两种方式在Flutter页面加载完成后，获取指定Widget的位置和尺寸信息，用来显示弹窗或者新手引导。

方法一: Notification

1. 需要自定义的class

   ///数据封装
   class PositionedNotification extends Notification {
     Offset? offset;
     Size? size;
   }
   
   class PositionedNotificationNotifier extends SingleChildRenderObjectWidget {
     const PositionedNotificationNotifier({Widget? child, Key? key})
         : super(key: key, child: child);
   
     @override
     _PositionedCallback createRenderObject(BuildContext context) {
       ///分发通知
       return _PositionedCallback(onLayoutChangedCallback: (offset, size) {
         (PositionedNotification()
               ..offset = offset
               ..size = size)
             .dispatch(context);
       });
     }
   }
   
   class _PositionedCallback extends RenderProxyBox {
     final Function(Offset?, Size?) onLayoutChangedCallback;
     Offset? tmpOffset;
     Size? tempSize;
   
     _PositionedCallback({RenderBox? child, required this.onLayoutChangedCallback})
         : super(child);
   
     @override
     void performLayout() {
       super.performLayout();
       ///widget绘制完成后，分发通知
       SchedulerBinding.instance?.addPostFrameCallback((time) {
         tmpOffset = localToGlobal(Offset.zero);
         tempSize = paintBounds.size;
         onLayoutChangedCallback(tmpOffset, tempSize);
       });
     }
   }
   

2. 使用

   class App extends StatelessWidget {
     const App({Key? key}) : super(key: key);
   
     @override
     Widget build(BuildContext context) {
       return NotificationListener<PositionedNotification>(
           onNotification: (notification) {
             print("Offset->${notification.offset}");
             print("Size->${notification.size}");
             return true;
           },
           child: const PositionedNotificationNotifier(child: Text("目标Widget")));
     }
   }
   

方法二: GlobalKey

1. 定义GlobalKey

   GlobalKey globalKey = GlobalKey();
   

2. 使用GlobalKey

   Container(
   key: globalKey,
   ...
   )
   

3. 获取数据

     @override
     void initState() {
           ...
       SchedulerBinding.instance?.addPostFrameCallback((time) {
        BuildContext? context = globalKey.currentContext;
        RenderBox? renderBox = context?.findRenderObject() as RenderBox?;
        Offset? offset = renderBox?.localToGlobal(Offset.zero);
        Size? size = renderBox?.paintBounds.size;
       });
     }
   

4. 如果无法直接引用GlobalKey，可以使用BuildContext.findAncestorStateOfType()

   class GlobalKeyExampleState extends State<GlobalKeyExample> {
     GlobalKey globalKey = GlobalKey();
   
     static GlobalKeyExampleState? of(BuildContext context) {
       final GlobalKeyExampleState? result =
           context.findAncestorStateOfType<GlobalKeyExampleState>();
       if (result != null) {
         return result;
       }
       return null;
     }
     ...
   }
   
   
   class ChildWidget extends StatefulWidget {
     const ChildWidget({Key? key}) : super(key: key);
   
     @override
     _ChildWidgetState createState() => _ChildWidgetState();
   }
   
   class _ChildWidgetState extends State<ChildWidget> {
     @override
     Widget build(BuildContext context) {
       GlobalKey? globalKey = GlobalKeyExampleState.of(context)?.globalKey;
       return Container(
         key: globalKey,
         ...
       );
     }
   }  
   

源码

完整示例代码见GitHub