Stop Unnecessary Rerenders: Mastering Techniques to Flutter Prevent Widget Rebuild

In every Flutter developer's journey, the challenge to optimally manage the widget tree arrives sooner or later. The focus is to explore how we, as Flutter developers, can prevent an unwanted widget rebuild. It's crucial to comprehend Flutter's widget lifecycle and that every time you modify a state, Flutter rebuilds the widget tree. But unnecessary rebuilds can be a problem. So, how can we "Flutter prevent widget rebuild"?

Understanding Widget Rebuild in Flutter

In the world of Flutter, widgets are the building blocks of the UI. They form the widget tree, with each widget having its parent widget and possibly child widgets. Each widget holds its state, which, when changed, triggers a build method call and, consequently, a rebuild of the widget. However, when Flutter rebuilds the widget, it's not merely the same widget but a new widget instance. The lifecycle of a widget in Flutter involves creation, update, and destruction phases, and it's during the update phase when the unwanted widget build may occur.

So, our aim, as Flutter developers, is to avoid unnecessary widget rebuilds and efficiently manage the update phase in the widget lifecycle.

Techniques to Prevent Widget Rebuild in Flutter

We must understand a few crucial techniques to manage the widget tree and prevent unnecessary widget rebuilds effectively. These methods help Flutter devs handle the internal state of widgets and avoid creating new widgets continuously.

The very first technique we dive into is using the const keyword. We will then explore using StatefulWidget to preserve the state and avoid creating new widgets. Lastly, we'll look at how to reduce Flutter widget rebuild with the help of Provider and ChangeNotifier.

Use of 'const' Constructor

In Flutter, constructors can be declared const, meaning their objects are immutable. Once a const widget (i.e., const constructor) is created, it won't change throughout its lifespan. Flutter uses this to its advantage to prevent widget rebuilds.

1Widget build(BuildContext context) {
2  return Column(
3    children: const <Widget>[
4      Text('Hello'),
5      Text('World'),
6    ],
7  );
8}

In the above Flutter code snippet, the Text widgets are created with a const keyword. This means that unless its parent widget requests the UI to rebuild, the Text widget isn't going to rebuild. Hence, using the const keyword in suitable places can prevent unnecessary widget rebuilds in our Flutter applications.

StatefulWidget and the Reuse of State

In those scenarios where the data (state) can change over time, like user input or a data change from a network request, using the 'const' keyword is impossible. For these cases, Flutter provides a type of widget called StatefulWidget.

A StatefulWidget, as the name suggests, carries state. It's segmented into two classes - Widget class and State class. The key here is that the State object, carrying the mutable information, persists over widget rebuilds. A new widget instance is created but connected to the same State object.

1class MyHomePage extends StatefulWidget {
2  MyHomePage({Key key, this.title}) : super(key: key);
3  final String title;
4
5  @override
6  _MyHomePageState createState() => _MyHomePageState();
7}

Whenever the internal state of your app changes, for example, the user presses a button, changing a value, you can call setState(), which triggers a build call, creating a new widget. But, crucially, the state object is reused.

Provider and ChangeNotifier

Provider and ChangeNotifier are powerful tools for managing state and preventing unnecessary widget rebuilds.

The Provider serves as a middleman, providing data from the top of the tree (where you declare the provider) down to any desired child widget. The ChangeNotifier, on the other hand, is a simple class that can be extended or mixed in to provide change notifications to its listeners.

The primary essence of Provider combined with ChangeNotifier lies in the ability to listen to changes in a specific part of your widget tree and only rebuild those that rely on that state.

Consider the following Flutter example:

1class Counter with ChangeNotifier {
2  int value = 0;
3
4  void increment() {
5    value += 1;
6    notifyListeners();
7  }
8}
9
10class MyApp extends StatelessWidget {
11  @override
12  Widget build(BuildContext context) {
13    return ChangeNotifierProvider(
14      create: (context) => Counter(),
15      child: MaterialApp(
16        home: MyHomePage(),
17      ),
18    );
19  }
20}

In the above code, any changes in Counter will notify the listeners in the MyHomePage widget and prompt a rebuild, but won't affect other widgets.

Other Advanced Techniques

Apart from the aforementioned techniques, various advanced solutions exist to achieve your desired goal of preventing unnecessary widget rebuilds. Bloc, Riverpod, and Redux are popular Flutter state management techniques that offer high versatility for controlling widget rebuilds and managing complex state-related scenarios.

Utilizing Riverpod or Bloc, for instance, offers more explicit state management and actions, while Redux enforces a unidirectional data flow to make state mutations predictable. While they present a steeper learning curve, mastering these could beneficially impact your Flutter development journey.

Conclusion

Achieving optimized performance is important for any Flutter application, and to reach that goal, keeping control over widget rebuilds plays a crucial role. We can significantly minimise unwanted widget rebuilds by utilizing Flutter's built-in capabilities, like the const keyword or StatefulWidget, and other state management techniques, such as Provider and ChangeNotifier.

So, Flutter developers, remember these techniques and utilize them to optimize your Flutter app performance next time you work on your widget tree. With these tools, you can generate optimized, efficient, high-performance Flutter applications.

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