NSResponder Modifications: Swipe, Rotate, and Magnify

called from NSApplication's -(void)sendEvent:(NSEvent *)anEvent method.

Just to confirm this assumption, I overrode this method and just logged events from it. Turns out the only events that come through this method are gestures. Awesome. Interestingly enough, there is a small observation to be made here. Since every gesture passes through here, one could actually replace these gesture events with any other event, so you could get some free custom gestures by replacing the gestures with say, a keyboard shortcut event before passing it down the responder chain. Nothing really fantastic, but nice to note if you feel like playing with private methods.

So finally, we have everything we need to implement gestures for any NSResponder based object. Simply override the appropriate method for your gesture (swipe, pinch [magnify], or rotate), you can grab the value out of the NSEvent using the corresponding methods:

Swipe : -(float)deltaY; -(float)deltaX;

Magnification : -(float)deltaZ; -(float)magnification;

Rotation : -(float)rotation;

These methods return various values for each type of event depending on the type of event, velocity of movement and/or direction of movement. Some interesting notes here - swipes can be detected to the left or right as expected, but also up and down. Pretty cool. I also suspect that the two methods under magnification actually return the same value, but I haven't checked. The value logged with the event is deltaZ, however. Rotation is pretty self explanatory. The actual values these methods return is a little bit more of a mystery. Swipe is straight forward, 1.0 for right, -1.0 for left, 1.0 for up, and -1.0 for down, depending on whether you check deltaX or deltaY respectively. Magnification and rotation both return various positive or negative values depending on the rotation or scale of magnification. You'll just kind of have to play with these, I couldn't discern a good pattern. However, there are two methods on NSEvent: -(float)standardRotationThreshold and -(float)standardMagnificationThreshold which I'm sure return some magic values that allow you to determine the angle and scale for the gestures, which I imagine can help you figure out what you need to do to your responder as a result of the gesture. Additionally, NSEvent adds -(BOOL)isGesture, which as expected, lets you determine if an event is a gesture. NSResponder also adds gesture masks via -(unsigned long long)gestureEventMask and -(void)setGestureEventMask:(unsigned long long)aMask which presumably deal with the specific masks regarding events.

So, that should pretty much do it. You should be armed and ready to go to write some amazing code to take advantage of the trackpad inside the Macbook Air, and hopefully coming soon to a Macbook and Macbook Pro near you. Before anyone asks, this code will NOT work on current Macbook models, at least not on the current build of Leopard. It's possible when this AppKit version begins shipping standard (10.5.2, I suspect) that we may be able to fake these events with the old trackpads, depending on how much information we can squeeze out of AppKit.

Finally, I've built a small project based on the all of this code that will allow those of you lucky enough to have your Macbook Air already to see it all in action. It's fairly simple, just filling a simple custom view with a color based on the gesture you perform, but it is set up as a great test bed for playing around with third party gestures. In addition it has a commented out private method from NSApplication to log all the gesture events if you want to see them. Obviously if you uncomment this method, the events won't be processed and the color won't change since it's not sending them down the chain. I'll leave it as an exercise to the reader to add that. ;)

Thanks to everyone at my local Apple Store for their help, and the Macbook Air. Hopefully this will help everyone get their applications ready for multi-touch trackpads before they are everywhere.

Be back soon.