Solving the Bad state: Future already completed Error in Flutter WebView Apps

Описание: Learn how to resolve the `Bad state: Future already completed` error in Flutter when using multiple WebView widgets within a CupertinoTabView setup.
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This video is based on the question https://stackoverflow.com/q/63487029/ asked by the user 'klados' ( https://stackoverflow.com/u/709745/ ) and on the answer https://stackoverflow.com/a/63489038/ provided by the user 'klados' ( https://stackoverflow.com/u/709745/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Solving the Bad state: Future already completed Error in Flutter WebView Apps

Creating a smooth and functional app with Flutter is an exciting endeavor, especially when it involves integrating services like web views. However, developers often face errors that can be frustrating and time-consuming to resolve. One such error that you might encounter while working with WebView and CupertinoTabView is the "Bad state: Future already completed" error. In this post, we’ll break down this problem and provide a clear solution.

The Problem

Imagine you’re developing a Flutter app that includes multiple CupertinoTabViews, each containing a different WebView for displaying various content. The app successfully launches and displays the first WebView correctly. However, upon clicking a tab to switch to another WebView, you encounter the dreaded error:

Unhandled Exception: Bad state: Future already completed.

This occurs primarily due to the way Completer is utilized in your implementation. Let’s explore the code to understand exactly what’s happening.

Code Overview

In your main.dart, you have set up your main application logic. When the user clicks a tab to switch views, it tries to complete the Completer that has already been completed with an initial WebViewController. Here’s a brief snippet from the relevant code:

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

The issue arises because the _controller, which is a static instance, can only be completed once. When the user navigates back and forth between tabs, the existing Completer is already completed and can throw an error if you try to complete it again.

The Solution

The solution to this problem is quite simple and effective. The best practice is to move the _controller declaration inside the PageWeb class. This ensures that each instance of PageWeb has its own Completer, thus avoiding conflicts when users switch between tabs.

Updated Code Implementation

Here's how you can implement the fix:

1. Move _controller inside PageWeb

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

2. Use this controller for web view actions within the state class.

This change will ensure that each WebView has a distinct controller, and switching tabs won’t cause any state conflicts.

Conclusion

The "Bad state: Future already completed" error is a common pitfall when dealing with multiple WebView widgets in a Flutter application. By understanding the structure of your code and the behavior of Completer, you can resolve this and create a seamless experience for your users.

If you’re ever stuck, remember: sometimes the solution is as simple as adjusting your variable scope! Happy coding!