Flutter Stateful Widget 重新创建 State

Question

我正在开发一个 flutter 应用程序，并发现状态管理出现意外行为。我创建了一个示例应用来重现该行为，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将按预期创建一次。当我再次向上滚动时，Flutter 会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(但不是容器小部件)。我希望 flutter 会在不重新创建整个状态对象的情况下检索先前的状态。我在这里做错了吗？

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

这是意料之中的，因为项目在离开屏幕时会被卸载。

如果你不想这样，你会想要使用我们所说的“保持活力”。你可以通过在你的 State 类中添加一个 mixin 来做到这一点:

class _MyHomePageState extends State<MyHomePage> with AutomaticKeepAliveClientMixin {

  bool get wantKeepAlive => true;


  // ...
 }