Flutter：在 SliverAppbar 上预加载图像

Question

在我的 Flutter 应用程序中，我使用了一个 SliverAppbar，其中包含标题和图像。首次加载屏幕时，图像（资产文件夹中的本地图像）尚未出现，并且在加载后几乎不出现。 有没有办法预加载图像或在加载后淡入？

Answer 1

请使用precacheImage函数参考这个https://alex.domenici.net/archive/preload-images-in-a-stateful-widget-on-flutter

代码sn-p

  Image image1;

  @override
  void initState() {
    // adjust the provider based on the image type
    super.initState();
    image1 = Image.asset('assets/images/background.jpg',fit: BoxFit.cover);
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();

    precacheImage(image1.image, context);
  }

完整代码

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: Sample2(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

class Sample2 extends StatefulWidget {
  @override
  _Sample2State createState() => _Sample2State();
}

class _Sample2State extends State<Sample2> {
  Image image1;

  @override
  void initState() {
    // adjust the provider based on the image type
    super.initState();
    image1 = Image.asset('assets/images/background.jpg',fit: BoxFit.cover);
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();

    precacheImage(image1.image, context);
  }

  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Material(
        child: CustomScrollView(
          slivers: [
            SliverPersistentHeader(
              delegate: MySliverAppBar(expandedHeight: 200, image1: image1),
              pinned: true,
            ),
            SliverList(
              delegate: SliverChildBuilderDelegate(
                    (_, index) => ListTile(
                  title: Text("Index: $index"),
                ),
              ),
            )
          ],
        ),
      ),
    );
  }
}

class MySliverAppBar extends SliverPersistentHeaderDelegate {
  final double expandedHeight;
  final Image image1;

  MySliverAppBar({@required this.expandedHeight, this.image1});

  @override
  Widget build(
      BuildContext context, double shrinkOffset, bool overlapsContent) {
    return Stack(
      fit: StackFit.expand,
      overflow: Overflow.visible,
      children: [
        image1,
        Center(
          child: Opacity(
            opacity: shrinkOffset / expandedHeight,
            child: Text(
              "MySliverAppBar",
              style: TextStyle(
                color: Colors.white,
                fontWeight: FontWeight.w700,
                fontSize: 23,
              ),
            ),
          ),
        ),
        Positioned(
          top: expandedHeight / 2 - shrinkOffset,
          left: MediaQuery.of(context).size.width / 4,
          child: Opacity(
            opacity: (1 - shrinkOffset / expandedHeight),
            child: Card(
              elevation: 10,
              child: SizedBox(
                height: expandedHeight,
                width: MediaQuery.of(context).size.width / 2,
                child: FlutterLogo(),
              ),
            ),
          ),
        ),
      ],
    );
  }

  @override
  double get maxExtent => expandedHeight;

  @override
  double get minExtent => kToolbarHeight;

  @override
  bool shouldRebuild(SliverPersistentHeaderDelegate oldDelegate) => true;
}