为什么父 StatefulWidget 中的 setState 不更新子 StatefulWidget

Question

我知道，从父 StatefulWidget 更新子 StatefulWidget 的正确方法是使用子 GlobalKey 实例。

但我不明白，为什么 setState(() {}) 不这样做！我没有找到任何关于此的文档或解释。

Calling setState notifies the framework that the internal state of this object has changed in a way that might impact the user interface in this subtree, which causes the framework to schedule a build for this State object.

If you just change the state directly without calling setState, the framework might not schedule a build and the user interface for this subtree might not be updated to reflect the new state.

通过文档，我希望整个小部件子树都会更新。但它没有。

请解释一下，为什么下面的示例无法按预期工作（更改 ChildStatefulWidget 的文本）。谢谢！

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

class ParentStatefulWidget extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return ParentState();
  }
}

class ParentState extends State<ParentStatefulWidget> {
  int counter = 0;

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: [
        GestureDetector(
          child: Container(
              color: Colors.lightBlue,
              padding: EdgeInsets.all(10),
              child: Text(
                "Tap me",
                style: TextStyle(color: Colors.white, fontSize: 24),
              )),
          onTap: () {
            setState(() {
              counter = counter + 1;
            });
          },
        ),
        ChildStatefulWidget(
          text: "Count: $counter",
        )
      ],
    );
  }
}

class ChildStatefulWidget extends StatefulWidget {
  final String text;

  ChildStatefulWidget({Key key, this.text = ""}) : super(key: key);

  @override
  State<StatefulWidget> createState() {
    return ChildState(text);
  }
}

class ChildState extends State<ChildStatefulWidget> {
  final String text;

  ChildState(this.text);

  @override
  Widget build(BuildContext context) {
    return Text(
      text,
      style: TextStyle(fontSize: 18),
    );
  }
}

Answer 1

您不需要在 childState 中创建“文本”——只需使用“widget.text”:

class ChildState extends State<ChildStatefulWidget> {    
  @override
  Widget build(BuildContext context) {
    return Text(
      widget.text,
      style: TextStyle(fontSize: 18),
    );
  }
}

您的代码不起作用，因为您发送值抛出构造函数，但每次更改状态时构造函数都不会调用。所以得到的抛出构造函数值没有改变。

为什么父 StatefulWidget 中的 setState 不更新子 StatefulWidget

Why setState in parent StatefulWidget do not update child StatefulWidget

dart

flutter

statefulwidget

flutter-layout