flutter显示出底部控件的引导页

需求：同一个页面的两个不同的入口，同一个控件的位置有变化，显示引导页时对应这个控件的引导内容的位置也需要改变；同时半透明底部显示出真实的页面内容。
这样的需要如果切图然后再往页面上贴位置无法精确的对准。
思路：先绘制一层半透明遮罩覆盖页面，在需要显示的控件位置绘制为全透明，然后再将引导内容绘制在遮罩上面（共有三层，真实页面、透明遮罩。控件对应的那些说明内容）,获取控件的位置确定在哪里全透明。

入口一进入时：

入口二进入时：

import 'package:common/sp_util.dart';
import 'package:jade/configs/CommonConfig.dart';
import 'package:jade/configs/PathConfig.dart';
import 'package:jade/utils/JadeColors.dart';
import 'package/jade/utils/Utils.dart';
import 'package:util/navigator_util.dart';
import 'package:flutter/material.dart';
import 'package:flutter_screenutil/flutter_screenutil.dart';
/*
* 引导页
* */
class SharedPurchaseGuidePage extends StatelessWidget {final double width;final double height;final Offset offset;const SharedPurchaseGuidePage({this.width,this.height,this.offset});@overrideWidget build(BuildContext context) {return WillPopScope(child: Stack(children: [CustomPaint(size: Size(Utils().screenWidth(context), Utils().screenHeight(context)), // 自定义Widget的大小painter: MyCustomPainter(width,height,offset),),Positioned(left: offset.dx + width,top: offset.dy,child: _leftView(context))],),onWillPop: () async => false,);}_leftView(context){return Container(width: Utils().screenWidth(context) * 0.68,height: 210.w,padding: EdgeInsets.symmetric(vertical: 28.w),decoration: BoxDecoration(image: DecorationImage(image: AssetImage(PathConfig.imageSharedPurchaseGuideBg),fit: BoxFit.fill)),child: Column(crossAxisAlignment: CrossAxisAlignment.end,children: [Row(mainAxisAlignment: MainAxisAlignment.end,children: [Text('点击了解共享购',style: TextStyle(color: Colors.white,fontSize: 28.sp,fontWeight: FontWeight.w600)),SizedBox(width: 54.w),GestureDetector(child: Container(width: 100.w,height: 40.w,alignment: Alignment.center,margin: EdgeInsets.only(right: 20.w),decoration: BoxDecoration(color: Colors.white,borderRadius: BorderRadius.circular(20),),child: Text('知道了',style: TextStyle(color: JadeColors.blue,fontSize: 22.sp,fontWeight: FontWeight.w600),),),onTap: (){SpUtil.putBool(CommonConfig.havePostDetailSharedPurchaseGuide, true);NavigatorUtil.pop();},)],),Container(margin: EdgeInsets.only(right: 20.w,top: 26.w),child: Text('为促进交易，本版块加入了独特的共\n享购功能，与传统的团购与拼单很不\n一样，欢迎体验！',style: TextStyle(color: Colors.white,fontSize: 22.sp)))],),);}
}class MyCustomPainter extends CustomPainter {final double width;final double height;final Offset offset;const MyCustomPainter(this.width,this.height,this.offset);@overridevoid paint(Canvas canvas, Size size) {//背景Paint backgroundPaint = Paint()..color = Colors.black45..style = PaintingStyle.fill;//添加背景一个路径Path path = Path()..addRect(Rect.fromLTWH(0, 0, size.width, size.height));//留白的圆角矩形路径Path holePath = Path()// ..addRect(Rect.fromLTWH(offset.dx, offset.dy, width, height));..addRRect(RRect.fromRectAndRadius(Rect.fromLTWH(offset.dx-5, offset.dy, width+5, height), Radius.circular(5)));Path combinedPath = Path.combine(PathOperation.difference, path, holePath);canvas.drawPath(combinedPath, backgroundPaint);}@overridebool shouldRepaint(covariant CustomPainter oldDelegate) {return false; // 不需要重绘，因为我们只是绘制一次，并没有动画或状态更改}
}

GlobalKey _sharedPurchaseKey = new GlobalKey();
//需要全透明的控件
Row(key: _sharedPurchaseKey,children: [Text(S.current.sharedPurchase,style: TextStyle(color: JadeColors.blue,fontSize: 24.sp,fontWeight: FontWeight.bold)),GestureDetector(child: Container(color: Colors.transparent,padding: EdgeInsets.all(4),child: Image.asset(PathConfig.iconQuestion,width: 34.w,height: 34.w),),onTap: (){NavigatorUtil.push(SharedPurchaseDesc());},),],),//获取共享购文字加问号的坐标，跳转（这个方法需要在页面加载完成后调用）_getOffset() {if(_sharedPurchaseKey.currentContext != null){_sharedPurchaseWidth = _sharedPurchaseKey.currentContext.size.width;_sharedPurchaseHeight = _sharedPurchaseKey.currentContext.size.height;_offset = WidgetUtil.getWidgetLocalToGlobal(_sharedPurchaseKey.currentContext);bool sharedPurchaseGuide = SpUtil.getBool(CommonConfig.havePostDetailSharedPurchaseGuide,defValue: false);if(!sharedPurchaseGuide){//跳转一个透明页面NavigatorUtil.pushTransparentRoute(SharedPurchaseGuidePage(width: _sharedPurchaseWidth,height: _sharedPurchaseHeight,offset: _offset));}}}///Get the coordinates of the widget on the screen.Widgets must be rendered completely.///获取widget在屏幕上的坐标,widget必须渲染完成static Offset getWidgetLocalToGlobal(BuildContext context) {RenderBox box = context.findRenderObject();return box == null ? Offset.zero : box.localToGlobal(Offset.zero);}