iPhone UIScrollView Speed Check

Question

I know how to get the contentOffset on movement for a UIScrollView, can someone explain to me how I can get an actual number that represents the current speed of a UIScrollV

Answer 1

2017...

It's very easy to do this with modern Swift/iOS:

var previousScrollMoment: Date = Date()
var previousScrollX: CGFloat = 0

func scrollViewDidScroll(_ scrollView: UIScrollView) {
        
    let d = Date()
    let x = scrollView.contentOffset.x
    let elapsed = Date().timeIntervalSince(previousScrollMoment)
    let distance = (x - previousScrollX)
    let velocity = (elapsed == 0) ? 0 : fabs(distance / CGFloat(elapsed))
    previousScrollMoment = d
    previousScrollX = x
    print("vel \(velocity)")

Of course you want the velocity in points per second, which is what that is.

Humans drag at say 200 - 400 pps (on 2017 devices).

1000 - 3000 is a fast throw.

As it slows down to a stop, 20 - 30 is common.

So very often you will see code like this ..

    if velocity > 300 {
    
        // the display is >skimming<
        some_global_doNotMakeDatabaseCalls = true
        some_global_doNotRenderDiagrams = true
    }
    else {
    
        // we are not skimming, ok to do calculations
        some_global_doNotMakeDatabaseCalls = false
        some_global_doNotRenderDiagrams = false
    }

This is the basis for "skimming engineering" on mobiles. (Which is a large and difficult topic.)

Note that that is not a complete skimming solution; you also have to care for unusual cases like "it has stopped" "the screen just closed" etc etc.