Flutter Stateful Widget重新创建状态

Question

我正在开发一个Flutter应用程序，并认识到状态管理的意外行为。我创建了一个示例应用程序来重现此行为，您可以在下面找到代码和日志输出。

该应用程序包含一个简单的ListView，其中包含10个有状态的容器（文本+装饰）。 当我向下滚动时，每个容器及其容器状态都会像预期的那样创建一次。当我再次向上滚动时，Flutter为再次出现在显示屏上的每个容器小部件重新创建每个状态（而不是容器小部件）。 我希望flutter能够检索先前的状态而无需重新创建整个状态对象。 我在这里做错什么了吗？

示例代码：

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key) {
    print("MyHomePage constructor");
  }

  @override
  _MyHomePageState createState() {
    print("createState");
    return _MyHomePageState();
  }
}

class _MyHomePageState extends State<MyHomePage> {
  _MyHomePageState() {
    print("_MyHomePageState contructor");
  }

  void initState() {
    super.initState();

    print("_MyHomePageState initState");
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: ListView.builder(
          itemBuilder: (context, index) {
            return ContainerWidget(index, key: ValueKey(index));
          },
          itemCount: 10,
        ));
  }
}

class ContainerWidget extends StatefulWidget {
  int index;

  ContainerWidget(this.index, {key}) : super(key: key) {
    print("ContainerWidget constructor for index $index");
  }

  @override
  State<StatefulWidget> createState() {
    print("ContainerWidget createState for index $index");
    return _ContainerState();
  }
}

class _ContainerState extends State<ContainerWidget> {
  _ContainerState() {
    print("_ContainerState constructor");
  }

  void initState() {
    super.initState();

    print("_ContainerState initState for index ${widget.index}");
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Center(
        child: Text("Index: ${widget.index}"),
      ),
      height: 200,
      decoration: BoxDecoration(
        border: Border(
          bottom: BorderSide(color: Colors.green),
        ),
      ),
    );
  }
}

日志输出：

I/flutter (22400): createState
I/flutter (22400): _MyHomePageState contructor
I/flutter (22400): _MyHomePageState initState
I/flutter (22400): ContainerWidget constructor for index 0
I/flutter (22400): ContainerWidget createState for index 0
I/flutter (22400): _ContainerState constructor
I/flutter (22400): _ContainerState initState for index 0
I/flutter (22400): ContainerWidget constructor for index 1
I/flutter (22400): ContainerWidget createState for index 1
I/flutter (22400): _ContainerState constructor
I/flutter (22400): _ContainerState initState for index 1
I/flutter (22400): ContainerWidget constructor for index 2
I/flutter (22400): ContainerWidget createState for index 2
I/flutter (22400): _ContainerState constructor
I/flutter (22400): _ContainerState initState for index 2
I/flutter (22400): ContainerWidget constructor for index 3
I/flutter (22400): ContainerWidget createState for index 3
I/flutter (22400): _ContainerState constructor
I/flutter (22400): _ContainerState initState for index 3
I/flutter (22400): ContainerWidget constructor for index 4
I/flutter (22400): ContainerWidget createState for index 4
I/flutter (22400): _ContainerState constructor
I/flutter (22400): _ContainerState initState for index 4
I/flutter (22400): ContainerWidget constructor for index 5
I/flutter (22400): ContainerWidget createState for index 5
I/flutter (22400): _ContainerState constructor
I/flutter (22400): _ContainerState initState for index 5
I/flutter (22400): ContainerWidget createState for index 1
I/flutter (22400): _ContainerState constructor
I/flutter (22400): _ContainerState initState for index 1
I/flutter (22400): ContainerWidget createState for index 0
I/flutter (22400): _ContainerState constructor
I/flutter (22400): _ContainerState initState for index 0

Answer 1

这是预期的，因为这些物品离开屏幕时会被卸载。

如果您不想这么做，则可以使用我们所谓的“保持活跃”。 您可以通过在State类中添加混合来实现：

class _MyHomePageState extends State<MyHomePage> with AutomaticKeepAliveClientMixin {

  bool get wantKeepAlive => true;


  // ...
 }