concurrency

Swift supports multithreading and Dart supports isolates, 
which are similar to lightweight threads
and won't be covered here.
Each isolate has its own event loop.
For more information, see [How isolates work][].

[How isolates work]: /language/concurrency#isolates

### Futures

Vanilla Swift doesn't have a counterpart to Dart's `Future`.
However, you might still know this object if you are familiar
with Apple's Combine framework, or third party libraries
like RxSwift or PromiseKit.

In a nutshell, a future represents the result
of an asynchronous operation, which becomes available
at a later time. If you have a function that returns
a `Future` of a `String` (`Future<String>`)
rather than just a `String`,
you are basically receiving a value that might exist
some time later&mdash;in the future. 

When a future's asynchronous operation completes,
the value becomes available. You should keep in mind,
however, that a future can also complete with an error
instead of a value.

An example of this would be if you made an HTTP request,
and immediately received a future as the response.
Once the result comes in, the future completes
with that value. However, if the HTTP request fails,
say because the internet connection was interrupted,
the future completes with an error instead. 

Futures can be created manually, too.
The easiest way to create a future is by
defining and calling an `async` function,
which is discussed in the next section.
When you have a value that needs to be a `Future`,
you can easily turn it into one using the `Future` class: 

```dart
String str = 'String Value';
Future<String> strFuture = Future<String>.value(str);
```

#### Async/await

While futures aren't part of vanilla Swift,
the `async/await` syntax in Dart has a Swift counterpart,
and works in a similar way, though without `Future` objects. 

As in Swift, functions can be marked as `async`.
The difference in Dart is that any `async` function
always implicitly returns a `Future`.
For example, if your function returns a `String`,
the async counterpart to this function returns a
`Future<String>`.

The `throws` keyword that's placed after the
`async` keyword in Swift
(but only if the function is throwable),
doesn't exist in Dart's syntax because Dart
exceptions and errors aren't checked by the compiler.
Rather, if an exception occurs in an async function,
the returned `Future` fails with the exception,
which can then be appropriately handled. 

```dart
// Returns a future of a string, as the method is async
Future<String> fetchString() async {
  // Typically some other async operations would be done here.
  
  Response response = await makeNetworkRequest();
  if (!response.success) {
    throw BadNetwork();
  }

  return 'String Value';
}
```

This async function can then be called as follows:

```dart
String stringFuture = await fetchString();
print(str); // "String Value"
```

The equivalent async function in Swift:

```swift
func fetchString() async throws -> String {
  // Typically some other async operations would be done here.
  let response = makeNetworkRequest()
  if !response.success {
    throw BadNetwork()
  }
  
  return "String Value"
}
```

Similarly, any exception that occurs in the `async`
function can be handled in the same way as handling
a failed `Future`, using the `catchError` method.

In Swift, an async function can't be invoked from
a non-async context. In Dart, you are allowed to do so,
but you must handle the resulting `Future` properly.
It's considered bad practice to call an async function
from a non-async context unnecessarily.

Like Swift, Dart also has the `await` keyword.
In Swift, `await` is only usable when calling
`async` functions, but Dart's `await` works with
the `Future` class. As a result, `await` also
works with `async` functions because all `async`
functions return futures in Dart. 

Awaiting a future suspends execution of the current
function and returns control to the event loop,
which can work on something else until the future
completes either with a value or an error.
At some time after that, the `await` expression
evaluates to that value or throws that error.

When it completes, the future's value is returned.
You can only `await` in an `async` context, as in Swift.

```dart
// We can only await futures within an async context.
asyncFunction() async {
  String returnedString = await fetchString();
  print(returnedString); // 'String Value'
}
```

When the awaited future fails, an error object
is thrown on the line with the `await` keyword.
You can handle this using a regular `try-catch` block:

```dart
// We can only await futures within an async context.
Future<void> asyncFunction() async {
  String? returnedString;
  try {
    returnedString = await fetchString();
  } catch (error) {
    print('Future encountered an error before resolving.');
    return;
  }
  print(returnedString);
}
```

For more information and interactive practice,
check out the [Asynchronous programming][] tutorial.

[Asynchronous programming]: /libraries/async/async-await

### Streams

Another tool in Dart's async toolbox is the `Stream` class.
While Swift has its own concept of streams,
those in Dart are similar to `AsyncSequence` in Swift.
Similarly, if you know `Observables` (in RxSwift) or
`Publishers` (in Apple's Combine framework),
Dart's streams should feel familiar. 

For those not familiar with `Streams`,
`AsyncSequence`, `Publishers`, or `Observables`,
the concept is as follows: a `Stream`
essentially acts like a `Future`,
but with multiple values spread out over time,
like an event bus. Streams can be listened to,
to receive value or error events,
and they can be closed when no further events will be sent.

#### Listening

To listen to a stream, you can combine a stream
with a `for-in` loop in an `async` context.
The `for` loop invokes the callback method
for each item emitted, and ends when the stream
completes or errors out:

```dart
Future<int> sumStream(Stream<int> stream) async {
  var sum = 0;
  try { 
    await for (final value in stream) {
      sum += value;
    }
  } catch (error) {
    print('Stream encountered an error! $err');
  }
  return sum;
}
```

If an error occurs when listening to a stream,
the error is thrown at the line containing the
`await` keyword, which you can handle with a
`try-catch` statement:

```dart
try {
  await for (final value in stream) { ... }
} catch (err) {
  print('Stream encountered an error! $err');
}
```

This isn't the only way to listen to a stream:
you can also call its `listen` method and
provide a callback, which is called whenever
the stream emits a value:

```dart
Stream<int> stream = ...
stream.listen((int value) {
  print('A value has been emitted: $value');
});
```

The `listen` method has some optional callbacks
for error handling, or for when the stream completes:

```dart
stream.listen(
  (int value) { ... },
  onError: (err) {
    print('Stream encountered an error! $err');
  },
  onDone: () {
    print('Stream completed!');
  },
);
```

The `listen` method returns an instance of a
`StreamSubscription`, which you can use to
stop listening to the stream:

```dart
StreamSubscription subscription = stream.listen(...);
subscription.cancel();
```

#### Creating streams

As with futures, you have several different
ways to create a stream. The two most common ways
use an async generator or a `SteamController`. 

##### Async generators

An async generator function has the same syntax
as a synchronous generator function,
but uses the `async*` keyword instead of `sync*`,
and returns a `Stream` instead of an `Iterable`.
This approach is similar to the `AsyncStream`
struct in Swift.

In an async generator function, the `yield` keyword
emits the given value to the stream.
The `yield*` keyword, however,
works with streams instead of other iterables.
This allows events from other streams to be
emitted to this stream. In the following example,
the function only continues once the newly yielded
stream has completed:

```dart
Stream<int> asynchronousNaturalsTo(int n) async* {
  int k = 0;
  while (k < n) yield k++;
}

Stream<int> stream = asynchronousNaturalsTo(5); 
```

You can also create a stream using the
`StreamController` API. For more information,
see [Using a StreamController][].

[Using a StreamController]: /libraries/async/creating-streams#using-a-streamcontroller

## Doc comments

Regular comments work the same in Dart as they do in Swift.
Using a double backslash (`//`) comments out everything
beyond the double slash for the rest of the line,
and `/* ... */` blocks comments spanning multiple lines.

In addition to regular comments,
Dart also has [doc comments][] that work in tandem
with [`dart doc`][]: a first-party tool that generates
HTML documentation for Dart packages.
It's considered best practice to place doc comments
above all declarations for public members.
You might notice that this process is similar to how
you add comments for various documentation generation
tools in Swift.

[doc comments]: /effective-dart/documentation
[`dart doc`]: /tools/dart-doc

As in Swift, you define a doc comment by
using three forward slashes instead of two (`///`):

```dart
/// The number of characters in this chunk when unsplit.
int get length => ...
```

Surround types, parameter-, and method names
with square brackets within doc comments. 

```dart
/// Returns the [int] multiplication result of [a] * [b].
multiply(int a, int b) => a * b;
```

While there is support for JavaDoc-style doc comments,
you should avoid them and use the `///` syntax. 

```dart
/** 
 * The number of characters in this chunk when unsplit. 
 * (AVOID USING THIS SYNTAX, USE /// INSTEAD.)
 */
int get length => ...
```

## Libraries and visibility

Dart's visibility semantics are similar to Swift's,
with Dart libraries being roughly equivalent to
Swift modules.

Dart offers two levels of access control:
public and private. Methods and variables
are public by default. Private variables
are prefixed with the underline character (`_`),
and are enforced by the Dart compiler.

```dart
final foo = 'this is a public property';
final _foo = 'this is a private property';

String bar() {
  return 'this is a public method';
}
String _bar() {
  return 'this is a private method';
}

// Public class
class Foo {
}

// Private class
class _Foo {
},
```

Private methods and variables are scoped
to their library in Dart, and to a module in Swift.
In Dart, you can define a library in a file,
while in Swift you must create a new build target
for your module. This means that in a single Dart
project you can define `n` libraries,
but in Swift you must create `n` modules. 

All files that are part of a library can gain access
to all private objects in that library.
But for security reasons, a file still needs
to allow specific files to gain access to its
private objects, otherwise any file&mdash;even from
outside of your project&mdash;could register itself to your
library and gain access to possibly sensitive data.
In other words,
private objects aren't shared across libraries.

```dart title="animal.dart"
library animals;

part 'parrot.dart';

class _Animal {
  final String _name;

  _Animal(this._name);
}
```

```dart title="parrot.dart"
part of animals;

class Parrot extends _Animal {
  Parrot(String name) : super(name);

  // Has access to _name of _Animal
  String introduction() {
    return 'Hello my name is $_name';
  }
}
```

For more information, check out
[creating packages][].

[creating packages]: /tools/pub/create-packages#organizing-a-package

## Next steps

This guide has introduced you to the major differences
between Dart and Swift. At this point,
you might consider moving to the general documentation
for [Dart][] or [Flutter][]
(an open-source framework that uses Dart for
building beautiful, natively compiled,
multiplatform applications from a single codebase),
where you'll find in-depth information about the
language and practical ways of getting started. 

[Dart]: /docs
[Flutter]: https://docs.flutter.dev


