In Swift, how do I have a UIScrollView subclass that has an internal and external delegate?

Question

I\'m subclassing UIScrollView to add some features such as double tap to zoom and an image property for gallery purposes. But in order to do the image part my s

Answer 1

Here's a simple working Playground version in Swift 3 that acts purely as an observer rather than only as an interceptor like the other answers here.

The distinction is that the original scroll view delegate should have all of its delegate methods called like normal versus them being hijacked by another delegate.

(You can copy/paste this into a playground and run it to test)

import UIKit

final class ScrollViewObserver: NSObject, UIScrollViewDelegate {

    // MARK: - Instantiation

    init(scrollView: UIScrollView) {
        super.init()

        self.scrollView = scrollView
        self.originalScrollDelegate = scrollView.delegate
        scrollView.delegate = self
    }

    deinit {
        self.remove()
    }

    // MARK: - API

    /// Removes ourselves as an observer, resetting the scroll view's original delegate
    func remove() {
        self.scrollView?.delegate = self.originalScrollDelegate
    }

    // MARK: - Private Properties

    fileprivate weak var scrollView: UIScrollView?
    fileprivate weak var originalScrollDelegate: UIScrollViewDelegate?

    // MARK: - Forwarding Delegates

    /// Note: we forward all delegate calls here since Swift does not support forwardInvocation: or NSProxy

    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        // Run any custom logic or send any notifications here
        print("proxy did scroll")

        // Then, forward the call to the original delegate
        self.originalScrollDelegate?.scrollViewDidScroll?(scrollView)
    }

    func scrollViewDidZoom(_ scrollView: UIScrollView) {
        self.originalScrollDelegate?.scrollViewDidZoom?(scrollView)
    }

    func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
        self.originalScrollDelegate?.scrollViewWillBeginDragging?(scrollView)
    }

    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer) {
        self.originalScrollDelegate?.scrollViewWillEndDragging?(scrollView, withVelocity: velocity, targetContentOffset: targetContentOffset)
    }

    func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
        self.originalScrollDelegate?.scrollViewDidEndDragging?(scrollView, willDecelerate: decelerate)
    }

    func scrollViewWillBeginDecelerating(_ scrollView: UIScrollView) {
        self.originalScrollDelegate?.scrollViewWillBeginDecelerating?(scrollView)
    }

    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        self.originalScrollDelegate?.scrollViewDidEndDecelerating?(scrollView)
    }

    func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
        self.originalScrollDelegate?.scrollViewDidEndScrollingAnimation?(scrollView)
    }

    func viewForZooming(in scrollView: UIScrollView) -> UIView? {
        return self.originalScrollDelegate?.viewForZooming?(in: scrollView)
    }

    func scrollViewWillBeginZooming(_ scrollView: UIScrollView, with view: UIView?) {
        self.originalScrollDelegate?.scrollViewWillBeginZooming?(scrollView, with: view)
    }

    func scrollViewDidEndZooming(_ scrollView: UIScrollView, with view: UIView?, atScale scale: CGFloat) {
        self.originalScrollDelegate?.scrollViewDidEndZooming?(scrollView, with: view, atScale: scale)
    }

    func scrollViewShouldScrollToTop(_ scrollView: UIScrollView) -> Bool {
        return self.originalScrollDelegate?.scrollViewShouldScrollToTop?(scrollView) == true
    }

    func scrollViewDidScrollToTop(_ scrollView: UIScrollView) {
        self.originalScrollDelegate?.scrollViewDidScrollToTop?(scrollView)
    }

}

final class TestView: UIView, UIScrollViewDelegate {

    let scrollView = UIScrollView()
    fileprivate(set) var scrollObserver: ScrollViewObserver?

    required init() {
        super.init(frame: .zero)

        self.scrollView.delegate = self
        self.scrollObserver = ScrollViewObserver(scrollView: self.scrollView)
    }

    required init?(coder aDecoder: NSCoder) { fatalError("init(coder:) has not been implemented") }

    public func scrollViewDidScroll(_ scrollView: UIScrollView) {
        print("view's original did scroll delegate method called")
    }

}

let testView = TestView()
testView.scrollView.setContentOffset(CGPoint(x: 0, y: 100), animated: true)
testView.scrollObserver?.remove()
print("removed the observer")
testView.scrollView.setContentOffset(CGPoint(x: 0, y: 200), animated: true)
testView.scrollView.setContentOffset(CGPoint(x: 0, y: 300), animated: true)

This prints

proxy did scroll

view's original did scroll delegate method called

removed the observer

view's original did scroll delegate method called

view's original did scroll delegate method called