1、为什么在Activity的onKeyDown方法中写Cocos的back按钮响应代码是无效的？
下面是官方AIP：
public boolean onKeyDown (int keyCode,KeyEvent event)
Since: API Level 1
Called when a key was pressed down and not handled by any of the views inside of the activity. So,for example,key presses while the cursor is inside a TextView will not trigger the event (unless it is a navigation to another object) because TextView handles its own key presses.
If the focused view didn't want this event,this method is called.
可见onKeyDown 方法执行的前提是当前拥有焦点的View没有“吞并”这个点击事件。在Activity的onKeyDown方法中写Cocos的back按钮响应代码是无效的原因正是游戏界面吞并了这个事件。
既然Cocos有意吞并这个事件，那么Cocos肯定有处理这个事件的机制。
Cocos实现对back键（菜单键也是如此）的响应实现方式如下：
1、在需要响应Scene的h文件中添加虚函数如下：
virtual void onKeyReleased(EventKeyboard::KeyCode keyCode,Event* event);
2、cpp中实现这个函数例如：
{
cocos2d::log("----------->%d",keyCode);
if (keyCode == EventKeyboard::KeyCode::KEY_BACK)//back键
{
//...
}}
3、init()函数中加入：
this->setKeypadEnabled(true);
