flutter 监听返回键

### 监听手机返回键（双击退出）
```
import 'package:fluttertoast/fluttertoast.dart';  //提示插件

class WillPopScopeTestRoute extends StatefulWidget {
  @override
  WillPopScopeTestRouteState createState() {
    return new WillPopScopeTestRouteState();
  }
}
class WillPopScopeTestRouteState extends State<WillPopScopeTestRoute> {
  DateTime _lastPressedAt; //上次点击时间
  var status = false;
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onDoubleTap: (){return status=true;},
      child: WillPopScope(
        onWillPop: () async {
          if (_lastPressedAt == null) {
            Fluttertoast.showToast(
              msg: "双击退出程序...",
              gravity: ToastGravity.BOTTOM,
              timeInSecForIos: 1,
              backgroundColor: Colors.grey[400],
              textColor: Colors.white,
              fontSize: ScreenUtil().setWidth(12),
            );
          }
          if (_lastPressedAt == null ||
              DateTime.now().difference(_lastPressedAt) >
                  Duration(seconds: 1)) {
            //两次点击间隔超过1秒则重新计时
            _lastPressedAt = DateTime.now();
            return false;
          }
          return true;
        },
        child: Pages(),
      ),
    );
  }
}

main-MyApp中使用
    home: WillPopScopeTestRoute(),
```

### 监听手机返回键(单击返回桌面不退出app)
```
// MainActivity
import android.os.Bundle;
import android.view.KeyEvent;

import io.flutter.app.FlutterActivity;
import io.flutter.plugin.common.MethodCall;
import io.flutter.plugin.common.MethodChannel;
import io.flutter.plugins.GeneratedPluginRegistrant;

 //通讯名称,回到手机桌面
 private  final String CHANNEL = "android/back/desktop";

// onCreate方法下
new MethodChannel(getFlutterView(), CHANNEL).setMethodCallHandler(
	new MethodChannel.MethodCallHandler() {
		@Override
		public void onMethodCall(MethodCall methodCall, MethodChannel.Result result) {
			//通过methodCall可以获取参数和方法名  执行对应的平台业务逻辑即可
			if (methodCall.method.equals("backDeskTop")) {//设置 当前flutter页返回到手机桌面
				result.success(true);
　　　　　　　　　　　　　　　　　　　moveTaskToBack(false);  // 调用返回事件
			}
		}
	}
);

// android_back_top文件

import 'package:flutter/services.dart';
import 'package:flutter/material.dart';

class AndroidBackTop {
  //初始化通信管道-设置退出到手机桌面
  static const String CHANNEL = "android/back/desktop";

  //设置回退到手机桌面
  static Future<bool> backDeskTop() async {
    final platform = MethodChannel(CHANNEL);
    //通知安卓返回,到手机桌面
    try {
      final bool out = await platform.invokeMethod('backDeskTop');
      if (out) {
        debugPrint('设置成功');
      }
    } on PlatformException catch (e) {
      debugPrint("通信失败(设置回退到安卓手机桌面:设置失败)");
      print(e.toString());
    }
    return Future.value(false);
  }
}

//  main文件下使用
home: WillPopScope(
    onWillPop: () async {
      AndroidBackTop.backDeskTop();
      return false;
    },
    child: Pages(),
),
```