Question

我正在努力使用其GlobalKey获取Widget的高度。在呈现Layout之后调用获取高度的函数以确保上下文可用但key.currentState和key.currentContext仍然返回null。

import 'package:flutter/material.dart';

class TestPage extends StatefulWidget{
  @override
  State<StatefulWidget> createState() => new TestPageState();
}

class TestPageState extends State<TestPage>{
  final TestWidget testWidget = new TestWidget();

  @override
  initState() {
    //calling the getHeight Function after the Layout is Rendered
    WidgetsBinding.instance
        .addPostFrameCallback((_) => getHeight());

    super.initState();
  }

  void getHeight(){
    final GlobalKey key = testWidget.key;

    //returns null:
    final State state = key.currentState;
    //returns null:
    final BuildContext context = key.currentContext;

    //Error: The getter 'context' was called on null.
    final RenderBox box = state.context.findRenderObject();

    print(box.size.height);
    print(context.size.height);
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: testWidget,
    );
  }
}

class TestWidget extends StatefulWidget{
  @override
  Key get key => new GlobalKey<TestWidgetState>();

  @override
  State<StatefulWidget> createState() => new TestWidgetState();
}

class TestWidgetState extends State<TestWidget>{
  @override
  Widget build(BuildContext context) {
    return new Container(
      child: new Text("Test", style: const TextStyle(fontSize: 32.0, fontWeight: FontWeight.bold),),
    );
  }
}

Answer 1

您需要使用窗口小部件构造函数中的super将该键分配给窗口小部件。不要将其添加为字段。 Key也必须保持不变。

import 'package:flutter/material.dart';

class TestPage extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => new TestPageState();
}

class TestPageState extends State<TestPage> {
  final key = new GlobalKey<TestWidgetState>();

  @override
  initState() {
    //calling the getHeight Function after the Layout is Rendered
    WidgetsBinding.instance.addPostFrameCallback((_) => getHeight());

    super.initState();
  }

  void getHeight() {
    //returns null:
    final State state = key.currentState;
    //returns null:
    final BuildContext context = key.currentContext;

    //Error: The getter 'context' was called on null.
    final RenderBox box = state.context.findRenderObject();

    print(box.size.height);
    print(context.size.height);
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new TestWidget(key: key),
    );
  }
}

class TestWidget extends StatefulWidget {
  TestWidget({Key key}) : super(key: key);

  @override
  State<StatefulWidget> createState() => new TestWidgetState();
}

class TestWidgetState extends State<TestWidget> {
  @override
  Widget build(BuildContext context) {
    return new Container(
      child: new Text(
        "Test",
        style: const TextStyle(fontSize: 32.0, fontWeight: FontWeight.bold),
      ),
    );
  }
}

Answer 2

这样定义您的构造函数：

const MyWidget(GlobalKey key) : super(key:key);。

框架将BuildContext和State对象存储在Widget.key字段中，该字段由构造函数而不是任意的key字段传递给对象。

在颤动中使用其GlobalKey获取Widget的高度

2 个答案: