在 Flutter 中将数据发送到父 Widget

问题描述

我正在尝试将文本从子小部件设置为父小部件.但是文本没有反映在父小部件中.

I'm trying to set the text from child widget to parent widget. But the text is not reflecting in parent widget.

也尝试使用 setState() 但仍然无法获得预期结果.

Tried to use setState() also but still unable to get expected result.

以下是我的代码:

void main() => runApp(new TestApp());

class TestApp extends StatefulWidget {
  @override
  _TestState createState() => new _TestState();
}

class _TestState extends State<TestApp>{

  String abc = "";

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new Scaffold(
        body: new Column(
          children: <Widget>[
            new Text("This is text $abc"),
            TestApp2(abc)
          ],
        ),
      ),
    );
  }
}


class TestApp2 extends StatefulWidget {

  String abc;

  TestApp2(this.abc);

  @override
  _TestState2 createState() => new _TestState2();
}

class _TestState2 extends State<TestApp2>{
  @override
  Widget build(BuildContext context) {
    return new Container(
      width: 150.0,
      height: 30.0,
      margin: EdgeInsets.only(top: 50.0),
      child: new FlatButton(
          onPressed: (){
            setState(() {
              widget.abc = "RANDON TEXT";
            });
          },
        child: new Text("BUTTON"),
        color: Colors.red,
      ),
    );
  }
}

我错过了什么吗?

推荐答案

在您的示例中，做了一些假设.我会一一删除.

In your example, a few assumptions were made. I will try to remove one by one.

1. 您将 abc 从父级传递给子级，并在按下按钮时改变了子级值.由于原始类型在 dart 中是传值，因此改变 child 中 abc 的值不会改变 parent abc 的值.请参阅以下代码段.

1. You pass abc from parent to child and you mutated the child value on press on button. As primitive types are pass by value in dart, change in the value of abc in child will not change the value of parent abc. Refer the below snippet.

void main() {
  String abc = "oldValue";
  changeIt(abc);
  print(abc); // oldValue
}

void changeIt(String abc) {
  abc = "newValue";
  print(abc); //newValue
}

让我们假设第一个是错误的(为了理解目的).然后改变child中abc的值会改变parent中abc的值.但是如果不在 parent 的 setState 中调用它，parent 将不会反映更改.在您的情况下，如果您更改如下代码，它将在单击时单独更改按钮文本(因为调用了 child 的 setState).

Let's assume the first one is wrong(for understanding purpose). Then changing the value of abc in child will change the value of abc in parent. But without calling that inside setState of parent, parent will not reflect the change. In your case if you change the code as below, it will change the button text alone on click (as setState of child is called).

  new FlatButton(
    onPressed: () {
      setState(
        () {
          widget.abc = "RANDON TEXT";
        },
      );
    },
    child:
        new Text(widget.abc), // setting the text based on abc
    color: Colors.red,
  ),

我建议使用 callbacks，而不是使用 globalState(随着应用程序的增长而很难维护/调试).请参考以下代码.

Instead of using globalState which will be very difficult to maintain/debug as app grows, I would recommend using callbacks. Please refer the below code.

    void main() => runApp(new TestApp());

    class TestApp extends StatefulWidget {
      @override
      _TestState createState() => new _TestState();
    }

    class _TestState extends State<TestApp> {
      String abc = "bb";

      callback(newAbc) {
        setState(() {
          abc = newAbc;
        });
      }

      @override
      Widget build(BuildContext context) {
        var column = new Column(
          children: <Widget>[
            new Text("This is text $abc"),
            TestApp2(abc, callback)
          ],
        );
        return new MaterialApp(
          home: new Scaffold(
            body: new Padding(padding: EdgeInsets.all(30.0), child: column),
          ),
        );
      }
    }

    class TestApp2 extends StatefulWidget {
      String abc;
      Function(String) callback;

      TestApp2(this.abc, this.callback);

      @override
      _TestState2 createState() => new _TestState2();
    }

    class _TestState2 extends State<TestApp2> {
      @override
      Widget build(BuildContext context) {
        return new Container(
          width: 150.0,
          height: 30.0,
          margin: EdgeInsets.only(top: 50.0),
          child: new FlatButton(
            onPressed: () {
              widget.callback("RANDON TEXT"); //call to parent
            },
            child: new Text(widget.abc),
            color: Colors.red,
          ),
        );
      }
    }

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