Скачать или смотреть How to Properly Cancel a StreamSubscription in Flutter

Learn how to halt a StreamSubscription in Flutter properly to ensure you stop receiving unwanted data.
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This video is based on the question https://stackoverflow.com/q/62337304/ asked by the user 'DrkStr' ( https://stackoverflow.com/u/2544048/ ) and on the answer https://stackoverflow.com/a/62421142/ provided by the user 'Michel Feinstein' ( https://stackoverflow.com/u/1227166/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Ensuring Smooth Streaming: How to Cancel a StreamSubscription in Flutter

In Flutter development, working with streams is a common practice, especially for dealing with asynchronous data such as events from a USB device. However, one of the common challenges developers face is canceling a StreamSubscription effectively. In this guide, we’ll explore a specific scenario involving a StreamSubscription related to a USB input stream, discuss the problem at hand, and delve into the solution for ensuring that you stop receiving data when you no longer need it.

The Problem: Unwanted Data Flow

You have a StreamSubscription set up to listen to incoming data from a USB device. Here’s a simplified version of your implementation:

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

The input stream is defined as a broadcast stream, allowing multiple listeners. However, once you decide to stop listening, you call the cancel function:

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

Despite calling this, you continue to receive messages in your onDataReceivedFromUSBSerial method. This raises a vital question: How do you effectively stop listening to your usbStream to prevent further data from being processed?

The Solution: Awaiting the Cancellation

The solution lies in ensuring that you properly await the asynchronous cancellation of your stream subscription. When you call cancel(), you must await its completion before assuming that the subscription is stopped. Here’s how you can modify your code:

Step-by-step Guide

Await the Cancellation:
Modify your cancellation line of code to properly handle the asynchronous operation. This ensures that the subscription is fully canceled before proceeding. Here’s how it should look:

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

Check for Completion:
After you cancel the subscription, it might be a good idea to check whether the listening part of your program reflects that change. This ensures no further callbacks are invoked on your method.

Testing:
Consider implementing test cases to ensure that once you cancel the StreamSubscription, no further data is processed.

Example Code

Here is your updated subscription cancellation code, reflecting the necessary changes:

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

Conclusion: Taking Control of Your Streams

In Flutter, especially when dealing with streams, proper management is crucial for maintaining performance and preventing unexpected behavior. By ensuring that you await the cancellation of your StreamSubscription, you can effectively stop receiving data when it’s no longer needed. This simple change in your implementation can save you from headaches down the line and lead to cleaner, more efficient code.

If you find yourself in a similar situation, remember to always await asynchronous calls to ensure you have complete control over your subscriptions. Happy coding!