Stack这个widget主要是用来把几个widget进行叠加，Positioned则相当于父组件定位。
下来我们实现一个类似下面的雪花效果:

image.png

代码：

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData.light(),
      home: Stack(
        children: <Widget>[
          SizedBox.expand(  // 默认填满父元素
            child: Container(
              decoration: BoxDecoration(
                color: Colors.lightBlue,
              ),
            ),
          ),
          Positioned(
              top: 250.0,  // 距离父元素上方的距离
              right: 20.0, // 距离父元素右方的距离
              child: Container(
                child: Icon(
                  Icons.ac_unit,
                  color: Colors.white,
                  size: 32.0,
                ),
                decoration: BoxDecoration(
                  shape: BoxShape.circle,
                  gradient: RadialGradient(colors: [  // 添加渐变色
                    Color.fromRGBO(7, 102, 255, 1.0),
                    Color.fromRGBO(200, 200, 200, 0.5),
                  ]),
                ),
              )),
          Positioned(
            top: 70.0,
            right: 10.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
          Positioned(
            top: 80.0,
            right: 150.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
          Positioned(
            top: 90.0,
            right: 40.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
          Positioned(
            top: 80.0,
            right: -20.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
          Positioned(
            top: 200.0,
            right: 300.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
          Positioned(
            top: 314.0,
            right: 400.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
          Positioned(
            top: 400.0,
            left: 40.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
          Positioned(
            top: 420.0,
            right: 250.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
          Positioned(
            top: 500.0,
            right: 230.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
          Positioned(
            top: 530.0,
            right: 50.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
          Positioned(
            top: 570.0,
            right: 260.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
          Positioned(
            top: 590.0,
            right: 222.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
          Positioned(
            top: 620.0,
            right: 290.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
          Positioned(
            top: 700.0,
            right: 150.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
          Positioned(
            top: 800.0,
            left: 40.0,
            child: Icon(
              Icons.ac_unit,
              color: Colors.white,
              size: 32.0,
            ),
          ),
        ],
      ),
    );
  }
}