How do I measure the velocity of a swipe?

Question

I'm developing a game that will use the force of the swipe as a variable user input.

I read from the documentation that on the touchesEnded event, I can get the allTouches array which is a list of the user touches collected from touchesBegan. From this I plan to get the last two touches to get the direction of the swipe. I will also get the time interval between touchesBegan and touchesEnded, from which I will get the speed of the swipe. I will use the direction and the speed to calculate the force of the swipe.

What I'd like to know is: is there a better way to do this? Is this already encapsulated in a library call somewhere?

Thanks in advance.

Answer 1

Rather than using allTouches on the UIEvent, you should use the touches set that you get in the call to your UIView's (or other UIResponder's) touchesEnded:withEvent: method. This ensures that you don't get touches that belong to other views.

Since the iPhone is a multi-touch device, the set contains all the touches that are associated with that event. In other words, if the user is touching the screen with two fingers, there should be two UITouch objects in the set, etc.

This means that the set does not contain all the points traversed since a touch began until it ended. To track that, you have to save the start point and time in touchesBegan:withEvent:, and then when the touch ends you calculate the speed based on that.

Note that if the set contains several points (which means the user is touching the screen with several fingers), you have to try to keep track of which UITouch object corresponds to which finger. You will want to do this in touchesMoved:withEvent:.

Since you get the touches in a set, you can't use an index or some key value to keep track of the touches you are interested in. I believe that the recommended way of doing this is to just assume that the touch that is closest to the point you saved on the previous event comes from the same finger. If you want to be more exact you can also use the UITouch's previousLocationInView: method.

If you're lazy, you can also simply do [[touches allObjects] objectAtIndex:0] and hope that this will give you the right touch (ie the one originating from the same finger) on each event. This actually often works, but I don't think you should use it in production code, especially not if you're trying to create an app with multi-touch functionality.

Answer 2

I've accomplished something along these lines rather simply by just comparing [touch locationInView:self] to [touch previousLocationInView:self] in touchesEnded of the Class (subclass of UIView) of the object that will be moved. This will give you a vector with location, direction & rough sense of velocity at the moment user released finger from the iPhone.