iPhone UIScrollView Speed Check - iphone

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 UIScrollView while it is tracking, or decelerating?

Answer 1

There's an easier way: check the UISCrollview's pan gesture recognizer. With it, you can get the velocity like so:
CGPoint scrollVelocity = [[_scrollView panGestureRecognizer] velocityInView:self];

Answer 2

Have these properties on your UIScrollViewDelegate
CGPoint lastOffset;
NSTimeInterval lastOffsetCapture;
BOOL isScrollingFast;
Then have this code for your scrollViewDidScroll:
- (void) scrollViewDidScroll:(UIScrollView *)scrollView {
CGPoint currentOffset = scrollView.contentOffset;
NSTimeInterval currentTime = [NSDate timeIntervalSinceReferenceDate];
NSTimeInterval timeDiff = currentTime - lastOffsetCapture;
if(timeDiff > 0.1) {
CGFloat distance = currentOffset.y - lastOffset.y;
//The multiply by 10, / 1000 isn't really necessary.......
CGFloat scrollSpeedNotAbs = (distance * 10) / 1000; //in pixels per millisecond
CGFloat scrollSpeed = fabsf(scrollSpeedNotAbs);
if (scrollSpeed > 0.5) {
isScrollingFast = YES;
NSLog(#"Fast");
} else {
isScrollingFast = NO;
NSLog(#"Slow");
}
lastOffset = currentOffset;
lastOffsetCapture = currentTime;
}
}
And from this i'm getting pixels per millisecond, which if is greater than 0.5, i've logged as fast, and anything below is logged as slow.
I use this for loading some cells on a table view animated. It doesn't scroll so well if I load them when the user is scrolling fast.

Answer 3

Converted #bandejapaisa answer to Swift 5:
Properties used by UIScrollViewDelegate:
var lastOffset: CGPoint = .zero
var lastOffsetCapture: TimeInterval = .zero
var isScrollingFast: Bool = false
And the scrollViewDidScroll function:
func scrollViewDidScroll(scrollView: UIScrollView) {
let currentOffset = scrollView.contentOffset
let currentTime = Date.timeIntervalSinceReferenceDate
let timeDiff = currentTime - lastOffsetCapture
let captureInterval = 0.1
if timeDiff > captureInterval {
let distance = currentOffset.y - lastOffset.y // calc distance
let scrollSpeedNotAbs = (distance * 10) / 1000 // pixels per ms*10
let scrollSpeed = fabsf(Float(scrollSpeedNotAbs)) // absolute value
if scrollSpeed > 0.5 {
isScrollingFast = true
print("Fast")
} else {
isScrollingFast = false
print("Slow")
}
lastOffset = currentOffset
lastOffsetCapture = currentTime
}
}

Answer 4

For a simple speed calculation (All the other answers are more complicated):
- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
CGFloat scrollSpeed = scrollView.contentOffset.y - previousScrollViewYOffset;
previousTableViewYOffset = scrollView.contentOffset.y;
}

Answer 5

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.

Answer 6

May be this would be helpful
- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset

Answer 7

You can see PageControl sample code about how to get the contentOffset of scrollview.
The contentOffset on movement can be obtained from UIScrollViewDelegate method, named - (void)scrollViewDidScroll:(UIScrollView *)scrollView, by querying scrollView.contentOffset. Current speed can be calculated by delta_offset and delta_time.
Delta_offset = current_offset - pre_offset;
Delta_time = current_time - pre_time;

Answer 8

Here is another smart way to do this in SWIFT :-
func scrollViewWillEndDragging(scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
if velocity.y > 1.0 || velocity.y < -1.0 && self.sendMessageView.isFirstResponder() {
// Somthing you want to do when scrollin fast.
// Generally fast Vertical scrolling.
}
}
So if you scrolling vertically you should use velocity.y and also if you are scrolling horizontally you should use velocity.x . Generally if value is more than 1 and less than -1, it represent generally fast scrolling. So you can change the speed as you want. +value means scrolling up and -value means scrolling down.