Understanding Why Exceptions Are Not Caught in async Functions in Dart

Discover why exceptions in Dart's `async` functions aren't caught as you'd expect, and learn how to properly handle them using `await`.
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This video is based on the question https://stackoverflow.com/q/70196704/ asked by the user 'Daniel Dai' ( https://stackoverflow.com/u/1089262/ ) and on the answer https://stackoverflow.com/a/70197063/ provided by the user 'Alexandre Ardhuin' ( https://stackoverflow.com/u/634863/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Why Exceptions Are Not Caught in async Functions in Dart

Dart's asynchronous programming model makes it easy to handle operations that take time, such as loading data from a database or calling an API. However, one common stumbling block developers face is catching exceptions in async functions. In this post, we will explore why certain exceptions aren’t caught in async functions as you might expect and how to effectively handle them.

The Problem: Uncaught Exceptions in Async Functions

Consider the following Dart function that is designed to fetch a user's order. The function simulates a delay, during which it throws an exception if a user ID is invalid:

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When running this code, you might expect the exception to be caught and printed. However, it isn’t, leading to the following output:

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This behavior raises the question: Why is the exception not caught in the catch block?

The Explanation: Awaiting Futures in Dart

The crux of the problem lies in the usage of Futures and how exceptions are managed in Dart's async functions. Here’s how it works:

Try-Catch Only Catches Awaited Futures

In Dart, a try-catch block can only catch exceptions from awaited Futures. In the original code, the Future.delayed function is not awaited, so the exception thrown inside it escapes the try-catch block. To fix this issue, you must use the await keyword before the Future.delayed call:

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Key Changes Made

Added await: By preceding the Future.delayed call with await, you ensure that the calling function waits for the completion of the Future before continuing execution.

Catching Exceptions: With await, if an exception is thrown within that Future, it will be caught by the surrounding try-catch.

Summary

To effectively handle exceptions in Dart's async functions, always remember to use await with your Future-returning function calls within a try-catch block. By making this small change, you can ensure that exceptions are properly handled, leading to more robust and error-resilient applications. If you find yourself encountering uncaught exceptions in your async code, check if you're properly awaiting your Futures!

In conclusion, mastering the handling of exceptions in async functions is essential for writing efficient Dart code. With the proper understanding and handling of Futures, you can enhance your coding practices and avoid running into these common pitfalls.

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