sencha touch中处理Android返回按钮事件

应用中只有一个NavigationView的情况：

// 处理一下android返回按钮
		document.addEventListener("deviceready", onDeviceReady, false);

		function onDeviceReady() {
			document.addEventListener("backbutton", onBackKeyDown, false);
		}

		function onBackKeyDown() {
			var canPop = false;
			var id = Ext.Viewport.getActiveItem().id;
			if (id.indexOf("login") != -1) {
			} else if (id.indexOf("mainview") != -1) {
				var mainview = Ext.Viewport.getActiveItem();
				var length = mainview.getItems().length;
				if (length > 2) {
					canPop = true;
				}
			}
			if (canPop) {
				console.log("canPop");
				ChinaFertilizer.app.getController("MainController").pop(1);
			} else {
				Ext.Msg.confirm("提示", "您确定要退出应用吗?", function(e) {
					if (e == "yes") {
						navigator.app.exitApp();
					}
				}, this);
			}

		}

在Ext.tab.Panel下有多个NavigationView的情况：

// 处理一下android返回按钮
		document.addEventListener("deviceready", onDeviceReady, false);

		function onDeviceReady() {
			document.addEventListener("backbutton", onBackKeyDown, false);
		}

		function onBackKeyDown() {
			var item = Ext.Viewport.getActiveItem();
			var id = item.id;
			if (id.indexOf("mainView") != -1) {
				 var mainview = Ext.Viewport.getActiveItem();
//				 var mainview = Ext.getCmp('moreView').getActiveItem();
				 var length = mainview.getActiveItem().getItems().length;
				 console.log("length = " + length);
				 if(length > 2){
					 if(app.viewId == 0){
						 app.getController("phone.MyAppControllerP").pop(1);
					 }else if(app.viewId == 1){
						 app.getController("phone.TodoControllerP").pop(1);
					 }else if(app.viewId == 2){
						 app.getController("phone.AppCenterControllerP").pop(1);
					 }else if(app.viewId ==3){
						 app.getController("phone.MoreControllerP").pop(1);
					 }
				 }else{
					 if (typeof WL === 'undefined') {
						 Ext.Msg.confirm("提示", "您确定要退出应用吗?", function(e) {
							 if (e == "yes") {
								 navigator.app.exitApp();
							 }
						 }, this);
					 } else {
						 WL.SimpleDialog.show("提示", "您确定要退出应用吗?", [ {
							 text : "确定",
							 handler : function() {
								 navigator.app.exitApp();
							 }
						 }, {
							 text : "取消",
							 handler : function() {
							 }
						 } ]);
					 }
					 
				 }
			 }else{
				 if (typeof WL === 'undefined') {
					 Ext.Msg.confirm("提示", "您确定要退出应用吗?", function(e) {
						 if (e == "yes") {
							 navigator.app.exitApp();
						 }
					 }, this);
				 } else {
					 WL.SimpleDialog.show("提示", "您确定要退出应用吗?", [ {
						 text : "确定",
						 handler : function() {
							 navigator.app.exitApp();
						 }
					 }, {
						 text : "取消",
						 handler : function() {
						 }
					 } ]);
				 }
			 }