iOS方向：尝试过并尝试过。

#import \"iPadVGAOutputTestAppDelegate.h\"

@implementation iPadVGAOutputTestAppDelegate

@synthesize deviceWindow;
@synthesize consoleTextView;
@synthesize externalWindow;

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation {
    // Overriden to allow any orientation.
    return YES;
}


- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{

    externalWindow.hidden = YES;


    // Make iPad window visible.
    [deviceWindow makeKeyAndVisible];

    // Check for external screen.
    if ([[UIScreen screens] count] > 1) {
        [self log:@\"Found an external screen.\"];

        // Internal display is 0, external is 1.
        externalScreen = [[[UIScreen screens] objectAtIndex:1] retain];
        [self log:[NSString stringWithFormat:@\"External screen: %@\", externalScreen]];

        screenModes = [externalScreen.availableModes retain];
        [self log:[NSString stringWithFormat:@\"Available modes: %@\", screenModes]];

        // Allow user to choose from available screen-modes (pixel-sizes).
        UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:@\"External Display Size\" 
                                                         message:@\"Choose a size for the external display.\" 
                                                        delegate:self 
                                               cancelButtonTitle:nil 
                                               otherButtonTitles:nil] autorelease];
        for (UIScreenMode *mode in screenModes) {
            CGSize modeScreenSize = mode.size;
            [alert addButtonWithTitle:[NSString stringWithFormat:@\"%.0f x %.0f pixels\", modeScreenSize.width, modeScreenSize.height]];
        }
        [alert show];

    } else {
        [self log:@\"External screen not found.\"];
    }

    return YES;
}


- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
    UIScreenMode *desiredMode = [screenModes objectAtIndex:buttonIndex];
    [self log:[NSString stringWithFormat:@\"Setting mode: %@\", desiredMode]];
    externalScreen.currentMode = desiredMode;

    [self log:@\"Assigning externalWindow to externalScreen.\"];
    externalWindow.screen = externalScreen;

    [screenModes release];
    [externalScreen release];

    CGRect rect = CGRectZero;
    rect.size = desiredMode.size;
    externalWindow.frame = rect;
    externalWindow.clipsToBounds = YES;

    [self log:@\"Displaying externalWindow on externalScreen.\"];
    externalWindow.hidden = NO;
    [externalWindow makeKeyAndVisible];
}


- (void)log:(NSString *)msg
{
    [consoleTextView setText:[consoleTextView.text stringByAppendingString:[NSString stringWithFormat:@\"%@\\r\\r\", msg]]];
}


- (void)dealloc
{
    [consoleTextView release];
    [deviceWindow release];
    [externalWindow release];

    [super dealloc];
}


@end