Flutter 解决Navigator跳转导致Rebuild的一种方案

Flutter开发者都知道，页面每次push和pop 时都会build当前页，滑动位置等数据因此被销毁，这或许是Flutter团队的一种调优策略，但在实际业务中反而造成了一些困扰，github关于此问题也争论了很久，相关issue，个人认为rebuild问题还是需要解决下的（或许是没有完全适应Flutter的UI模式，至少目前需要解决）

/// ！解决rebuild问题 ！
///
/// 相关issue：https://github.com/flutter/flutter/issues/11655?tdsourcetag=s_pcqq_aiomsg
///
/// 所有State类请继承[XCState]，勿继承[State]
///
/// 所有State类请重写[XCState.shouldBuild]，勿重写[State.build];
///
/// 刷新请使用[XCState.reload]，勿使用[State.setState]

源码

XCShouldBuild.dart

import 'package:flutter/widgets.dart';

typedef XCShouldBuildFunction = bool Function(
    T oldSubstance, T newSubstance);

class XCShouldBuild extends StatefulWidget {
  final T substance; // substance
  final XCShouldBuildFunction shouldBuildFunction;
  final WidgetBuilder builder;
  XCShouldBuild(
      {this.substance, this.shouldBuildFunction, @required this.builder})
      : assert(() {
          if (builder == null) {
            throw FlutterError.fromParts([
              ErrorSummary('error in XCShouldBuild: builder must exist')
            ]);
          }
          return true;
        }());
  @override
  _XCShouldBuildState createState() => _XCShouldBuildState();
}

class _XCShouldBuildState extends State {
  Widget oldWidget;
  T oldSubstance;

  bool _isInit = true;

  @override
  Widget build(BuildContext context) {
    final newSubstance = widget.substance;

    if (_isInit ||
        (widget.shouldBuildFunction == null
            ? true
            : widget.shouldBuildFunction(oldSubstance, newSubstance))) {
      _isInit = false;
      oldSubstance = newSubstance;
      oldWidget = widget.builder(context);
    }
    return oldWidget;
  }

  @override
  void dispose() {
    super.dispose();
    debugPrint("_XCShouldBuildState dispose");
  }
}

XCState.dart

import 'package:flutter/widgets.dart';
import 'XCShouldBuild.dart';

/// ## Sumarry

/// - this is a way of resolving the issue of rebuilding stateful widget when a navigator is pushing or popping

/// - associated issue：https://github.com/flutter/flutter/issues/11655?tdsourcetag=s_pcqq_aiomsg

///  ## Usage

/// 1. import XCShouldBuild.dart and XCState.dart

/// 2. make all classes of State inherit from [XCState]，do not inherit from [State]

/// 3. override [XCState.shouldBuild]，do not override [State.build]

/// 4. use [XCState.reload] to reload，do not use [State.setState] to reload
abstract class XCState extends State {
  bool _isShouldBuild = false;

  @override
  Widget build(BuildContext context) {
    bool willUseSubstance = useSubstance();

    return XCShouldBuild(
        substance: willUseSubstance ? substance() : null,
        shouldBuildFunction: (oldSubstance, newSubstance) {
          bool willReload;
          if (_isShouldBuild) {
            willReload = true;
          } else {
            if (willUseSubstance) {
              willReload = oldSubstance != newSubstance;
            } else {
              willReload = false;
            }
          }
          return willReload;
        },
        builder: (BuildContext context) {
          _isShouldBuild = false;
          return shouldBuild(context);
        });
  }

  Widget shouldBuild(BuildContext context);

  bool useSubstance() => false;

  T substance() => null;

  void reload([VoidCallback fn]) {
    if (!mounted) return;
    setState(() {
      _isShouldBuild = true;
      if (fn != null) {
        fn();
      }
    });
  }

  @override
  void dispose() {
    super.dispose();
    debugPrint("XCState dispose");
  }
}

场景1：

在Push或Pop时不rebuild（默认）

class _XCTestViewState extends XCState {

  @override
  Widget shouldBuild(BuildContext context) {
    return Center(
        child: Text(
            "_XCTestView, if you use `shouldBuild`, it can avoid rebuilding when pushing or popping"));
  }
    
  void aFunc() {
    reload()
  }

}

场景2：

也可以重写参照物方法，决定Push或Pop时是否rebuild

class _XCUseSubstanceTestViewState extends XCState {

  var _aCountAsSubstance = 0;

  @override
  Widget shouldBuild(BuildContext context) {
    return Center(
        child: Text(
            "_XCUseSubstanceTestView, if you use `shouldBuild`, it can avoid rebuilding when pushing or popping, _aCountAsSubstance = $_aCountAsSubstance "));
  }

  /// only effect to pushing or poping, no effect to [XCState.reload]
  @override
  bool useSubstance() {
    return true;
  }

  /// only effect to pushing or poping, no effect to [XCState.reload]
  @override
  substance() {
    return _aCountAsSubstance;
  }

  void aFunc() {
    reload();
  }
}

附GitHubDemo

2021.04.16 updated

Flutter官方已改进了跳转时的rebuild方案：
详情见：
flutter release-notes-1.17.0中的这几个pr48900、49366、49376