Question

I’ve been feverishly searching for a method by which to limit the user’s mouse to one display in a multi-display setup on a Mac.

I’ve stumbled upon this question: Cocoa: Limit mouse to screen, I promise I have not duplicated this question.

The question did, however, spark an idea in my mind that it might be possible to write a simple application using Cocoa to restrict the mouse to one screen, run this application in the background, and still use my game which has been developed in AS3/Adobe AIR/Flash.

The game is a full-screen game, and will always be at the same resolution on the same monitor. The other monitor will also always have the same resolution, but is to be just a non-interactive display of information. I need the user to be able to interact with the game, but not accidentally move the mouse off of the game’s screen.

Question Summary:
Can I create a basic application for Mac OS X (Lion) using Cocoa/Objective C that will restrict the mouse to one monitor that can be run IN THE BACKGROUND and prevent users from moving the mouse outside of the monitor that will have a full-screen game running on it?

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[EDIT:]
I found the basic code necessary to run a loop for the Quartz Event Filter, courtesy of this answer: Modify NSEvent to send a different key than the one that was pressed

I am going to use that code for testing purposes, and I have modified it a bit to detect mouse events like so:

CGEventRef mouse_filter(CGEventTapProxy proxy, CGEventType type, CGEventRef event, void *refcon) {

    NSPoint point = CGEventGetLocation(event);
    NSPoint target = NSMakePoint(100,100);
    if (point.x >= 500){
        CGEventSetLocation(event,target);

    }
    return event;
}

int main(int argc, char *argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    CFRunLoopSourceRef runLoopSource;

    CFMachPortRef eventTap = CGEventTapCreate(kCGHIDEventTap, kCGHeadInsertEventTap, kCGEventTapOptionDefault, kCGEventMouseMoved, mouse_filter, NULL);

    if (!eventTap) {
        NSLog(@"Couldn't create event tap!");
        exit(1);
    }

    runLoopSource = CFMachPortCreateRunLoopSource(kCFAllocatorDefault, eventTap, 0);

    CFRunLoopAddSource(CFRunLoopGetCurrent(), runLoopSource, kCFRunLoopCommonModes);

    CGEventTapEnable(eventTap, true);

    CFRunLoopRun();

    CFRelease(eventTap);
    CFRelease(runLoopSource);
    [pool release];

    exit(0);
}

This, however, doesn’t seem to work quite right. It is certainly detecting mouse events properly, and it moves the events properly as well. For instance, if I drag a window past 500 pixels from the left of the screen, the window drag event gets moved to 100,100. However, the mouse itself doesn’t get moved to the new location when it is simply being moved around the screen, performing no other actions. IE: You can still move the mouse all around the screen, instead of just on the left 500 pixel column.

Any ideas?

Answer 1

Use a Quartz Event Tap to filter mouse moved and mouse dragged events. In the tap callback, use CGEventSetLocation to modify the location of mouse events that would otherwise move off the main screen.

You may need to run the program as root, or have assistive device access enabled in System Preferences.

Quartz Event Services Reference