添加一个半透明"引导"

guide_mask.dart

import 'package:flutter/material.dart';

class Mask extends StatefulWidget {
  final Function onTap;
  final Rect rect;
  const Mask({Key key, this.onTap, this.rect}) : super(key: key);

  @override
  _MaskState createState() => _MaskState();
}

class _MaskState extends State<Mask> {
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: widget.onTap,
      child: Stack(
        children: <Widget>[
          ClipPath(
            child: Container(
              width: double.infinity,
              height: double.infinity,
              color: Colors.black.withOpacity(0.5),
            ),
            clipper: getClip(),
          ),
          GestureDetector(
            behavior: HitTestBehavior.opaque,
            child: Container(
              width: widget.rect.width,
              height: widget.rect.height,
              margin: EdgeInsets.only(
                left: widget.rect.left,
                top: widget.rect.top,
              ),
            ),
            onTap: widget.onTap,
          ),
        ],
      ),
    );
  }

  CustomClipper<Path> getClip() {
    return MyClipper(widget.rect);
  }
}

class MyClipper extends CustomClipper<Path> {
  Rect rect;
  MyClipper(this.rect);

  @override
  Path getClip(Size size) {
    print(size);
    return Path()
      ..addRect(Rect.fromLTRB(0, 0, size.width, rect.top))
      ..addRect(Rect.fromLTRB(0, 0, rect.left, size.height))
      ..addRect(Rect.fromLTRB(rect.right, 0, size.width, size.height))
      ..addRect(Rect.fromLTRB(0, rect.bottom, size.width, size.height));
  }

  @override
  bool shouldReclip(CustomClipper<Path> oldClipper) {
    return true;
  }
}

void showMask(GlobalKey key) {
  var rect = findGlobalRect(key);

  final overlay = Overlay.of(key.currentContext);
  OverlayEntry entry;
  entry = OverlayEntry(builder: (BuildContext context) {
    return Mask(
      rect: rect,
      onTap: () {
        entry.remove();
      },
    );
  });

  overlay.insert(entry);
}

Rect findGlobalRect(GlobalKey key) {
  RenderBox renderObject = key.currentContext?.findRenderObject();
  if (renderObject == null) {
    return null;
  }

  var globalOffset = renderObject?.localToGlobal(Offset.zero);

  if (globalOffset == null) {
    return null;
  }

  var bounds = renderObject.paintBounds;
  bounds = bounds.translate(globalOffset.dx, globalOffset.dy);
  return bounds;
}

需要配合GlobalKey使用
可以结合首帧监听或延迟几百毫秒使用

上面截图对应例子:

main.dart

import 'package:flutter/material.dart';

import 'guide_mask.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  var key = GlobalKey();

  @override
  void initState() {
    super.initState();
    Future.delayed(Duration(seconds: 1), () {
      showMask(key);
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        key: key,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}