layout - 当 FutureBuilder 在 LayoutBuilder 中时 flutter 无限循环

Question

我在 LayoutBuilder 中有一个 PageView 来获取小部件的大小。由于它取决于小部件的大小，因此在构建小部件之前我不知道会有多少页面。所以我在LayoutBuilder里面加了FutureBuilder，这样就可以异步计算页数了。这是我的代码，但它在 ConnectionState.waiting 中无限期等待。代码中有什么问题，如何解决？

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(home: ScrollableTabsDemo());
  }
}

class _Page {
  const _Page({this.icon, this.text});
  final IconData icon;
  final String text;
}

const List<_Page> _allPages = <_Page>[
  _Page(text: 'tab 1'),
  _Page(text: 'tab 2'),
  _Page(text: 'tab 3'),
];

class ScrollableTabsDemo extends StatefulWidget {
  static const String routeName = '/material/scrollable-tabs';

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

class ScrollableTabsDemoState extends State<ScrollableTabsDemo>
    with SingleTickerProviderStateMixin {
  TabController _controller;

  @override
  void initState() {
    super.initState();
    _controller = TabController(vsync: this, length: _allPages.length);
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    final Color iconColor = Theme.of(context).accentColor;
    return Scaffold(
      appBar: AppBar(
        title: const Text('Scrollable tabs'),
        bottom: TabBar(
          controller: _controller,
          isScrollable: true,
          tabs: _allPages.map<Tab>((_Page page) {
            return Tab(text: page.text);
          }).toList(),
        ),
      ),
      body: TabBarView(
        controller: _controller,
        children: _allPages.map<Widget>((_Page page) {
          return SafeArea(
            top: false,
            bottom: false,
            child: LayoutBuilder(builder: (context, constraints) {
              return FutureBuilder<int>(
                  future: getPageCount(),
                  builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
                    switch (snapshot.connectionState) {
                      case ConnectionState.none:
                        print("none ${snapshot.data}");
                        return Text('none');
                      case ConnectionState.active:
                        print("active ${snapshot.data}");
                        return Text('active');
                      case ConnectionState.waiting:
                        print("waiting ${snapshot.data}");
                        return Center(child: CircularProgressIndicator());
                      case ConnectionState.done:
                        print("done ${snapshot.data}");
                        return buildPageView(snapshot.data);
                    }
                  });
            }),
          );
        }).toList(),
      ),
    );
  }

  Future<int> getPageCount() => Future.delayed(Duration(seconds: 3), () => 5);

  Widget buildPageView(int pageCount) {
    return PageView.builder(
      itemBuilder: (context, position) {
        return Container(child: Center(child: Text(position.toString())));
      },
      itemCount: pageCount,
    );
  }
}

Answer 1

FutureBuilder 的文档声明您不应在每次构建时都创建 future :https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html

要么在 initState 中运行 getPageCount，在字段初始化器中，要么准备一个类似于 FutureBuilder 的有状态小部件，它需要一个 future 创建函数和在它自己的 initState 中调用此函数。

编辑:使用此助手，您可以自动缓存 future

class CachingFutureBuilder<T> extends StatefulWidget {
  final Future<T> Function() futureFactory;
  final AsyncWidgetBuilder<T> builder;

  const CachingFutureBuilder(
      {Key key, @required this.futureFactory, @required this.builder})
      : super(key: key);
  @override
  _CachingFutureBuilderState createState() => _CachingFutureBuilderState<T>();
}

class _CachingFutureBuilderState<T> extends State<CachingFutureBuilder<T>> {
  Future<T> _future;

  @override
  void initState() {
    _future = widget.futureFactory();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder<T>(
      future: _future,
      builder: widget.builder,
    );
  }
}

如果您的 getPageCount 需要大小作为输入，您可以像下面的代码一样使用该辅助小部件。诀窍是使用 ValueKey 以便 Flutter 知道重新初始化 CachingFutureBuilder。确保您没有任何大小更改动画，因为它会导致 Future 在每一帧都重新加载。

class ScrollableTabsDemo extends StatelessWidget {
  Future<int> getPageCount(Size size) {
    return Future.value(1);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: LayoutBuilder(
          builder: (context, constraints) {
            final size = Size(constraints.maxHeight, constraints.maxWidth);
            return CachingFutureBuilder<int>(
              key: ValueKey(size),
              futureFactory: () => getPageCount(size),
              builder: (context, snapshot) {
                return Text(snapshot.toString());
              },
            );
          },
        ),
      ),
    );
  }
}