Intro to async and HTTP

In this chapter, you'll explore asynchronous programming in Dart, allowing your applications to perform multiple tasks concurrently. You'll learn how to fetch data from the internet using the http package, to retrieve an article summary from Wikipedia.

• Completion of Chapter 2, which covered basic Dart syntax and command-line interaction. You should have a dartpedia project set up.
• Familiarity with the concept of APIs (Application Programming Interfaces) as a way to retrieve data.

In this chapter, you will modify the existing dartpedia CLI application to fetch and display an article summary using the http package and asynchronous programming techniques.

Before you can make HTTP requests, you need to add the http package as a dependency to your project.

1. Open the dartpedia/pubspec.yaml file within your project. This file is called the pubspec, and it manages your Dart project's metadata, dependencies (like the http package), and assets.

2. Locate the dependencies section.

3. Add http: ^1.3.0 (or the latest stable version) under dependencies. The ^ symbol allows compatible versions to be used.

   yaml

   dependencies:
     http: ^1.3.0
   

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4. Save the pubspec.yaml file.

5. Run dart pub get in your terminal from the dartpedia/cli directory. This command fetches the newly added dependency and makes it available for use in your project.

   You should see output similar to this:

   bash

   Resolving dependencies...
   Downloading packages...
   + http 1.4.0
     lints 5.1.1 (6.0.0 available)
   Changed 1 dependency!
   1 package has newer versions incompatible with dependency constraints.
   Try `dart pub outdated` for more information.
   

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Now that you've added the http package, you need to import it into your Dart file to use its functionalities.

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

2. Add the following import statement at the top of the file, along with the existing dart:io import:

   dart

   import 'dart:io';
   import 'package:http/http.dart' as http; // Add this line
   

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   This line imports the http package and gives it the alias http. After you do this, you can refer to classes and functions within the http package using http. (for example, http.Client, http.get). The as http part is a standard convention to avoid naming conflicts if another imported library also has a similarly named class or function.

Now create a new function called getWikipediaArticle that handles fetching data from an external API. This function will be async because network requests are asynchronous operations.

1. Define the function signature: Below your main function (and printUsage function), add the following function signature.

   dart

   // ... (your existing printUsage() function)
   ​
   Future<String> getWikipediaArticle(String articleTitle) async {
     //You'll add more code here soon
   }
   

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   Highlights from the preceding code:

   • The Future<String> return type indicates that this function will eventually produce a String result, but not immediately, because it's an asynchronous operation.
   • The async keyword marks the function as asynchronous, allowing you to use await inside it.

2. Construct the API URL and http.Client: Inside your new getWikipediaArticle function, create an http.Client() instance and a Uri object. The Uri represents the endpoint of the Wikipedia API you'll be calling to get an article summary.

   Add these lines inside the getWikipediaArticle function:

   dart

   Future<String> getWikipediaArticle(String articleTitle) async {
     final client = http.Client(); // Create an HTTP client
     final url = Uri.https(
       'en.wikipedia.org', // Wikipedia API domain
       '/api/rest_v1/page/summary/$articleTitle', // API path for article summary
     );
     // ...
   }
   

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3. Make the HTTP Request and handle the response: Now, use the http client to make an HTTP GET request to the URL you just constructed. The await keyword pauses the execution of getWikipediaArticle until the client.get(url) call completes and returns an http.Response object.

   After the request completes, check the response.statusCode to ensure the request was successful (a status code of 200 means OK). If successful, return the response.body, which contains the fetched data (in this case, raw JSON). If the request fails, return an informative error message.

   Add these lines after the Uri construction within getWikipediaArticle:

   dart

   Future<String> getWikipediaArticle(String articleTitle) async {
     final client = http.Client();
     final url = Uri.https(
       'en.wikipedia.org',
       '/api/rest_v1/page/summary/$articleTitle',
     );
     final response = await client.get(url); // Make the HTTP request
   ​
     if (response.statusCode == 200) {
       return response.body; // Return the response body if successful
     }
   ​
     // Return an error message if the request failed
     return 'Error: Failed to fetch article "$articleTitle". Status code: ${response.statusCode}';
   }
   

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You'll integrate the API call into searchWikipedia. This function will house the core logic for handling the wikipedia command.

1. Update searchWikipedia to use async: Locate your searchWikipedia function and update its signature to be async as it will now perform asynchronous operations.

   Your searchWikipedia function should now look like this (initial part):

   dart

   // ... (your existing main function)
   ​
   void searchWikipedia(List<String>? arguments) async { // Added 'async'
     late 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.');
       articleTitle = stdin.readLineSync(); // Await input from the user
       // You'll add error handling for null input here in a moment
     } 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")');
   }
   ​
   // ... (your existing printUsage() function)
   

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   Highlights from the preceding code:

   • void searchWikipedia(List<String>? arguments) async: The function is now async. This is essential because it will call getWikipediaArticle, which is an async function itself and will need to await its result.

2. Add null and empty string checks for user input: Inside searchWikipedia, refine the if block that handles the case where no arguments are provided. If stdin.readLineSync() returns null (for example, if the user presses Ctrl+D/Ctrl+Z) or an empty string, print a message and exit the function.

   dart

   void searchWikipedia(List<String>? arguments) async {
     late String? articleTitle;
   ​
     if (arguments == null || arguments.isEmpty) {
       print('Please provide an article title.');
       final inputFromStdin = stdin.readLineSync(); // Read input
       if (inputFromStdin == null || inputFromStdin.isEmpty) {
         print('No article title provided. Exiting.');
         return; // Exit the function if no valid input
       }
       articleTitle = inputFromStdin;
     } else {
       articleTitle = arguments.join(' ');
     }
   ​
     print('Looking up articles about "$articleTitle". Please wait.');
     print('Here ya go!');
     print('(Pretend this is an article about "$articleTitle")');
   }
   

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3. Call getWikipediaArticle and print the result: Now, modify the searchWikipedia function to call your new getWikipediaArticle function and print the result. Replace the previous placeholder print statements with the actual API call.

   dart

   // ... (beginning of searchWikipedia function, after determining articleTitle)
   ​
   void searchWikipedia(List<String>? arguments) async {
     late String? articleTitle;
     if (arguments == null || arguments.isEmpty) {
       print('Please provide an article title.');
       final inputFromStdin = stdin.readLineSync();
       if (inputFromStdin == null || inputFromStdin.isEmpty) {
         print('No article title provided. Exiting.');
         return;
       }
       articleTitle = inputFromStdin;
     } else {
       articleTitle = arguments.join(' ');
     }
   ​
     print('Looking up articles about "$articleTitle". Please wait.');
   ​
     // Call the API and await the result
     var articleContent = await getWikipediaArticle(articleTitle);
     print(articleContent); // Print the full article response (raw JSON for now)
   }
   

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   Highlights from the preceding code:

   • await getWikipediaArticle(articleTitle): Because getWikipediaArticle is an async function, you need to await its result. This pauses the searchWikipedia function until the Future<String> returned by getWikipediaArticle resolves into a String containing the article's contents.
   • print(articleContent): Prints the fetched article summary as a raw JSON string to the console.

Finally, update your main function to call the new searchWikipedia function when the wikipedia command is used.

1. Locate the else if block in your main function that currently handles the search command. Change the command name from search to wikipedia and update the function call.

   In the sample code, main does not await the call to searchWikipedia, meaning main itself does not need to be marked async.

   Your main function should now look like this:

   dart

   // ... (existing const version declaration and printUsage function)
   ​
   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') { // Changed to 'wikipedia'
       // Pass all arguments *after* 'wikipedia' to searchWikipedia
       final inputArgs = arguments.length > 1 ? arguments.sublist(1) : null;
       searchWikipedia(inputArgs); // Call searchWikipedia (no 'await' needed here for main)
     } else {
       printUsage(); // Catch all for any unrecognized command.
     }
   }
   

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   • arguments.sublist(1): This extracts all elements from the arguments list starting from the second element (index 1). This effectively removes the wikipedia command itself, so searchWikipedia only receives the actual article title arguments.
   • searchWikipedia(inputArgs): This calls searchWikipedia directly. Since main doesn't need to do anything after searchWikipedia completes, you don't need to await it from main (and therefore main doesn't need to be async).

Now that you've implemented the http request and integrated it into your application, test it out.

1. Open your terminal run the following command:

   dart

   dart run bin/cli.dart wikipedia "Dart_(programming_language)"
   

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2. Check to make sure that the application fetched the summary of the "Dart" article from the Wikipedia API and print the raw JSON response to the console. You might see something like:

   bash

   Looking up articles about "Dart_(programming_language)". Please wait.
   {
     "type": "standard",
     "title": "Dart (programming language)",
     "displaytitle": "<span class=\"mw-page-title-main\">Dart (programming language)</span>",
     "namespace": {
         "id": 0,
         "text": ""
       }
   
     // ... (rest of the JSON output will be present but truncated here)
   
   }
   

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3. Next, try running without arguments (type or paste in "Flutter_(software)" when prompted):

   bash

   dart run bin/cli.dart wikipedia
   

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   bash

   Please provide an article title.
   Flutter_(software)
   Looking up articles about "Flutter_(software)". Please wait.
   {
     "type": "standard",
     "title": "Flutter (software)",
     "displaytitle": "<span class=\"mw-page-title-main\">Flutter (software)</span>",
     "namespace": {
         "id": 0,
         "text": ""
     }
   
   // ... (rest of the JSON output will be present but truncated here)
   
   }
   

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   You have now successfully implemented the basic wikipedia command that fetches real data from an external API!

In this chapter, you learned about:

• Asynchronous programming: Understanding Futures, async, and await for operations that take time, like network requests.
• External packages: How to add dependencies using pubspec.yaml and import them into your Dart files.
• HTTP requests: Making network calls using the package:http library.
• API interaction: Fetching data from a public API (Wikipedia) and handling its response.
• Code organization: Refactoring logic into a dedicated searchWikipedia function for better structure.

Question 1: What keyword is used to mark a function as asynchronous in Dart?

• A) sync
• B) async
• C) future
• D) thread

Question 2: What does the await keyword do?

• A) Cancels the execution of an asynchronous function.
• B) Declares a new Future object.
• C) Pauses the execution of the current async function until a Future completes.
• D) Creates a new thread.

Question 3: Which package is used in this guide to make HTTP requests?

• A) dart:io
• B) dart:html
• C) package:http
• D) package:async

In the next chapter, You'll focus on organizing our code into reusable libraries and packages. You'll refactor our application to improve its structure and maintainability by creating a separate package for handling command-line arguments.