Question

I’m using CGEventTapCreate to “steal” media keys from iTunes when my app is running. The code inside of the callback that I pass to CGEventTapCreate examines the event, and if it finds that it’s one of the media keys, posts an appropriate notification to the default notification center.

Now, this works fine if I post a notification for the “key up” event. If I do that for “key down” events, eventually my app stops getting media key events and iTunes takes over. Any ideas on what can be causing this? The relevant part of the code is below

enum { 
...
  PlayPauseKeyDown = 0x100A00,
  PlayPauseKeyUp = 0x100B00,
...
};

static CGEventRef event_tap_callback(CGEventTapProxy proxy,
                                     CGEventType type,
                                     CGEventRef event,
                                     void *refcon)
{
  if (!(type == NX_SYSDEFINED) || (type == NX_KEYUP) || (type == NX_KEYDOWN))
      return event;

  NSEvent* keyEvent = [NSEvent eventWithCGEvent: event];
  if (keyEvent.type != NSSystemDefined) return event;

  switch(keyEvent.data1)
  {
    case PlayPauseKeyUp:  // <--- this works reliably
    //case PlayPauseKeyDown:  // <--- this will break eventually
      post_notification(@"PlayPauseMediaKeyPressed", nil, nil);
      return NULL;

    ... and so on ...

Answer 1

Does something kill my event tap if the callback takes too long?

Some people suspect that Snow Leopard has a bug that sometimes disables event taps even if they don’t take too long. To handle that, you can watch for the event type kCGEventTapDisabledByTimeout, and respond by re-enabling your tap with CGEventTapEnable.