Скачать или смотреть Understanding Why Your Test with FakeAsync Hangs on Await

Discover why your tests using `FakeAsync` in Dart hang on the `await` statement and learn a solution to prevent deadlocks.
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Understanding Why Your Test with FakeAsync Hangs on Await

If you've been writing tests using the FakeAsync feature in Dart and have run into a situation where the execution hangs on an await, you're not alone. Many developers encounter this issue, and it can be frustrating to diagnose. In this guide, we'll dissect this problem and provide a clear solution to ensure your tests run smoothly.

The Issue at Hand

In your test setup, you’re calling a future that seems to complete, but your await never progresses past that point. Here's a simplified version of the scenario you might be experiencing:

[[See Video to Reveal this Text or Code Snippet]]

Output

When you run the above code, your output may look something like this:

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It’s clear from the output that the future completes, but your test hangs on the await for the variable a.

Why Does This Happen?

The key to understanding this problem lies in how Dart handles asynchronous calls with futures and microtasks. Here's what's happening under the hood:

Future Completion: The future you defined completes, and the associated callbacks are queued to the microtask queue.

Await Behavior: The await statement must process the microtask queue before it can continue. However, since you do not have control over the microtask processing after the await, the function hangs indefinitely.

In simpler terms, the await call requires further action from the microtask queue, which isn't happening—leading to a deadlock.

The Solution: Continuously Pumping the Microtask Queue

To resolve this issue, you can introduce a helper function that keeps pumping the microtask queue while your code runs. Here’s how you can implement this:

[[See Video to Reveal this Text or Code Snippet]]

Explanation:

runFakeAsync Function: This function wraps the FakeAsync.run call and continuously flushes the microtasks.

Flushing Microtasks: Within a while loop, time.flushMicrotasks() is called to ensure that any queued microtasks are processed before returning from the function.

Avoiding Deadlocks: This method prevents the deadlock situation by ensuring that the microtask queue is in constant circulation, allowing your await statements to complete and your tests to run without hanging.

Final Thoughts

With this solution, you can avoid hanging await calls in your tests using FakeAsync. Understanding the interplay between futures and async processing in Dart can save you from frustrating debugging sessions. Now, with the runFakeAsync function, your tests can proceed smoothly without deadlocks.

Try incorporating this approach into your testing strategy, and you'll find your asynchronous tests running significantly more efficiently!

Happy coding!