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.

Answer 2

UPDATE: my answer below no longer works. See the answer here.

Usually to do this you'd need to use a Quartz Event Tap, although the touch events don't seem to be "officially" supported by the CGEvent API. The non-multitouch event types in NSEvent.h seem to map to the CGEvent types in CGEventTypes.h, so the multitouch ones will probably work, even if they're not documented.

In order to block the events from propagating, you need to return NULL from the event tap callback.

You'd need some code like this:

#import <ApplicationServices/ApplicationServices.h>

//assume CGEventTap eventTap is an ivar or other global

void createEventTap(void)
{
 CFRunLoopSourceRef runLoopSource;

 //listen for touch events
 //this is officially unsupported/undocumented
 //but the NSEvent masks seem to map to the CGEvent types
 //for all other events, so it should work.
 CGEventMask eventMask = (
  NSEventMaskGesture       |
  NSEventMaskMagnify       |
  NSEventMaskSwipe         |
  NSEventMaskRotate        |
  NSEventMaskBeginGesture  |
  NSEventMaskEndGesture
 );

 // Keyboard event taps need Universal Access enabled, 
 // I'm not sure about multi-touch. If necessary, this code needs to 
 // be here to check whether we're allowed to attach an event tap
 if (!AXAPIEnabled()&&!AXIsProcessTrusted()) { 
  // error dialog here 
  NSAlert *alert = [[[NSAlert alloc] init] autorelease];
  [alert addButtonWithTitle:@"OK"];
  [alert setMessageText:@"Could not start event monitoring."];
  [alert setInformativeText:@"Please enable \"access for assistive devices\" in the Universal Access pane of System Preferences."];
  [alert runModal];
  return;
 } 


 //create the event tap
 eventTap = CGEventTapCreate(kCGHIDEventTap, //this intercepts events at the lowest level, where they enter the window server
        kCGHeadInsertEventTap, 
        kCGEventTapOptionDefault, 
        eventMask,
        myCGEventCallback, //this is the callback that we receive when the event fires
        nil); 

 // Create a run loop source.
 runLoopSource = CFMachPortCreateRunLoopSource(kCFAllocatorDefault, eventTap, 0);

 // Add to the current run loop.
 CFRunLoopAddSource(CFRunLoopGetCurrent(), runLoopSource, kCFRunLoopCommonModes);

 // Enable the event tap.
 CGEventTapEnable(eventTap, true);
}


//the CGEvent callback that does the heavy lifting
CGEventRef myCGEventCallback(CGEventTapProxy proxy, CGEventType type, CGEventRef theEvent, void *refcon)
{
 //handle the event here
 //if you want to capture the event and prevent it propagating as normal, return NULL.

 //if you want to let the event process as normal, return theEvent.
 return theEvent;
}