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I'm writing an iPad application that needs to time the finger movement very precisely, so I am keen on getting the best performance and accuracy I can. I have two questions about this:

  1. For intercepting finger position on screen, must I use touchesMoved(), or is there a lower-level API?

  2. Does anyone know whether touchesMoved() returns the exact position on screen, or does the iPad perform some interpolation?

Thanks!

user809803
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2 Answers2

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You have 3 ways to intercept finger positions: a) UIResponder Delegate metods:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event;
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event;
- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event;
- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event;

b) UIGestureRecognizer's methods

c) You also can override - (void)sendEvent:(UIEvent *)event; within UIApplicationDelegate or UIWindow to catch UIEvents - but this method is not recommended.

All this methods work with UITouch that contain touch point. This point calculated by device hardware, so you can't affect precision.

And the last: minimal recommended touch area is 44x44. If you touch area will be smaller then users will fill yourself discomfortable working with your software.

SVGreg
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Touches are not very precise, because fingers are not very precise (much less so than a mouse cursor), and because the tracking hardware isn't very precise (because it doesn't need to, because of finger size - it does not work without significant touched area).

That being said, the iDevices have very good touchscreens compared to many other capacitive touch devices. I'm just not sure pixel-precision is attainable in practice.

It's easy to experiment with this, though.

fzwo
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