Flutter-为什么在每次重新构建父窗口小部件时都未调用子窗口小部件的initState()? [英] Flutter - Why is child widget's initState() is not called on every rebuild of Parent widget?

问题描述

我有一个 Parent 小部件，它具有某种状态，在这种情况下为计数器.

I have a Parent widget, which holds some state, a counter in this case.

此状态通过其构造函数传递给 Child 小部件.

This state is passed on to a Child widget using its constructor.

现在，据我所知， Child 应该在每次 Parent 状态更改时重新构建，因为它位于 build()内部父母的函数，并且每次状态更改时都会调用 build().

Now, how I understand it, the Child should get re-built every-time Parent's state changes, since it's inside the build() function of Parent, and build() gets called every-time the state changes.

这个概念使我相信，每次计数器更改时，都会输出 INIT STATE！消息.事实并非如此！

This concept lead me to believe that the INIT STATE! message would be printed every time the counter changes. But it is not the case!

我本质上希望每当 Child 的构造函数参数( message )更改时，都将仅触发一次钩子".

I essentially want a "hook" that gets fired only once, whenever the Child's constructor arguments (message) change.

有人可以解释为什么会这样吗，以及使用上述挂钩"的正确方法是什么?

Can someone please explain why this is the case, and what is the right way to have the aforementioned "hook"?

class Child extends StatefulWidget {
  final String message;

  const Child(this.message, {Key key}) : super(key: key);

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

class _ChildState extends State<Child> {
  @override
  void initState() {
    print("INIT STATE!");
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Text(widget.message),
    );
  }
}

class Parent extends StatefulWidget {
  @override
  _ParentState createState() => _ParentState();
}

class _ParentState extends State<Parent> {
  int _counter = 0;

  @override
  void initState() {
    Timer.periodic(Duration(seconds: 1), (_) {
      if (mounted) {
        setState(() {
          _counter += 1;
        });
      }
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Child(_counter.toString());
  }
}

推荐答案

好吧，这看起来在

Ok, looks like this was clearly mentioned in the State class' docs.

正确的方法是重写 State 子类中的 didUpdateWidget 方法.

The correct way to do this is to override the didUpdateWidget method in the State subclass.

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