Скачать или смотреть Understanding the async and then in Dart

Dive deep into the Dart `async` function and learn how to accurately utilize `then` with futures without blocking your application's execution!
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Understanding the async and then in Dart: A Simple Guide

When coding in Dart, particularly when using Flutter, it’s crucial to understand how async functions work alongside futures and the then method. Many developers encounter issues that stem from simple misunderstandings, especially when it comes to blocking operations. In this post, we’ll explore a common problem encountered while using async, followed by a clear solution. Let’s dive in!

The Problem

Consider the following Dart code:

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In the code snippet above, we have an asynchronous function f() that simulates a delay of 3 seconds before returning the integer 7. Meanwhile, in the main() function, we initialize a variable a with the value 3. We call function f() and store its returned future in b. The expectation here is that we should see 3 initially, followed by b value received: 7, and finally 7, followed by end at the end. However, the output is not as expected:

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What Went Wrong?

The confusion arises from the use of the sleep function. Unlike await, which only pauses the execution of a single async function, sleep blocks the entire app, halting all asynchronous operations. This means that while the app is asleep, no values can be processed or printed from your future. Thus, the outcome is that we only see the initial value of a and not the updated one after the future completes.

The Solution

To solve this issue and achieve the expected output, we need to avoid using sleep. Instead, we can utilize await Future.delayed. Here’s how you can rewrite the code:

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Expected Output

Running the updated code will produce the expected results:

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Conclusion

By avoiding the blocking sleep function and properly using await to manage asynchronous code execution, we can ensure our application behaves as expected. Understanding the differences between blocking and non-blocking calls is essential, especially when working with futures in Dart.

When faced with async challenges, remember to keep your code non-blocking, and always favor await over sleep to maintain the flow of your application!

Happy coding!