Organizing Dart code with packages and libraries

In this chapter, you'll level up from basic Dart syntax to building command-line applications "the Dart way," embracing best practices. You'll learn to refactor your code into reusable components by creating a dedicated package for handling command-line arguments. This step sets you up for building a more advanced command-line application in future chapters, which will integrate specialized packages for Wikipedia logic and a robust command_runner framework. This chapter helps you understand Dart libraries, export statements, and how to structure your project for better organization and maintainability.

• Completion of Chapter 3, which covered asynchronous programming and HTTP requests.

In this chapter, you'll be refactoring the existing dartpedia CLI application by extracting the command-line argument parsing logic into a separate package called command_runner. This will improve the structure of your project, making it more modular and maintainable.

First, create a new Dart package to house the command-line argument parsing logic.

1. Navigate to the root directory of your project (/dartpedia).

2. Run the following command in your terminal:

   bash

   dart create -t package command_runner
   

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   This command creates a new directory named command_runner with the basic structure of a Dart package. You should now see a new folder command_runner in your project root, alongside cli.

Now that you have created the command_runner package, add a placeholder class that will eventually handle the command-line argument parsing logic.

1. Open the command_runner/lib/command_runner.dart file. Remove any existing placeholder code and add the following:

   dart

   /// A simple command runner to handle command-line arguments.
   ///
   /// More extensive documentation for this library goes here.
   library;
   ​
   export 'src/command_runner_base.dart';
   // TODO: Export any other libraries intended for clients of this package.
   

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   Highlights from the preceding code:

   • library; declares this file as a library, which helps define the boundaries and public interface of a reusable unit of Dart code.
   • export 'src/command_runner_base.dart'; is a crucial line that makes declarations from command_runner_base.dart available to other packages that import the command_runner package. Without this export statement, the classes and functions within command_runner_base.dart would be private to the command_runner package, and you wouldn't be able to use them in your dartpedia application.

2. Open the file command_runner/lib/src/command_runner_base.dart.

3. Remove any existing placeholder code and add the following CommandRunner class to command_runner/lib/src/command_runner_base.dart:

   dart

   class CommandRunner {
     /// Runs the command-line application logic with the given arguments.
     Future<void> run(List<String> input) async {
       print('CommandRunner received arguments: $input');
     }
   }
   

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   Highlights from the preceding code:

   • CommandRunner is a class that serves as a simplified stand-in for now. Its run method currently just prints the arguments it receives. In later chapters, you'll expand this class to handle complex command parsing.
   • Future<void> is a return type that indicates that this method might perform asynchronous operations, but doesn't return a value.

Now that you've created the command_runner package and added a placeholder CommandRunner class, you need to tell your cli application that it depends on command_runner. Because the command_runner package is located locally within your project, use the path dependency option.

1. Open the cli/pubspec.yaml file.

2. Locate the dependencies section. Add the following lines:

   yaml

   dependencies:
     http: ^1.3.0 # Keep your existing http dependency
     command_runner:
       path: ../command_runner # Points to your local command_runner package
   

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   This section tells the cli application to depend on the command_runner package, and specifies that the package is located in the ../command_runner directory (relative to the cli directory).

3. Run dart pub get in the /dartpedia/cli directory of your terminal to fetch the new dependency.

Now that you've added command_runner as a dependency, you can import it into your cli application and replace your existing argument-handling logic with the new CommandRunner class. This step also fixes the program exit behavior discussed at the end of Chapter 3.

1. Open the cli/bin/cli.dart file.

2. Add the following import statement at the top of the file, alongside your other imports:

   dart

   import 'package:command_runner/command_runner.dart';
   

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   This statement imports the command_runner package, making the CommandRunner class available for use.

3. Refactor the main function and remove old logic: Currently, your main function from Chapter 3 directly handles commands like version, help, and wikipedia, and then calls searchWikipedia. You'll now replace all of this custom command-handling logic with a single call to the new CommandRunner class.

   Your cli/bin/cli.dart file (from Chapter 3) should currently look like this:

   dart

   import 'dart:io';
   import 'package:http/http.dart' as http;
   import 'package:command_runner/command_runner.dart';
   ​
   const version = '0.0.1';
   ​
   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 == 'wikipedia') {
       final inputArgs = arguments.length > 1 ? arguments.sublist(1) : null;
       searchWikipedia(inputArgs);
     } else {
       printUsage();
     }
   }
   ​
   void searchWikipedia(List<String>? arguments) async { /* ... existing logic ... */ }
   void printUsage() { /* ... existing logic ... */ }
   Future<String> getWikipediaArticle(String articleTitle) async { /* ... existing logic ... */ }
   

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   Now, replace the entire contents of cli/bin/cli.dart (except for the http import) with the following updated version:

   dart

   import 'dart:io';
   import 'package:http/http.dart' as http;
   import 'package:command_runner/command_runner.dart';
   ​
   void main(List<String> arguments) async { // main is now async and awaits the runner
     var runner = CommandRunner(); // Create an instance of your new CommandRunner
     await runner.run(arguments); // Call its run method, awaiting its Future<void>
   }
   

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   Highlights from the preceding code:

   • void main(List<String> arguments) async directly addresses the program not exiting cleanly issue from Chapter 3. Notice that main is now declared async. This is essential because runner.run() returns a Future, and main must await its completion to ensure the program waits for all asynchronous tasks to finish before exiting.
   • var runner = CommandRunner(); creates an instance of the CommandRunner class from your new command_runner package.
   • await runner.run(arguments); calls the run method on the CommandRunner instance, passing in the command-line arguments.

   Removed Functions:

   The printUsage, searchWikipedia, and getWikipediaArticle functions are now completely removed from cli/bin/cli.dart. Their logic will be redesigned and moved into the command_runner package in future chapters, as part of building the full command-line framework.

Now that you've refactored the code and updated the cli application to use the command_runner package, run the application to verify that everything is working correctly at this stage.

1. Open your terminal and navigate to the cli directory.

2. Run the wikipedia command:

   bash

   dart run bin/cli.dart wikipedia Computer_programming
   

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3. Ensure that the application now executes without errors and print the arguments to the console, demonstrating that the control has successfully transferred to your new command_runner package.

   bash

   CommandRunner received arguments: [wikipedia, Computer_programming]
   

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In this chapter, you learned about:

• Creating Dart packages using dart create -t package.
• Using export statements to make declarations from one library available in another.
• Adding local packages as dependencies using the path option in pubspec.yaml.
• Importing packages into your Dart code using import statements.
• Refactoring code to improve organization and maintainability, including making main async to correctly await asynchronous operations.

Question 1: What is the purpose of the export statement in a Dart library?

• A) To hide declarations from other libraries.
• B) To make declarations available to other libraries.
• C) To specify the version of the Dart SDK required by the library.
• D) To define the entry point of the library.

Question 2: How do you add a local package as a dependency in pubspec.yaml?

• A) By specifying the package name and version.
• B) By specifying the package name and the path to the package.
• C) By using the git option and specifying the URL of the Git repository.
• D) By using the hosted option and specifying the URL of the package server.

In the next chapter, you'll dive into object-oriented programming (OOP) concepts in Dart. You'll learn how to create classes, define inheritance relationships, and build a more robust command-line argument parsing framework using OOP principles within your new command_runner package.