How can I make a fullscreen overlay on the OS X desktop?

问题

I want to make some kind of drawable surface that exists beneath the mouse cursor but above everything else rendered on the desktop. I am trying to create a "trail" behind the mouse.

How can I do this in Cocoa and Objective-C?

回答1:

You need to subclass NSWindow to create a borderless window and set its window level to something like NSScreenSaverWindowLevel - 1.

- (id)initWithContentRect:(NSRect)contentRect 
                styleMask:(NSUInteger)aStyle
                  backing:(NSBackingStoreType)bufferingType
                    defer:(BOOL)flag
{
    self=[super initWithContentRect:contentRect 
                          styleMask:NSBorderlessWindowMask 
                            backing:bufferingType
                              defer:flag];

    if(self!=nil)
    {
        [self setHasShadow:NO];
        [self setOpaque:NO];
        [self setBackgroundColor:[NSColor clearColor]];
        [self setLevel:NSScreenSaverWindowLevel - 1];
    }
    return self;
}

来源：https://stackoverflow.com/questions/7217931/how-can-i-make-a-fullscreen-overlay-on-the-os-x-desktop