Flex 实现全屏后无法捕获键盘事件

private function changeScreen():void {
	if ( stage.displayState == StageDisplayState.FULL_SCREEN ) {
		stage.displayState = StageDisplayState.NORMAL;
                  // other handle
		normalscreen.play([main]);
	} else if ( stage.displayState ==  StageDisplayState.NORMAL ) {
		stage.displayState = StageDisplayState.FULL_SCREEN;
                  // other handle
		fullscreen.play([main]);
	}
}