ionic框架对Android返回键的处理

       在HybridApp移动跨平台开发中，android平台会遇到一些比较特殊并难以解决的问题，这些问题在原生应用开发中很easy， Android的返回键处理就是问题之一，假如我们要实现一个在很多App中都有的在主页按返回键弹出对话框提示用户退出应用的功能，在原生应用开发中是很容易的，只要在onKeyUp事件里面对返回键事件进行处理就可以了。按2次返回键退出应用的Java代码如下：

private long exitTime = 0;

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if(keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN){   
        if((System.currentTimeMillis()-exitTime) > 2000){  
            Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();                                
            exitTime = System.currentTimeMillis();   
        } else {
            finish();
            System.exit(0);
        }
        return true;   
    }
    return super.onKeyDown(keyCode, event);
}

      在深入分析ionic框架源码，在与ionic论坛的国外开发者交流后，终于找到了一个比较后的解决方法。Ionic作为目前国外比较活跃的HybridApp移动开发框架，对Android平台的返回键的处理是有比较合理的解决方案的。ionic框架对android返回键处理的源码如下：

返回键优先级定义，主要用途是返回键行为的优先级定义，例如当有弹出框时（优先级400），按返回键取消弹出框，不回退页面（优先级100）

var PLATFORM_BACK_BUTTON_PRIORITY_VIEW = 100;
var PLATFORM_BACK_BUTTON_PRIORITY_SIDE_MENU = 150;
var PLATFORM_BACK_BUTTON_PRIORITY_MODAL = 200;
var PLATFORM_BACK_BUTTON_PRIORITY_ACTION_SHEET = 300;
var PLATFORM_BACK_BUTTON_PRIORITY_POPUP = 400;
var PLATFORM_BACK_BUTTON_PRIORITY_LOADING = 500;

        /**
         * @ngdoc method
         * @name $ionicPlatform#registerBackButtonAction
         * @description
         * Register a hardware back button action. Only one action will execute
         * when the back button is clicked, so this method decides which of
         * the registered back button actions has the highest priority.
         *
         * For example, if an actionsheet is showing, the back button should
         * close the actionsheet, but it should not also go back a page view
         * or close a modal which may be open.
         *
         * @param {function} callback Called when the back button is pressed,
         * if this listener is the highest priority.
         * @param {number} priority Only the highest priority will execute.
         * @param {*=} actionId The id to assign this action. Default: a
         * random unique id.
         * @returns {function} A function that, when called, will deregister
         * this backButtonAction.
         */
        $backButtonActions: {},
        registerBackButtonAction: function(fn, priority, actionId) {

          if(!self._hasBackButtonHandler) {
            // add a back button listener if one hasn't been setup yet
            self.$backButtonActions = {};
            self.onHardwareBackButton(self.hardwareBackButtonClick);
            self._hasBackButtonHandler = true;
          }

          var action = {
            id: (actionId ? actionId : ionic.Utils.nextUid()),
            priority: (priority ? priority : 0),
            fn: fn
          };
          self.$backButtonActions[action.id] = action;

          // return a function to de-register this back button action
          return function() {
            delete self.$backButtonActions[action.id];
          };
        },

.run(['$ionicPlatform', '$ionicPopup','$rootScope','$location', function ($ionicPlatform, $ionicPopup, $rootScope, $location) {
       
        //主页面显示退出提示框
        $ionicPlatform.registerBackButtonAction(function (e) {

            e.preventDefault();

            function showConfirm() {
                var confirmPopup = $ionicPopup.confirm({
                    title: '<strong>退出应用?</strong>',
                    template: '你确定要退出应用吗?',
                    okText: '退出',
                    cancelText: '取消'
                });

                confirmPopup.then(function (res) {
                    if (res) {
                        ionic.Platform.exitApp();
                    } else {
                        // Don't close
                    }
                });
            }

            // Is there a page to go back to?
            if ($location.path() == '/home' ) {
                showConfirm();
            } else if ($rootScope.$viewHistory.backView ) {
                console.log('currentView:', $rootScope.$viewHistory.currentView);
                // Go back in history
                $rootScope.$viewHistory.backView.go();
            } else {
                // This is the last page: Show confirmation popup
                showConfirm();
            }

            return false;
        }, 101);

    }])