UIScreen

UIScreen 对象定义了与硬件显示的相关属性，IOS设备有一个主屏幕，0个或是多个附加屏幕，用这个类来获取附加到设备屏幕的对象，每一个Screen对象都定义了矩形边界和一些其余有趣的属性brightness。

处理通知

• - (void)handleScreenConnectNotification:(NSNotification*)aNotification {

•    UIScreen*    newScreen = [aNotification object];

•    CGRect        screenBounds = newScreen.bounds;

•    if (!_secondWindow) {

•        _secondWindow = [[UIWindow alloc] initWithFrame:screenBounds];

•        _secondWindow.screen = newScreen;

•        // Set the initial UI for the window and show it.

•        [self.viewController displaySelectionInSecondaryWindow:_secondWindow];

•        [_secondWindow makeKeyAndVisible];

•    }

• }

• - (void)handleScreenDisconnectNotification:(NSNotification*)aNotification {

•    if (_secondWindow) {

•        // Hide and then delete the window.

•        _secondWindow.hidden = YES;

•        [_secondWindow release];

•        _secondWindow = nil;

•        // Update the main screen based on what is showing here.

•        [self.viewController displaySelectionOnMainScreen];

•    }

• }