How to Use Non-Final Fields in a Stateful Widget Constructor in Flutter

Learn how to effectively manage non-final fields in Flutter stateful widgets by utilizing the `didUpdateWidget` method for dynamic updates.
---
This video is based on the question https://stackoverflow.com/q/70328009/ asked by the user 'darkstar' ( https://stackoverflow.com/u/13704230/ ) and on the answer https://stackoverflow.com/a/70328665/ provided by the user 'Stephan' ( https://stackoverflow.com/u/5647935/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: How to have non-final fields in Stateful Widget Constructor?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
How to Use Non-Final Fields in a Stateful Widget Constructor in Flutter

When working with Flutter, developers often encounter challenges related to state management, especially in the context of stateful widgets. One common dilemma arises when creating nested stateful widgets. The issue becomes particularly clear when you have a situation where the parent widget’s state changes, but the child's internal state does not update correctly because it relies on final field values. In this post, we will explore how to effectively manage non-final fields in stateful widget constructors and ensure that they update seamlessly when parent state changes.

Understanding the Problem

In a typical scenario where you have a parent WidgetOne and a child WidgetTwo, you might have fields that are set as final in WidgetTwo. This helps to avoid warnings from Flutter regarding immutability. However, when the state in WidgetOne changes and is updated using setState, you might find that WidgetTwo does not receive the updated values as expected. The underlying issue is that initState in WidgetTwo is called only once when the widget is created. As a result, it does not respond to state changes in the parent widget.

Key Points of the Problem:

initState runs only the first time a widget is created.

Parent widget changes do not trigger the re-initialization of child widget fields (staying final).

The didChangeDependencies() method fails to address this issue as well.

Proposed Solution

To solve this issue, we must leverage the didUpdateWidget method, which is invoked whenever the parent widget rebuilds and the properties of the child widget change. This method provides an opportunity to react to changes in the widget's properties and update internal state accordingly.

Implementing didUpdateWidget

Here is a step-by-step implementation to ensure that WidgetTwo updates its internal state based on changes from WidgetOne:

Change WidgetOne to Manage States:

The non-final fields are not declared in WidgetTwo, as it will receive updated values from WidgetOne. The changes will be handled directly in the build method of WidgetTwo.

Utilize didUpdateWidget:

In WidgetTwo, override the didUpdateWidget method to check for changes in the widget properties and respond accordingly.

Example Implementation

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

Conclusion

In conclusion, dealing with non-final fields in stateful widgets can present a challenge when trying to maintain responsive UIs in Flutter. By utilizing the didUpdateWidget method, you can create a robust solution that allows your child widget to respond dynamically to changes in the parent widget's state. This approach helps maintain Flutter's best practices while avoiding unnecessary warnings and ensuring proper state management throughout the widget lifecycle.

With this knowledge, you can confidently enhance your Flutter applications and create smooth, responsive user interfaces that dynamically adjust to changing state.