android让按键在弹起时有效

else if(code == KeyEvent.KEYCODE_LIGHT_DOWN){
                System.out.println("the key is light-down");                try{
                    mOldBrightness = Settings.System.getInt(mContext.getContentResolver(),
                            Settings.System.SCREEN_BRIGHTNESS);
                }catch(SettingNotFoundException snfe) {
                    mOldBrightness = 30;
                }                try{
                    IPowerManager power = IPowerManager.Stub.asInterface(
                            ServiceManager.getService("power"));                    if(power != null){
                        power.setBacklightBrightness(mOldBrightness - 10);
                    }                }catch(RemoteException doe) {                }                if((mOldBrightness - 10) >= 30)
                    Settings.System.putInt(mContext.getContentResolver(),Settings.System.SCREEN_BRIGHTNESS, mOldBrightness - 10);
                else
                    Settings.System.putInt(mContext.getContentResolver(),Settings.System.SCREEN_BRIGHTNESS, 30);            }
本来是想将背光亮度每按一次减10 的，可是按键在 按下和弹起时 都操作了，按一次减了20， 求大神解决，  最好贴源码  java 不是很懂 ...急急急...