UITouch的所有CGPoints

Question

Consider the case of a single tap of a finger on a UIView . I know you can get the CGPoint of the tap from the UITouch object, but is it possible to get all CGPoint s that were a part of the UITouch (ie all points under the finger)? The touch screen hardware must be detecting all points of the screen where a conductive material is touching, but does Apple make it available?

Answer 1

What you get is not only the computed center of the touch (slightly corrected, because people tend to touch slightly below the point which they target), but also a variable called majorRadius . In iOS8 this is given in increments of 10.45 mm, and the variable majorRadiusTolerance adds the variation of this measurement. Conveniently, in my measurements majorRadiusTolerance always came out as half the increment size, so a touch reported as 31.35 mm can be anywhere between 26.125 mm and 36.575 mm. Of course, this is only an approximation, because non-round objects will still provoke the point and the radius, nothing more.

I agree, more precision should be possible, but then where do you draw the boundary? The capacitive signal wanes the more you move away from the center of the touch, and where it ends is an arbitrary decision in the end.