How to determine true end velocity of pan gesture?

Question

When using UIPanGestureRecognizer and detecting UIGestureRecognizerStateEnded, then the velocity of the gesture is not the true velocity. Instead, it\'

Answer 1

2019 how-to ...

This is the only way to really know the velocity when the finger comes up:

have some variables...

var cat: CADisplayLink? = nil
var prevTime = CFAbsoluteTimeGetCurrent()
var lastKnownPosition: CGFloat = 0
var lastKnownActualVelocity: Double = 0

and then ...

@objc func _checkVelocityEveryTrueFrame() {
    let newTime = CFAbsoluteTimeGetCurrent()
    let frameTime = newTime - prevTime
    prevTime = newTime

    let newPos = yourConstraint.constant
    lastKnownActualVelocity = Double(newPos - lastKnownPosition) / frameTime
    lastKnownPosition = newPos
    print("- \(frameTime) \(lastKnownPosition) \(lastKnownActualVelocity)")
}

@objc func dragOrFlick(_ p: UIPanGestureRecognizer) {
    if p.state == .began {
        cat?.invalidate()
        cat = nil
        cat = CADisplayLink(target: self,
             selector: #selector(_checkVelocityEveryTrueFrame))
        cat?.add(to: .main, forMode: .common)
    }

    if p.state == .changed {
        ... p.translation(in: ...
        yourConstraint.constant = new position...
    }

    if p.state == .ended {
        cat?.invalidate()
        cat = nil
        let trueFinalVelocity = lastKnownActualVelocity
        print("trueFinalVelocity is truly \(trueFinalVelocity)")
    }
}

That's it. As far as I know there's no simpler way.

⁺ Footnote. As any game programmer will tell you, even this is a bit shoddy; it gives the platonic velocity over one frame: purists would smooth it a little over a discussable amount of frames :/ It's a tricky issue.