实现这个功能主要用到了Stack、Positioned、Draggable等几个组件
直接上代码：

1.main.dart文件

import 'package:flutter/material.dart';
import 'FloatBtn.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: FloatBtn(),
    );
  }
}

2. FloatBtn.dart 文件

import 'package:flutter/material.dart';

class FloatBtn extends StatefulWidget {
  FloatBtn({Key key}) : super(key: key);

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

class _FloatBtnState extends State<FloatBtn> {
  
  Offset offsetA = Offset(20, kToolbarHeight + 100);//按钮的初始位置
  
  @override
  Widget build(BuildContext context) {

    final size = MediaQuery.of(context).size;
    final width = size.width;
    final height = size.height;

    return Scaffold(
       body: Stack(
         children:<Widget>[
          Positioned(
            left: offsetA.dx,
            top: offsetA.dy,
            child: Draggable(
                //创建可以被拖动的Widget
                child:  FloatingActionButton(
                  tooltip: 'Increment',
                  child: Icon(Icons.add), onPressed: () {},
                ),
                //拖动过程中的Widget
                feedback: FloatingActionButton(
                  tooltip: 'Increment',
                  child: Icon(Icons.add), onPressed: () {},
                ),
                //拖动过程中，在原来位置停留的Widget，设定这个可以保留原本位置的残影，如果不需要可以直接设置为Container()
                childWhenDragging: Container(),
                
                // FloatingActionButton(
                //   tooltip: 'Increment',
                //   child: Icon(Icons.add), onPressed: () {},
                // ),
                //拖动结束后的Widget
                onDraggableCanceled: (Velocity velocity, Offset offset) {
                  // 计算组件可移动范围  更新位置信息
                  setState(() {
                    var x = offset.dx;
                    var y = offset.dy;
                    if(offset.dx<0){
                      x = 20;
                    }

                    if(offset.dx>375){
                      x = 335;
                    }

                    if(offset.dy<kBottomNavigationBarHeight){
                      y = kBottomNavigationBarHeight;
                    }

                    if(offset.dy>height-100){
                      y = height - 100;
                    }

                    offsetA = Offset(x, y);
                  });

                }
              )
          )
         ]
       ),
    );
  }
}