如何相对于中心将小部件放置在Stack中？

Question

Stack(
  children: [  
      Positioned(
        left: ??? + 20,
        child: Text('This text is 20 pixel left to the center of the stack'),
      ),
  ],
),

我正在尝试将窗口小部件相对于中心放置在Stack中。

如何获得上述代码中的???？

Answer 1

这里是一种方法

     Positioned(
        left: MediaQuery.of(context).size.width / 2 + 20,
        child:
            Text('This text is 20 pixel left to the center of the stack'),
      ),

Answer 2

您将需要使用LayoutBuilder小部件，该小部件将在布局时构建并提供父小部件的约束。

尝试类似的东西

class HomeScreen extends StatefulWidget {
  @override
  _HomeScreenState createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  double _constIconSize = 50;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: LayoutBuilder(
        builder: (BuildContext context, BoxConstraints constraints) {
          print(constraints);
          return Stack(
            children: <Widget>[
              //Exact center
              Positioned(
                top: constraints.constrainHeight() / 2 - _constIconSize / 2,
                left: constraints.constrainWidth() / 2 - _constIconSize / 2,
                child: Icon(
                  Icons.gps_fixed,
                  color: Colors.green,
                  size: _constIconSize,
                ),
              ),
              //100 right and 50  bottom from center
              Positioned(
                top: constraints.constrainHeight() / 2 + 100,
                left: constraints.constrainWidth() / 2 + 50,
                child: Icon(
                  Icons.face,
                  color: Colors.red,
                  size: _constIconSize,
                ),
              ),
            ],
          );
        },
      ),
    );
  }
}