KVO for scroll start & end in UIScrollView - ios

Question

Is there a way to use KVO (key value observing) to detect scrollViewWillBeginDragging and scrollViewDidEndDecelerating in swift?
Edit: I tried
scrollView.addObserver(self, forKeyPath: "dragging", options: NSKeyValueObservingOptions.New, context: nil)
But it's never called. If I observe for instance "contentOffset" in the same way, it's called. Is it KVC compliant?

Answer 1

I'm not sure whether it is sufficient for your question, but while you may not be able to observe isDragging or isDecelerating directly you can track them as your observing contentOffset. To find the start and end of dragging you can simply track when isDragging changes from false to true and vice versa by creating a global Boolean (here draggingInitiated) and including something like this in your observeValue:
if !draggingInitiated && scrollView.isDragging {
draggingInitiated = true
print("Started Dragging")
} else if draggingInitiated && !scrollView.isDragging {
draggingInitiated = false
print("Ended Dragging")
}

Answer 2

Answer based on #Philip De Vries
Swift 5
add observer
scrollView.addObserver(self, forKeyPath: "contentOffset", options: .new, context: nil)
observe
// MARK: - Observation
override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey: Any]?, context: UnsafeMutableRawPointer?) {
if let obj = object as? UIScrollView {
if obj == self.scrollView && keyPath == "contentOffset" {
scrollViewDidScroll(scrollView)
if !draggingInitiated && scrollView.isDragging {
draggingInitiated = true
onStartDraggin()
} else if draggingInitiated && !scrollView.isDragging {
draggingInitiated = false
onReleaseDragging()
}
}
}
}
cleanUp
private func removeObsrvers() {
scrollView.removeObserver(self, forKeyPath: "contentOffset")
}

Answer 3

You can add observers using NSNotification center.
In the viewDidLoad of your VC,
override func viewDidLoad() {
super.viewDidLoad()
NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(self.informBeginDragging), name: "begin", object: nil)
NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(self.informEndDragging), name: "end", object: nil)
}
You can do anything in the following two methods
func informBeginDragging() {
//do something
}
func informEndDragging() {
//do something
}
And these two methods will be triggered when scroll view begin dragging or end dragging
func scrollViewDidEndDragging(scrollView: UIScrollView, willDecelerate decelerate: Bool) {
NSNotificationCenter.defaultCenter().postNotificationName("end", object: nil)
}
func scrollViewWillBeginDragging(scrollView: UIScrollView) {
NSNotificationCenter.defaultCenter().postNotificationName("begin", object: nil)
}