UIWINDOW

• It is not a problem at all to add another UIWindow. Just create one and makeKeyAndVisible. Done.
• Remove it by making another window visible, then release the one you don't need anymore.
• The window that is "key" receives all the keyboard input.
• UIWindow covers everything, even modals, popovers, etc. Brilliant!
• UIWindow is always portrait implicitly. It does no rotate. You'll have to add a controller as the new window's root controller and let that handle rotation. (Just like the main window)
• The window's level determines how "high" it gets displayed. Set it to UIWindowLevelStatusBar to have it cover everything. Set its "hidden" property to NO.
• A 2nd UIWindow can be used to bring views on the screen that float on top of everything. Without creating a dummy controller just to embed that in a UIPopoverController.
• It can be especially useful on iPhone where there is no popover controller but where you might want to mimic something like it.
• And yes, it solved of course my problem: if the app resigns activation, add a cover window over whatever is currently shown to prevent iOS from taking a screenshot of your app's current content.

UIWindow *secondWindow = [[UIWindow alloc] initWithFrame:[[[UIApplication sharedApplication] keyWindow] frame]];
[secondWindow setWindowLevel:UIWindowLevelStatusBar];
[secondWindow makeKeyAndVisible];

UIViewController *aNewViewController = [[UIViewController alloc] initWithFrame:secondWindow.bounds];
[secondWindow setRootViewController:aNewViewController];

(secondWindow不能被釋放)

http://stackoverflow.com/questions/2508630/orientation-in-a-uiview-added-to-a-uiwindow/4960988#4960988

UIWindow* window = [UIApplication sharedApplication].keyWindow;
if (!window) 
    window = [[UIApplication sharedApplication].windows objectAtIndex:0];
[[[window subviews] objectAtIndex:0] addSubview:myView]; 

# 1 Add view to window

[[[UIApplication sharedApplication] keyWindow] addSubview:aView];

# 2 Add listener and update view

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(statusBarFrameOrOrientationChanged:) name:UIApplicationDidChangeStatusBarOrientationNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(statusBarFrameOrOrientationChanged:) name:UIApplicationDidChangeStatusBarFrameNotification object:nil];

# 3 Do the math

- (void)statusBarFrameOrOrientationChanged:(NSNotification *)notification
{
    /*
     This notification is most likely triggered inside an animation block,
     therefore no animation is needed to perform this nice transition.
     */
    [self rotateAccordingToStatusBarOrientationAndSupportedOrientations];
}

- (void)rotateAccordingToStatusBarOrientationAndSupportedOrientations
{
    UIInterfaceOrientation statusBarOrientation = [UIApplication sharedApplication].statusBarOrientation;
    CGFloat angle = UIInterfaceOrientationAngleOfOrientation(statusBarOrientation);
    CGFloat statusBarHeight = [[self class] getStatusBarHeight];

    CGAffineTransform transform = CGAffineTransformMakeRotation(angle);
    CGRect frame = [[self class] rectInWindowBounds:self.window.bounds statusBarOrientation:statusBarOrientation statusBarHeight:statusBarHeight];

    [self setIfNotEqualTransform:transform frame:frame];
}

- (void)setIfNotEqualTransform:(CGAffineTransform)transform frame:(CGRect)frame
{
    if(!CGAffineTransformEqualToTransform(self.transform, transform))
    {
        self.transform = transform;
    }
    if(!CGRectEqualToRect(self.frame, frame))
    {
        self.frame = frame;
    }
}

+ (CGFloat)getStatusBarHeight
{
    UIInterfaceOrientation orientation = [UIApplication sharedApplication].statusBarOrientation;
    if(UIInterfaceOrientationIsLandscape(orientation))
    {
        return [UIApplication sharedApplication].statusBarFrame.size.width;
    }
    else
    {
        return [UIApplication sharedApplication].statusBarFrame.size.height;
    }
}

+ (CGRect)rectInWindowBounds:(CGRect)windowBounds statusBarOrientation:(UIInterfaceOrientation)statusBarOrientation statusBarHeight:(CGFloat)statusBarHeight
{    
    CGRect frame = windowBounds;
    frame.origin.x += statusBarOrientation == UIInterfaceOrientationLandscapeLeft ? statusBarHeight : 0;
    frame.origin.y += statusBarOrientation == UIInterfaceOrientationPortrait ? statusBarHeight : 0;
    frame.size.width -= UIInterfaceOrientationIsLandscape(statusBarOrientation) ? statusBarHeight : 0;
    frame.size.height -= UIInterfaceOrientationIsPortrait(statusBarOrientation) ? statusBarHeight : 0;
    return frame;
}

CGFloat UIInterfaceOrientationAngleOfOrientation(UIInterfaceOrientation orientation)
{
    CGFloat angle;

    switch (orientation)
    {
        case UIInterfaceOrientationPortraitUpsideDown:
            angle = M_PI;
            break;
        case UIInterfaceOrientationLandscapeLeft:
            angle = -M_PI_2;
            break;
        case UIInterfaceOrientationLandscapeRight:
            angle = M_PI_2;
            break;
        default:
            angle = 0.0;
            break;
    }

    return angle;
}

UIInterfaceOrientationMask UIInterfaceOrientationMaskFromOrientation(UIInterfaceOrientation orientation)
{
    return 1 << orientation;
}