Mouse tracking daemon

Question

I need to write something using Cocoa to surface raw mouse movement data. Optimally, the app would just be a little daemon that would run, passing the data to a socket server which another application could tap into to gain access to the events.

Can anyone point me in the right direction with regard to approach and tools? I am not even sure where to begin with this right now.

Answer 1

The other simple way to do this is to add a global event monitor (10.6 only, however):

id eventHandler = [NSEvent addGlobalMonitorForEventsMatchingMask:NSMouseMovedMask handler:^(NSEvent * mouseEvent) {
  NSLog(@"Mouse moved: %@", NSStringFromPoint([mouseEvent locationInWindow]));
}];

Then when you're done tracking, you do:

[NSEvent removeMonitor:eventHandler];

Answer 2

Write an EventTap. Documentation can be found here.

In MacOS X every event (e.g. every keyboard key pressed, every mouse key pressed or mouse movement) creates an event that travels the following path:

Driver (Kernel) -> Window Server (privileged) -> User (Login) Session -> Active Application

Everywhere where I wrote an arrow (->) an EventTap can be placed to either only look at the event (a listen only EventTap) or to either modify or drop the event (an event filtering EventTap). Please note that to catch an event between Driver and WindowServer, your daemon must run with root privileges.

Here is some sample code:

// Compile with:
// gcc -framework ApplicationServices -o MouseWatcher MouseWatcher.c
//
// Start with:
// ./MouseWatcher
//
// Terminate by hitting CTRL+C

#include <ApplicationServices/ApplicationServices.h>


static CGEventRef myEventTapCallback (
    CGEventTapProxy proxy,
    CGEventType type,
    CGEventRef event,
    void * refcon
) {
    CGPoint mouseLocation;

    // If we would get different kind of events, we can distinguish them
    // by the variable "type", but we know we only get mouse moved events

    mouseLocation = CGEventGetLocation(event);
    printf(
        "Mouse is at x/y: %ld/%ld\n",
        (long)mouseLocation.x,
        (long)mouseLocation.y
    );
    // Pass on the event, we must not modify it anyway, we are a listener
    return event;
}


int main (
    int argc,
    char ** argv
) {
    CGEventMask emask;
    CFMachPortRef myEventTap;
    CFRunLoopSourceRef eventTapRLSrc;

    // We only want one kind of event at the moment: The mouse has moved
    emask = CGEventMaskBit(kCGEventMouseMoved);

    // Create the Tap
    myEventTap = CGEventTapCreate (
        kCGSessionEventTap, // Catch all events for current user session
        kCGTailAppendEventTap, // Append to end of EventTap list
        kCGEventTapOptionListenOnly, // We only listen, we don't modify
        emask,
        &myEventTapCallback,
        NULL // We need no extra data in the callback
    );

    // Create a RunLoop Source for it
    eventTapRLSrc = CFMachPortCreateRunLoopSource(
        kCFAllocatorDefault,
        myEventTap,
        0
    );

    // Add the source to the current RunLoop
    CFRunLoopAddSource(
        CFRunLoopGetCurrent(),
        eventTapRLSrc,
        kCFRunLoopDefaultMode
    );

    // Keep the RunLoop running forever
    CFRunLoopRun();

    // Not reached (RunLoop above never stops running)
    return 0;
}

The answer from Dave is the nicer Cocoa way of doing pretty much the same thing; basically Cocoa does the same as I do in my sample above behind the scenes, just wrapped into a static method. The code of Dave works only on 10.6, though, the above works in 10.4, 10.5 and 10.6.