防止back鍵作用

   public boolean onKeyDown(int keyCode, KeyEvent event) {
       if (keyCode == KeyEvent.KEYCODE_BACK ) {
           /*
            * Use this hook instead of onBackPressed(), because onBackPressed()
            * is not available in API 4.
            */
           return true;
       }

       return super.onKeyDown(keyCode, event);
   }// onKeyDown()