Скачать или смотреть Resolving the Flutter TypeError - Future Not a Subtype of Widget

Learn how to fix the `TypeError: type 'Future Null ' is not a subtype of type 'Widget'` in your Flutter login app by refactoring your widget build methods for seamless navigation.
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This video is based on the question https://stackoverflow.com/q/64746332/ asked by the user 'blackbird' ( https://stackoverflow.com/u/14236206/ ) and on the answer https://stackoverflow.com/a/64746695/ provided by the user 'Ravi Singh Lodhi' ( https://stackoverflow.com/u/9511650/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving the Flutter TypeError - Future Not a Subtype of Widget

While developing a login app with Azure AD in Flutter, you may encounter a frustrating error message: "The following TypeError was thrown ... type 'Future Null ' is not a subtype of type 'Widget'". In this guide, we will explore why this error occurs and how to effectively fix it, allowing your app to run smoothly and transition seamlessly between pages.

Understanding the Problem

The error you're experiencing is due to a mismatch in the expected widget types in your build method. Flutter requires widgets to be returned in certain contexts, and returning a Future instead leads to this type mismatch. To summarize, the issue arises in the _SignInState class where navigation logic is incorrectly placed.

Error Context

Here’s a brief backdrop of the context when the error is thrown:

You are building a simple login app.

Upon signing in, the app should redirect the user to another screen.

However, upon executing the navigation code, you receive the following output in your debug console:

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

This indicates there is an attempt to use a future operation where a widget is expected.

Breaking Down the Solution

Now, let's move on to the solution to this issue.

1. Identify the Problem Area

In the build method of your _SignInState class, you have the conditional statement:

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

This section tries to navigate to a new screen if the user is signed in by returning a Future instead of a widget.

2. Refactor the Widget Tree

To effectively address the error, utilize the Visibility widget. This widget allows you to show or hide other widgets based on a boolean value. For instance:

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

In this case, the button will only show if the user is not signed in.

3. Handle Navigation Properly

You should also move the navigation logic outside the widget tree. A good place to implement sign-in navigation is within the initState method of your StatefulWidget. Here’s how you can structure it:

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

Conclusion

By following the steps outlined above, you will be able to address the TypeError: type 'Future<Null>' is not a subtype of type 'Widget'. Refactoring your conditional rendering to utilize the Visibility widget and structuring navigation in the initState method ensures your Flutter app runs smoothly without crashes or unexpected errors.

Remember!

A Future cannot be used directly in a widget tree where a Widget is expected.

Always ensure your navigation logic is placed correctly in the lifecycle methods.

By implementing these changes, your app will enhance the user experience by eliminating abrupt errors during login. Happy coding!