kTouchSlop top-level constant
The distance a touch has to travel for the framework to be confident that the gesture is a scroll gesture, or, inversely, the maximum distance that a touch can travel before the framework becomes confident that it is not a tap.
A total delta less than or equal to kTouchSlop is not considered to be a drag, whereas if the delta is greater than kTouchSlop it is considered to be a drag.
Implementation
// This value was empirically derived. We started at 8.0 and increased it to
// 18.0 after getting complaints that it was too difficult to hit targets.
const double kTouchSlop = 18.0;