目录
description bug_report

LayoutBuilder optimization

目录

Summary

This guide explains how to migrate Flutter applications after the LayoutBuilder optimization.

Context

LayoutBuilder and SliverLayoutBuilder call the builder function more often than necessary to fulfill their primary goal of allowing apps to adapt their widget structure to parent layout constraints. This has led to less efficient and jankier applications because widgets are rebuilt unnecessarily.

This transitively affects OrientationBuilder as well.

In order to improve app performance the LayoutBuilder optimization was made, which results in calling the builder function less often.

Apps that rely on this function to be called with a certain frequency may break. The app may exhibit some combination of the following symptoms:

  • The builder function is not called when it would before the upgrade to the Flutter version that introduced the optimization.
  • The UI of a widget is missing.
  • The UI of a widget is not updating.

Description of change

Prior to the optimization the builder function passed to LayoutBuilder or SliverLayoutBuilder was called when any one of the following happened:

  1. LayoutBuilder is rebuilt due to a widget configuration change (this typically happens when the widget that uses LayoutBuilder rebuilds due to setState, didUpdateWidget or didChangeDependencies).
  2. LayoutBuilder is laid out and receives layout constraints from its parent that are different from the last received constraints.
  3. LayoutBuilder is laid out and receives layout constraints from its parent that are the same as the constraints received last time.

After the optimization the builder function is no longer called in the latter case. If the constraints are the same and the widget configuration did not change, the builder function is not called.

Your app can break if it relies on the relayout to cause the rebuilding of the LayoutBuilder rather than on an explicit call to setState. This usually happens by accident. You meant to add setState, but you forgot because the app continued functioning as you wanted, and therefore nothing reminded you to add it.

Migration guide

Look for usages of LayoutBuilder and SliverLayoutBuilder and make sure to call setState any time the widget state changes.

Example: in the example below the contents of the builder function depend on the value of the _counter field. Therefore, whenever the value is updated, you should call setState to tell the framework to rebuild the widget. However, this example may have previously worked even without calling setState, if the _ResizingBox triggers a relayout of LayoutBuilder.

Code before migration (note the missing setState inside the onPressed callback):

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
      ),
      home: Counter(),
    );
  }
}

class Counter extends StatefulWidget {
  Counter({Key key}) : super(key: key);

  @override
  _CounterState createState() => _CounterState();
}

class _CounterState extends State<Counter> {
  int _counter = 0;

  @override
  Widget build(BuildContext context) {
    return Center(child: Container(
      child: LayoutBuilder(
        builder: (BuildContext context, BoxConstraints constraints) {
          return _ResizingBox(
            TextButton(
                onPressed: () {
                  _counter++;
                },
                child: Text('Increment Counter')),
            Text(_counter.toString()),
          );
        },
      ),
    ));
  }
}

class _ResizingBox extends StatefulWidget {
  _ResizingBox(this.child1, this.child2);

  final Widget child1;
  final Widget child2;

  @override
  State<StatefulWidget> createState() => _ResizingBoxState();
}

class _ResizingBoxState extends State<_ResizingBox>
    with SingleTickerProviderStateMixin {
  Animation animation;

  @override
  void initState() {
    super.initState();
    animation = AnimationController(
      vsync: this,
      duration: const Duration(minutes: 1),
    )
      ..forward()
      ..addListener(() {
        setState(() {});
      });
  }

  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisSize: MainAxisSize.min,
      children: [
        SizedBox(
          width: 100 + animation.value * 100,
          child: widget.child1,
        ),
        SizedBox(
          width: 100 + animation.value * 100,
          child: widget.child2,
        ),
      ],
    );
  }
}

Code after migration (setState added to onPressed):

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
      ),
      home: Counter(),
    );
  }
}

class Counter extends StatefulWidget {
  Counter({Key key}) : super(key: key);

  @override
  _CounterState createState() => _CounterState();
}

class _CounterState extends State<Counter> {
  int _counter = 0;

  @override
  Widget build(BuildContext context) {
    return Center(child: Container(
      child: LayoutBuilder(
        builder: (BuildContext context, BoxConstraints constraints) {
          return _ResizingBox(
            TextButton(
                onPressed: () {
                  setState(() {
                    _counter++;
                  });
                },
                child: Text('Increment Counter')),
            Text(_counter.toString()),
          );
        },
      ),
    ));
  }
}

class _ResizingBox extends StatefulWidget {
  _ResizingBox(this.child1, this.child2);

  final Widget child1;
  final Widget child2;

  @override
  State<StatefulWidget> createState() => _ResizingBoxState();
}

class _ResizingBoxState extends State<_ResizingBox>
    with SingleTickerProviderStateMixin {
  Animation animation;

  @override
  void initState() {
    super.initState();
    animation = AnimationController(
      vsync: this,
      duration: const Duration(minutes: 1),
    )
      ..forward()
      ..addListener(() {
        setState(() {});
      });
  }

  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisSize: MainAxisSize.min,
      children: [
        SizedBox(
          width: 100 + animation.value * 100,
          child: widget.child1,
        ),
        SizedBox(
          width: 100 + animation.value * 100,
          child: widget.child2,
        ),
      ],
    );
  }
}

Watch for usages of Animation and LayoutBuilder in the same widget. Animations have internal mutable state that changes on every frame. If the logic of your builder function depends on the value of the animation, it may require a setState to update in tandem with the animation. To do that, add an animation listener that calls setState, like so:

Animation animation = … create animation …;
animation.addListener(() {
  setState(() {
    // Intentionally empty. The state is inside the animation object.
  });
});

Timeline

This change was released in Flutter v1.20.0.

References

API documentation:

Relevant issue:

Relevant PR: