Mastering Async and Await Keywords in Flutter

2.3.3 Async and Await Keywords

In the realm of modern mobile app development, handling asynchronous operations efficiently is crucial. Flutter, powered by Dart, provides robust support for asynchronous programming through the async and await keywords. These keywords transform complex asynchronous code into a more readable, maintainable format, resembling synchronous code execution. In this section, we will delve deeply into the mechanics of async and await, explore their usage, and highlight best practices to optimize your Flutter applications.

Simplifying Asynchronous Code

Asynchronous programming is essential for performing tasks such as network requests, file I/O, and database operations without blocking the main thread. Traditionally, handling asynchronous operations involved using callbacks or chaining Future methods like then(), which often led to what is known as “callback hell” or convoluted code structures. The introduction of async and await in Dart simplifies this by allowing developers to write asynchronous code that looks and behaves like synchronous code.

Consider the following traditional approach using then():

void fetchUserOrder() {
  getUserOrder().then((order) {
    print('Order: $order');
  }).catchError((error) {
    print('An error occurred: $error');
  });
}

This code can become cumbersome as more asynchronous operations are added. By using async and await, the same logic becomes more straightforward:

Future<void> fetchUserOrder() async {
  try {
    var order = await getUserOrder();
    print('Order: $order');
  } catch (e) {
    print('An error occurred: $e');
  }
}

Using async Functions

The async keyword is used to mark a function as asynchronous, allowing the use of await within it. An async function always returns a Future, even if the return type is void. This is because the function’s execution is paused at each await and resumes when the awaited Future completes.

Declaring an Async Function

Here is a simple example of declaring an async function:

Future<void> fetchUserOrder() async {
  var order = await getUserOrder();
  print('Order: $order');
}

In this example, fetchUserOrder is an asynchronous function that waits for getUserOrder() to complete before printing the order.

Using await

The await keyword is used to pause the execution of an async function until a Future completes. This allows you to write code that appears to execute sequentially, even though it is non-blocking.

Awaiting a Future

When you use await, the function execution is paused until the Future resolves. This is particularly useful for operations that depend on the result of an asynchronous task:

Future<void> fetchData() async {
  var data = await getDataFromApi();
  print(data);
}

In this example, the execution of fetchData pauses at await getDataFromApi(), resuming only after the data is retrieved.

Error Handling with Try-Catch

Handling errors in asynchronous code is crucial to ensure your application remains robust and user-friendly. Dart allows you to use try-catch blocks within async functions to handle exceptions gracefully.

Using Try-Catch in Async Functions

Here’s how you can implement error handling in an async function:

Future<void> fetchData() async {
  try {
    var data = await getDataFromApi();
    print(data);
  } catch (e) {
    print('An error occurred: $e');
  }
}

In this example, if getDataFromApi() throws an error, the catch block handles it, preventing the application from crashing and providing a user-friendly error message.

Avoiding Blocking UI

One of the key benefits of using async and await is that they do not block the UI thread. This means your application remains responsive while performing time-consuming operations in the background. Even though await pauses the execution of the async function, it does not freeze the UI, allowing animations, user interactions, and other tasks to continue seamlessly.

Best Practices

To make the most of async and await in your Flutter applications, consider the following best practices:

Consistent Use of Async and Await

• Maintain Consistency: Use async and await consistently throughout your codebase to improve readability and maintainability. Mixing then() and await styles can lead to confusion and errors.

• Avoid Blocking Operations: Ensure that any long-running operations are performed asynchronously to prevent blocking the UI thread.

• Error Handling: Always implement error handling in your asynchronous functions to manage exceptions gracefully.

• Return Types: Be mindful of the return types of your async functions. Even if a function does not return a value, it should return Future<void>.

Avoid Mixing Styles

Mixing then() and await in the same codebase can lead to inconsistencies and potential errors. Choose one style and stick with it for clarity and maintainability.

Practical Code Examples

Let’s explore some practical scenarios where async and await can be applied effectively.

Fetching Data from an API

Consider an application that fetches user data from a remote API. Using async and await, the code becomes clean and easy to follow:

Future<void> fetchUserData() async {
  try {
    var response = await http.get(Uri.parse('https://api.example.com/user'));
    if (response.statusCode == 200) {
      var userData = jsonDecode(response.body);
      print('User Data: $userData');
    } else {
      print('Failed to load user data');
    }
  } catch (e) {
    print('An error occurred: $e');
  }
}

Performing Multiple Asynchronous Operations

When multiple asynchronous operations are required, async and await help maintain a clear flow of execution:

Future<void> performOperations() async {
  try {
    var data = await fetchData();
    var processedData = await processData(data);
    await saveData(processedData);
    print('Operations completed successfully');
  } catch (e) {
    print('An error occurred: $e');
  }
}

In this example, each operation depends on the result of the previous one, and await ensures they are executed in order.

Diagrams and Visualizations

To better understand the flow of asynchronous operations, let’s visualize the process using a flowchart:

    graph TD;
	    A[Start] --> B[Call async function];
	    B --> C[Await Future];
	    C -->|Future completes| D[Resume Execution];
	    D --> E[Handle Result];
	    E --> F[End];
	    C -->|Error occurs| G[Catch Error];
	    G --> F;

This flowchart illustrates how an async function pauses at await, resumes upon completion, and handles errors if they occur.

Conclusion

Mastering the async and await keywords in Flutter is essential for building responsive, efficient applications. By transforming asynchronous code into a synchronous-like format, these keywords enhance code readability and maintainability. Remember to handle errors gracefully, avoid blocking the UI, and maintain consistency in your codebase. With these practices, you’ll be well-equipped to tackle complex asynchronous operations in your Flutter projects.

Quiz Time!