Make your CLI program interactive

In this chapter, you'll get hands-on practice with Dart syntax. You'll learn how to read user input, print usage information, and create a basic command-line interaction.

Before you begin this chapter, ensure you have:

• Completed Chapter 1 and have a working Dart development environment.
• Familiarity with basic programming concepts (variables, data types, control flow).

Add some basic functionality to your Dartpedia command-line application and then explore the Dart syntax for it.

1. Implement the version command in cli/bin/cli.dart: Add logic to handle a version command, which prints the current version of the CLI. Use an if statement to check if the first argument provided is version. You'll also need a version constant.

   First, above your main function, declare a const variable for the version. The value of a const variable can never be changed after it's been set:

   dart

   const version = '0.0.1'; // Add this line

   Next, modify your main function to check for the version argument:

   dart

   void main(List<String> arguments) {
     if (arguments.isEmpty) {
       print('Hello, Dart!');
     } else if (arguments.first == 'version') {
       print('Dartpedia CLI version $version');
     }
   }

   The $version syntax is called string interpolation. It lets you embed the value of the variable directly into a string by prefixing the variable name with a $ sign.

2. Test the version command: Run your application with the version argument:

   bash

   dart bin/cli.dart version

   You should now see:

   bash

   Dartpedia CLI version 0.0.1

   If you run your app without arguments, you'll still see "Hello, Dart!".

3. Add a printUsage function: To make the output more user-friendly, create a separate function to display usage information. Place this function outside and below your main function.

   dart

   void printUsage() { // Add this new function
     print(
       "The following commands are valid: 'help', 'version', 'search <ARTICLE-TITLE>'"
     );
   }

   search is the command that will eventually search from Wikipedia.

4. Implement the help command and refine main: Now, integrate the help command using an else if statement, and clean up the default behavior to call the printUsage function.

   Modify your main function to look like this:

   dart

   void main(List<String> arguments) {
     if (arguments.isEmpty || arguments.first == 'help') {
       printUsage(); // Change this from 'Hello, Dart!'
     } else if (arguments.first == 'version') {
       print('Dartpedia CLI version $version');
     } else {
       printUsage(); // Catch all for any unrecognized command.
     }
   }

5. Understand the if/else structure and variables: Now that you've implemented control flow in the main function, review the code that was added for it.

   • arguments.isEmpty checks if no command-line arguments were provided.
   • arguments.first accesses the very first argument, which you're using as our command.
   • version is declared as a const. This means its value is known at compile time and you can't change it during runtime.
   • arguments is a regular (non-constant) variable because its content can change during runtime based on user input.

   Run your application with the help argument. You should see the usage information printed:

   bash

   dart bin/cli.dart help

   Also, try running it without any arguments:

   bash

   dart bin/cli.dart

   Notice that it continues to display usage information. At this point, any command you haven't defined will also print usage information. This is expected behavior for now.

Next, implement a basic search command that takes an article title as input. As you build this functionality, you'll work with List manipulation, null checks, and string interpolation.

1. Integrate the search command into main: First, modify the main function in cli/bin/cli.dart to include an else if branch that handles the search command. For now, just print a placeholder message.

   dart

   void main(List<String> arguments) {
     if (arguments.isEmpty || arguments.first == 'help') {
       printUsage(); 
     } else if (arguments.first == 'version') {
       print('Dartpedia CLI version $version');
     } else if (arguments.first == 'search') {
       // Add this new block:
       print('Search command recognized!');
     } else {
       printUsage();
     }
   }

2. Test the new command: Run your application with the search command:

   bash

   dart bin/cli.dart search

   You should see:

   bash

   Search command recognized!

3. Define the searchWikipedia function: The search command will eventually run the core logic of your application by calling a function called searchWikipedia. For now, have searchWikipedia print the arguments passed into it with the search command. Place this new function below main.

   dart

   // ... (your existing main function)
   
   void searchWikipedia(List<String>? arguments) { // Add this new function and add ? to arguments type
     print('searchWikipedia received arguments: $arguments');
   }
   
   // ... (your existing printUsage() function)

   Highlights from the preceding code:

   • List<String>? arguments means that the arguments list itself can be null.

4. Call the searchWikipedia function from the main function: Now, modify the search command block in main to call searchWikipedia and pass it any arguments that come after the search command itself. Use arguments.sublist(1) to get all arguments starting from the second one. If no arguments are provided after search, pass null to searchWikipedia.

   dart

   void main(List<String> arguments) {
     if (arguments.isEmpty || arguments.first == 'help') {
       printUsage(); 
     } else if (arguments.first == 'version') {
       print('Dartpedia CLI version $version');
     } else if (arguments.first == 'search') {
       // Add this new block:
       final inputArgs = arguments.length > 1 ? arguments.sublist(1) : null;
       searchWikipedia(inputArgs);
     } else {
       printUsage();
     }
   }

   Highlights from the preceding code:

   • final variables can only be set once and are used when you never intend to change the variable again in the code.
   • arguments.sublist(1) creates a new list containing all elements of the arguments list after the first element (which was search).
   • arguments.length > 1 ? ... : null; is a conditional (ternary) operator. It ensures that if no arguments are provided after the search command, inputArgs becomes null, matching the sample code's behavior for searchWikipedia's arguments parameter of List<String>?.

5. Test searchWikipedia with arguments: Using the command line, run the application with a test article title:

   bash

   dart bin/cli.dart search Dart Programming

   You should see:

   bash

   searchWikipedia received arguments: [Dart, Programming]

   Next, run the same command without the extra arguments:

   bash

   dart bin/cli.dart search

   You should see:

   bash

   searchWikipedia received arguments: null

6. Handle the missing article title and user input with the stdin command: It's more user-friendly to prompt the user if they don't provide an article title on the command line. Use stdin.readLineSync() for this.

   First, add the necessary import at the top of your cli/bin/cli.dart file:

   dart

   import 'dart:io'; // Add this line at the top

   dart:io is core library in the Dart SDK, and provides APIs to deal with files, directories, sockets, and HTTP clients and servers, and more.

   Now, update your searchWikipedia function.

   dart

   void searchWikipedia(List<String>? arguments) {
     final String articleTitle;
   
     // If the user didn't pass in arguments, request an article title.
     if (arguments == null || arguments.isEmpty) {
       print('Please provide an article title.');
       // Await input and provide a default empty string if the input is null.
       articleTitle = stdin.readLineSync() ?? ''; 
     } else {
       // Otherwise, join the arguments into the CLI into a single string
       articleTitle = arguments.join(' ');
     }
   
     print('Current article title: $articleTitle');
   }

   This preceding code block introduces a few key concepts:

   • It declares a final String articleTitle variable. This allows static analysis to detect that articleTitle will be a String and won't be null.
   • An if/else statement then checks if command-line arguments for the search were provided.
   • If arguments are missing, it prompts the user, reads input using stdin.readLineSync(), and safely handles cases where no input is given.
   • If arguments are present, it uses arguments.join(' ') to combine them into a single search string.

   Highlights from the preceding code:

   • stdin.readLineSync() ?? '' reads the input from the user. While stdin.readLineSync() can return null, the null-coalescing operator (??) provides a default empty string (''). This is a concise way to ensure that the variable is a non-null String.
   • arguments.join(' '): concatenates all elements of the arguments list into a single string, using a space as the separator. For example, ['Dart', 'Programming'] becomes "Dart Programming". This is crucial for treating multi-word command-line inputs as a single search phrase.
   • Dart static analysis can detect that articleTitle is guaranteed to be initialized when the print statement is executed. No matter which path is taken through this function body, the variable is non-nullable.

7. Finish searchWikipedia to print mock search results: Update searchWikipedia to display messages that look like our program found something. This helps us see what our finished program will do without actually building everything right now. You'll only see these messages if you include a search query when you run the program.

   For example: dart bin/cli.dart search Dart Programming.

   dart

   void searchWikipedia(List<String>? arguments) {
     final String articleTitle;
   
     // If the user didn't pass in arguments, request an article title.
     if (arguments == null || arguments.isEmpty) {
       print('Please provide an article title.');
       // Await input and provide a default empty string if the input is null.
       articleTitle = stdin.readLineSync() ?? ''; 
     } else {
       // Otherwise, join the arguments into the CLI into a single string
       articleTitle = arguments.join(' ');
     }
   
     print('Looking up articles about "$articleTitle". Please wait.');
     print('Here ya go!');
     print('(Pretend this is an article about "$articleTitle")');
   }

8. Final Test Run with both scenarios:

   Now that the article simulation is set up, test the searchWikipedia function in a few different ways:

   bash

   dart bin/cli.dart search Dart Programming

   You should see:

   bash

   Looking up articles about "Dart Programming". Please wait.
   Here ya go!
   (Pretend this is an article about "Dart Programming")

   Run without arguments (type "Flutter Framework" when prompted):

   bash

   dart bin/cli.dart search

   bash

   Please provide an article title.
   Flutter Framework

   You have now successfully built the basic search command with user input handling, correctly treating multi-word command-line inputs as a single search phrase in the output.

In this chapter, you learned:

• Control flow: Using if/else statements to control the execution flow of your program.
• Variables and Constants: Declaring variables with var, const, and final String.
• Lists: Creating and manipulating lists using .isEmpty, .first, .sublist, and .join().
• Null Safety: Understanding nullability (?) and using null checks. Handling potential null values with the null-coalescing operator (??) to provide default values.
• Functions: Defining and calling functions.
• String interpolation: Embedding variables in strings using $.
• Input/Output: Reading user input from the console using stdin.readLineSync().

Question 1: Which keyword is used to declare a constant variable in Dart whose value is known at compile time?

• A) var
• B) final
• C) const
• D) static

Question 2: What is the primary purpose of stdin.readLineSync() in a CLI application?

• A) To print output to the console.
• B) To read a single line of text input from the user.
• C) To execute a command.
• D) To check if a file exists.

In the next chapter, you'll dive into asynchronous programming and learn how to fetch data from the Wikipedia API using the http package. This will allow your application to retrieve real data and display it to the user.