Capturing all multitouch trackpad input in Cocoa

Question

Using touchesBeganWithEvent, touchesEndedWithEvent, etc you can get the touch data from the multitouch trackpad, but is there a way to block that touch data from moving the

Answer 1

As noted by valexa, using NSEventMask for CGEventTap is a hack. Tarmes also notes that Rob Keniger's answer no longer works (OS X >= 10.8). Luckily, Apple has provided a way to do this quite easily by using kCGEventMaskForAllEvents and converting the CGEventRef to an NSEvent within the callback:

NSEventMask eventMask = NSEventMaskGesture|NSEventMaskMagnify|NSEventMaskSwipe|NSEventMaskRotate|NSEventMaskBeginGesture|NSEventMaskEndGesture;

CGEventRef eventTapCallback(CGEventTapProxy proxy, CGEventType type, CGEventRef eventRef, void *refcon) {
  // convert the CGEventRef to an NSEvent
  NSEvent *event = [NSEvent eventWithCGEvent:eventRef];

  // filter out events which do not match the mask
  if (!(eventMask & NSEventMaskFromType([event type]))) { return [event CGEvent]; }

  // do stuff
  NSLog(@"eventTapCallback: [event type] = %d", [event type]);

  // return the CGEventRef
  return [event CGEvent];
}

void initCGEventTap() {
  CFMachPortRef eventTap = CGEventTapCreate(kCGSessionEventTap, kCGHeadInsertEventTap, kCGEventTapOptionListenOnly, kCGEventMaskForAllEvents, eventTapCallback, nil);
  CFRunLoopAddSource(CFRunLoopGetCurrent(), CFMachPortCreateRunLoopSource(kCFAllocatorDefault, eventTap, 0), kCFRunLoopCommonModes);
  CGEventTapEnable(eventTap, true);
  CFRunLoopRun();
}

Note that the call to CFRunLoopRun() is included since this snippet was taken from a project which could not use NSApplication but instead had a bare-bones CFRunLoop. Omit it if you use NSApplication.