How to detect when a UIScrollView has finished scrolling

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
    [self stoppedScrolling];
}

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
    if (!decelerate) {
        [self stoppedScrolling];
    }
}

- (void)stoppedScrolling {
    // ...
}

I think scrollViewDidEndDecelerating is the one you want. Its UIScrollViewDelegates optional method:

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView

Tells the delegate that the scroll view has ended decelerating the scrolling movement.

UIScrollViewDelegate documentation

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
    scrollingFinished(scrollView: scrollView)
}

func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
    if decelerate {
        //didEndDecelerating will be called for sure
        return
    }
    scrollingFinished(scrollView: scrollView)        
}

func scrollingFinished(scrollView: UIScrollView) {
   // Your code
}

If you're into Rx, you can extend UIScrollView like this:

import RxSwift
import RxCocoa

extension Reactive where Base: UIScrollView {
    public var didEndScrolling: ControlEvent<Void> {
        let source = Observable
            .merge([base.rx.didEndDragging.map { !$0 },
                    base.rx.didEndDecelerating.mapTo(true)])
            .filter { $0 }
            .mapTo(())
        return ControlEvent(events: source)
    }
}

which will allow you to just do like this:

scrollView.rx.didEndScrolling.subscribe(onNext: {
    // Do what needs to be done here
})

This will take into account both dragging and deceleration.

To recap (and for newbies). It's not that painful. Just add the protocol, then add the functions you need for detection.

In the view (class) that contains the UIScrolView, add the protocol, then added any the functions from here to your view (class).

// --------------------------------
// In the "h" file:
// --------------------------------
@interface myViewClass : UIViewController  <UIScrollViewDelegate> // <-- Adding the protocol here

// Scroll view
@property (nonatomic, retain) UIScrollView *myScrollView;
@property (nonatomic, assign) BOOL isScrolling;

// Protocol functions
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView;
- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView;


// --------------------------------
// In the "m" file:
// --------------------------------
@implementation BlockerViewController

- (void)viewDidLoad {
    CGRect scrollRect = self.view.frame; // Same size as this view
    self.myScrollView = [[UIScrollView alloc] initWithFrame:scrollRect];
    self.myScrollView.delegate = self;
    self.myScrollView.contentSize = CGSizeMake(scrollRect.size.width, scrollRect.size.height);
    self.myScrollView.contentInset = UIEdgeInsetsMake(0.0,22.0,0.0,22.0);
    // Allow dragging button to display outside the boundaries
    self.myScrollView.clipsToBounds = NO;
    // Prevent buttons from activating scroller:
    self.myScrollView.canCancelContentTouches = NO;
    self.myScrollView.delaysContentTouches = NO;
    [self.myScrollView setBackgroundColor:[UIColor darkGrayColor]];
    [self.view addSubview:self.myScrollView];

    // Add stuff to scrollview
    UIImage *myImage = [UIImage imageNamed:@"foo.png"];
    [self.myScrollView addSubview:myImage];
}

// Protocol functions
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView {
    NSLog(@"start drag");
    _isScrolling = YES;
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
    NSLog(@"end decel");
    _isScrolling = NO;
}

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
    NSLog(@"end dragging");
    if (!decelerate) {
       _isScrolling = NO;
    }
}

// All of the available functions are here:
// https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIScrollViewDelegate_Protocol/Reference/UIScrollViewDelegate.html

An alternative would be to use scrollViewWillEndDragging:withVelocity:targetContentOffset which is called whenever the user lifts the finger and contains the target content offset where the scroll will stop. Using this content offset in scrollViewDidScroll: correctly identifies when the scroll view has stopped scrolling.

private var targetY: CGFloat?
public func scrollViewWillEndDragging(_ scrollView: UIScrollView,
                                      withVelocity velocity: CGPoint,
                                      targetContentOffset: UnsafeMutablePointer<CGPoint>) {
       targetY = targetContentOffset.pointee.y
}
public func scrollViewDidScroll(_ scrollView: UIScrollView) {
    if (scrollView.contentOffset.y == targetY) {
        print("finished scrolling")
    }

On some earlier iOS versions(like iOS 9, 10), scrollViewDidEndDecelerating won't be triggered if the scrollView is suddenly stopped by touching.

But in the current version (iOS 13), scrollViewDidEndDecelerating will be triggered for sure (As far as I know).

So, if your App targeted earlier versions as well, you might need a workaround like the one mentioned by Ashley Smart, or you can the following one.

    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        if !scrollView.isTracking, !scrollView.isDragging, !scrollView.isDecelerating { // 1
            scrollViewDidEndScrolling(scrollView)
        }
    }

    func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
        if !decelerate, scrollView.isTracking, !scrollView.isDragging, !scrollView.isDecelerating { // 2
            scrollViewDidEndScrolling(scrollView)
        }
    }

    func scrollViewDidEndScrolling(_ scrollView: UIScrollView) {
        // Do something here
    }

Explanation

UIScrollView will be stoped in three ways:
- quickly scrolled and stopped by itself
- quickly scrolled and stopped by finger touch (like Emergency brake)
- slowly scrolled and stopped

The first one can be detected by scrollViewDidEndDecelerating and other similar methods while the other two can't.

Luckily, UIScrollView has three statuses we can use to identify them, which is used in the two lines commented by "//1" and "//2".

I went through all the permutations and the best code I found was this. The interesting part is in 'scrollViewWillBeginDragging' and detecting if velocity is zero. This can occur when the user's finger is put on screen during an animating scroll to put the brakes on the scroll. For some reason, doing this does not trigger any of the other delegate 'stopping' API.

public func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {        
    let velocity = scrollView.panGestureRecognizer.velocity(in: scrollView.superview)
    if (velocity.equalTo(CGPoint.zero)){
        scrollViewDidEndScrollingAnimation(scrollView)
    }
}


public func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
    if (decelerate == false){
        scrollViewDidEndScrollingAnimation(scrollView)
    }
}

public func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
    scrollViewDidEndScrollingAnimation(scrollView)
}

public func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
    // do whatever when scrolling stops
}

Using Ashley Smart logic and is being converted into Swift 4.0 and above.

    func scrollViewDidScroll(_ scrollView: UIScrollView) {
         NSObject.cancelPreviousPerformRequests(withTarget: self)
        perform(#selector(UIScrollViewDelegate.scrollViewDidEndScrollingAnimation(_:)), with: scrollView, afterDelay: 0.3)
    }

    func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
        NSObject.cancelPreviousPerformRequests(withTarget: self)
    }

The logic above solve issues such as when user is scrolling off the tableview. Without the logic, when you scroll off the tableview, didEnd will be called but it will not execute anything. Currently using it in year 2020.

RXSwift version

commentsTableView.rx.didEndScrollingAnimation
    .bind(
        onNext: { _ in
            debugPrint(":DEBUG:", "didEndScrollingAnimation")
        }
    )
    .disposed(by: disposeBag)