Flutter开发：获取屏幕及 Widget 宽高两种方法

开发中的过程中通常都会获取屏幕或者 widget 的宽高用来做一些事情，在 Flutter 中，我们有两种方法来获取 widget 的宽高。文章源自菜鸟学院-https://www.cainiaoxueyuan.com/xcx/10471.html

MediaQuery

一般情况下，我们会使用如下方式去获取 widget 的宽高：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/xcx/10471.html

final size =(context).size;
final width =size.width;
final height =size.height; 

但是如果不注意，这种写法很容易报错，例如下面的写法就会报错：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/xcx/10471.html

import 'package:flutter/';

class GetWidgetWidthAndHeiget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final size =(context).size;
    final width =size.width;
    final height =size.height;
    print('width is $width; height is $height');
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Width & Height'),
        ),
        body: Container(
          width: width / 2,
          height: height / 2,
        ),
      ),
    );
  }
}

在代码中，我们是想获取屏幕的宽和高，然后将屏幕宽高的一半分别赋值给 Container 的宽和高，但上述代码并不能成功运行，会报如下错误：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/xcx/10471.html

flutter: The following assertion was thrown building GetWidgetWidthAndHeiget(dirty):
flutter: () called with a context that does not contain a MediaQuery.
flutter: No MediaQuery ancestor could be found starting from the context that was passed to ().
flutter: This can happen because you do not have a WidgetsApp or MaterialApp widget (those widgets introduce
flutter: a MediaQuery), or it can happen if the context you use comes from a widget above those widgets.

从错误异常中我们可以大概了解到有两种情况会导致上述异常：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/xcx/10471.html

1. 当没有 WidgetsApp or MaterialApp 的时候，我们使用 (context) 来获取数据。
2. 当我们在当前小部件中使用了上一个小部件的 context，来使用 (context) 获取数据的时候。

我们上述的代码很显然是属于第一种情况，也就是说我们在使用 (context) 的地方并没有一个 WidgetsApp or MaterialApp 来提供数据。文章源自菜鸟学院-https://www.cainiaoxueyuan.com/xcx/10471.html

解决方法就是将 (context) 挪到 MaterialApp 内，如下：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/xcx/10471.html

import 'package:flutter/';

class GetWidgetWidthAndHeiget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: HomePage(),
    );
  }
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final size = (context).size;
    final width = size.width;
    final height = size.height;
    print('width is $width; height is $height');
    return Scaffold(
      appBar: AppBar(
        title: Text('Width & Height'),
      ),
      body: Center(
        child: Container(
          color: Colors.redAccent,
          width: width / 2,
          height: height / 2,
        ),
      ),
    );
  }
}

运行效果及输出如下：文章源自菜鸟学院-https://www.cainiaoxueyuan.com/xcx/10471.html

flutter: width is 414.0; height is 896.0

上述代码中，我们获取的是 MaterialApp 的宽高，也就是屏幕的宽高文章源自菜鸟学院-https://www.cainiaoxueyuan.com/xcx/10471.html

class HomePage extends StatelessWidget {

  final GlobalKey globalKey = GlobalKey();

  void _getWH() {
    final containerWidth = globalKey.currentContext.size.width;
    final containerHeight = globalKey.currentContext.size.height;
    print('Container widht is $containerWidth, height is $containerHeight');
  }

  @override
  Widget build(BuildContext context) {
    final size = (context).size;
    final width = size.width;
    final height = size.height;
    print('width is $width; height is $height');
    return Scaffold(
      appBar: AppBar(
        title: Text('Width & Height'),
      ),
      body: Center(
        child: Container(
          key: globalKey,
          color: Colors.redAccent,
          width: width / 2,
          height: height / 2,
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _getWH,
        child: Icon(),
      ),
    );
  }
}

flutter: Container widht is 207.0, height is 448.0