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Learn how to effectively use `FutureBuilder` in Flutter to ensure your `build` function waits for async data retrieval.
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This video is based on the question https://stackoverflow.com/q/71674873/ asked by the user 'sokhib Abdurasulov' ( https://stackoverflow.com/u/15485056/ ) and on the answer https://stackoverflow.com/a/71679665/ provided by the user 'Loren.A' ( https://stackoverflow.com/u/15170515/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Make the build Function Wait for an Async Function in Flutter

When developing apps with Flutter, you might stumble upon scenarios where the widget's build function doesn't wait for an async function to complete, leading to unexpected behavior. This issue usually arises when fetching data asynchronously, such as user details, which the UI depends on.

In this guide, we will explore how to use the FutureBuilder widget to resolve this common problem, allowing your Flutter app to handle async data seamlessly.

The Problem

In the initial code example provided, the developers tried to fetch user data (a phone number and password) from SharedPreferences during the widget's lifecycle. However, the data was not immediately available when the build method executed. As a result, the URLs where the app navigates based on these values were constructed using null values before eventually updating after the async call finished.

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In such cases, the sequential nature of async calls conflicts with the immediate needs of the UI. The build function executes right away without waiting for the async operation to complete, which can lead to errors or empty states in your UI.

The Solution: Using FutureBuilder

To mitigate this issue, we can utilize the FutureBuilder widget, which allows you to build a widget based on the latest snapshot of interaction with a Future. The FutureBuilder provides a simple way to run an async function within the build method while controlling the UI state based on loading processes or errors.

Refactoring the Code

Here's how to refactor the given code by implementing FutureBuilder to ensure the UI waits for the async operation:

Step 1: Update the retrieveStringValue function

Instead of directly modifying UI state, we will create a function that returns a Map containing our data.

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Step 2: Implement FutureBuilder in the build method

Next, in the build method, we will wrap our WebView inside a FutureBuilder.

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Summary of Changes

Async Data Retrieval: Returning data in a format that's compatible with the FutureBuilder.

Dynamic UI: The UI reflects the loading state with a spinner until data is available.

Error Handling: This structure allows for potential error states where you can add additional feedback if needed.

Conclusion

By implementing FutureBuilder, you not only enhance the user experience but also manage asynchronous data more effectively in Flutter. This approach ensures that your build function waits for the necessary data to be retrieved, preventing null or incomplete values from breaking your application’s flow.

Now you can confidently build Flutter applications that wait for async functions to complete, improving the reliability of your UI interactions. Happy coding!