Скачать или смотреть Understanding Why Flutter await Doesn't Wait: A Guide for Firebase Users

Discover the common issue in Flutter where `await` doesn't function as expected, particularly when uploading images to Firebase Storage. This post provides solutions to ensure proper execution order in your code.
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Understanding Why Flutter await Doesn't Wait: A Guide for Firebase Users

When developing Flutter applications, you may come across unexpected behavior with the await keyword, particularly when working with asynchronous tasks. This can be especially troublesome when uploading files to services like Firebase Storage and saving data to Firestore. A common dilemma faced by developers is understanding why the print statements in their code execute in an unexpected order, leading to confusion and bugs in their applications.

The Problem: Execution Order Confusion

In a typical Flutter app, when you are uploading images and expecting the app to wait for these operations before proceeding, you may find, as one developer reported, that the console outputs "2" before "1". In their code, a function meant to upload images and subsequently save URLs to Firestore was producing this output, and the expected results simply weren't materializing.

Here's a quick glance at the situation:

The expectation was for print('1') to execute before print('2'), indicating that the image upload was complete before any Firestore updates.

The actual output was "2, 0, 1", confusing the developer regarding the state of their picUrlList, which should have contained data by that point.

Understanding the forEach Issue

The core of the problem lies in the use of the forEach method. When you use forEach with an asynchronous function, the outer method does not wait for the inner async functions to complete before proceeding to the next line of code. Consequently, your Firestore updates may execute before any image uploads finish.

Why Does This Happen?

The forEach loop initiates the asynchronous operations, but it does not pause execution of the surrounding code until those operations are complete.

Solutions to Ensure await Functions as Expected

Fortunately, there are simple solutions to this common issue:

1. Replace forEach with a for loop

Instead of using forEach, a standard for loop will allow you to await the completion of each asynchronous task before moving on. Here’s how you can implement it:

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

2. Use await Future.forEach

Another modern approach is to use Future.forEach, which allows you to await each item in the collection before continuing:

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

Conclusion

By understanding how Dart's forEach function works with asynchronous calls, you can avoid the confusion of unexpected execution orders in your Flutter applications. Implementing either a traditional for loop or using Future.forEach will ensure that your await calls work as intended, allowing for smoother code execution and better management of asynchronous operations.

Always remember to test after making changes and observe the console output to confirm that your code behaves as expected. Happy coding!