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Discover how to obtain the parent size in Flutter for animated transitions in your AnimatedContainer. Learn solutions using LayoutBuilder for better UI experiences.
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This video is based on the question https://stackoverflow.com/q/62487312/ asked by the user 'MostHated' ( https://stackoverflow.com/u/5821692/ ) and on the answer https://stackoverflow.com/a/62487442/ provided by the user 'Federick Jonathan' ( https://stackoverflow.com/u/12402503/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Use LayoutBuilder in Flutter for Animated Container Width Adjustments

When building a Flutter app, you may encounter situations where you want to animate the resizing of a widget. For instance, you might have a layout with two Expanded widgets that dynamically change their sizes based on user interactions. You want to implement an AnimatedContainer to smoothly transition between these sizes rather than having an abrupt change. But how do you get the width of the parent Expanded widget to use within the AnimatedContainer? Let’s dive into the solution.

Understanding the Problem

In your scenario, you have two Expanded widgets that start off with a flex size of 50% each. According to certain events, one might expand to 75% while the other contracts down to 25%. Without an AnimatedContainer, these changes happen instantly, which is not visually appealing.

The main challenge is determining the current width of the Expanded widget to pass it to the AnimatedContainer. Simply using an AnimatedContainer as a direct parent of Expanded leads to Flex errors.

The Solution: Using LayoutBuilder

Fortunately, Flutter provides a handy widget called LayoutBuilder that allows you to build custom widgets based on the parent's constraints. Here’s how you can implement it:

Method 1: Using LayoutBuilder

To set the AnimatedContainer's width dynamically based on the parent's size, you can wrap the AnimatedContainer in a LayoutBuilder which gives you access to the incoming constraints. Here's a code snippet illustrating this:

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

Explanation:

LayoutBuilder: This widget provides the dimensions of the parent, which you can utilize to set the width of your AnimatedContainer.

AnimatedContainer: You specify the width to be constraints.maxWidth, ensuring it adopts the current width of the parent.

Method 2: Simpler Approach with double.infinity

If you want a more straightforward implementation without additional complexity, you can set the width of AnimatedContainer to double.infinity. Here’s the snippet:

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

Explanation:

double.infinity: By doing this, your AnimatedContainer will automatically take the available width of the parent Expanded. This method avoids Flex errors entirely and allows the animation to function smoothly.

Conclusion

Animations can significantly enhance the user experience in your Flutter application. By using LayoutBuilder, you take full control of how your widgets respond to changes in size. Alternatively, setting width to double.infinity provides a quick and effective solution.

Feel free to experiment with these methods, and take your Flutter UI to the next level with beautifully animated transitions! Happy coding!