Preserving Widget State in Flutter: Techniques and Best Practices

5.4.1 Preserving Widget State

In the world of mobile app development, user experience is paramount. One critical aspect of delivering a seamless user experience is ensuring that the state of your application is preserved as users navigate through different screens. Imagine filling out a form or scrolling through a list, only to lose your progress when switching to another screen. This can be frustrating for users and can lead to a poor app experience. In this section, we will explore the importance of preserving widget state in Flutter applications, delve into the use of PageStorage, and understand the role of keys in maintaining state across navigation.

Why State Preservation Matters

State preservation is crucial for maintaining a consistent and intuitive user experience. When users interact with an app, they expect their actions to be remembered, even as they navigate between different parts of the application. Here are some scenarios where state preservation is essential:

• Form Inputs: Users may fill out lengthy forms, and losing this data upon navigating away can be frustrating.
• Scroll Positions: When scrolling through a list or a feed, users expect to return to the same position when they navigate back.
• User Preferences: Settings or preferences that users adjust should remain consistent across sessions.

Preserving state ensures that users can pick up right where they left off, enhancing the overall usability and satisfaction with the app.

Using PageStorage

Flutter provides a convenient way to preserve the state of widgets using PageStorage. This is particularly useful for storing page-related state information, such as scroll positions or form inputs, which need to be retained across navigation.

What is PageStorage?

PageStorage is a widget that stores the state of its descendants. It uses a PageStorageBucket to save and restore the state of widgets that have a PageStorageKey. This mechanism is especially useful for preserving the state of scrollable widgets like ListView or PageView.

Example: Preserving Scroll Position

Let’s consider an example where we want to preserve the scroll position of a ListView when navigating away and back to it.

import 'package:flutter/material.dart';

class MyListViewPage extends StatefulWidget {
  @override
  _MyListViewPageState createState() => _MyListViewPageState();
}

class _MyListViewPageState extends State<MyListViewPage> {
  ScrollController _scrollController;

  @override
  void initState() {
    super.initState();
    _scrollController = ScrollController();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Preserve Scroll Position')),
      body: ListView.builder(
        key: PageStorageKey('myListView'),
        controller: _scrollController,
        itemCount: 100,
        itemBuilder: (context, index) {
          return ListTile(
            title: Text('Item $index'),
          );
        },
      ),
    );
  }
}

In this example, the ListView is assigned a PageStorageKey. This key is used by PageStorage to save and restore the scroll position of the list. When the user navigates away and then returns to this page, the scroll position is preserved.

Applying Keys

Keys play a vital role in preserving the state of widgets in Flutter. They help Flutter identify which widgets need to be updated or preserved when the widget tree is rebuilt. There are different types of keys, each serving a specific purpose:

• ValueKey: This key uses a value to identify widgets. It’s useful when you have a unique identifier for each widget, such as an ID.
• ObjectKey: Similar to ValueKey, but it uses an object to identify widgets. This is useful when the object itself is the identifier.
• UniqueKey: This key generates a unique value every time it’s created. It’s useful when you want to ensure that a widget is always treated as a new widget.

Example: Using Keys

import 'package:flutter/material.dart';

class MyWidget extends StatelessWidget {
  final String id;

  MyWidget({Key key, this.id}) : super(key: ValueKey(id));

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Text('Widget with ID: $id'),
    );
  }
}

In this example, a ValueKey is used to uniquely identify the widget based on its id. This ensures that the widget’s state is preserved across rebuilds.

Best Practices

• Use Keys Sparingly: While keys are powerful, they should be used judiciously. Overusing keys can lead to performance issues and unexpected behavior.
• Be Consistent with Key Assignments: Ensure that keys are consistently assigned to widgets that need state preservation. Inconsistent key usage can lead to state loss.
• Understand the Purpose of Each Key Type: Choose the appropriate key type based on your use case. For instance, use ValueKey when you have a unique identifier, and UniqueKey when you want to force a widget to be treated as new.

Exercise: Preserve Scroll Position

As an exercise, try creating a list where the scroll position is preserved when navigating back to it. Use PageStorage and PageStorageKey to achieve this. Experiment with different key types and observe their effects on state preservation.

Conclusion

Preserving widget state is a fundamental aspect of creating a seamless user experience in Flutter applications. By leveraging PageStorage and understanding the role of keys, you can ensure that your app retains important state information across navigation events. This not only enhances user satisfaction but also contributes to a more polished and professional application.

For further exploration, consider diving into the official Flutter documentation on State Management and Navigation to deepen your understanding of these concepts.

Quiz Time!