UIScrollView+Behavior.1.swift

import UIKit

private final class ScrollViewBehaviorContainer: NSObject, UIScrollViewDelegate {
    // MARK: Properties

    var behaviors = [ScrollViewBehavior]()

    // MARK: UIScrollViewDelegate

    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        behaviors.forEach { $0.afterScrolling(scrollView) }
    }

    func scrollViewDidZoom(_ scrollView: UIScrollView) {
        behaviors.forEach { $0.afterZooming(scrollView) }
    }

    func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
        behaviors.forEach { $0.beforeDraggingBegins(scrollView) }
    }

    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
        behaviors.forEach { $0.beforeDraggingEnds(scrollView, velocity: velocity, targetContentOffset: targetContentOffset) }
    }

    func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
        behaviors.forEach { $0.afterDraggingEnds(scrollView, wilDecelerate: decelerate) }
    }

    func scrollViewWillBeginDecelerating(_ scrollView: UIScrollView) {
        behaviors.forEach { $0.beforeDeceleratingBegins(scrollView) }
    }

    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        behaviors.forEach { $0.afterDeceleratingEnds(scrollView) }
    }

    func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
        behaviors.forEach { $0.afterScrollingAnimation(scrollView) }
    }

    func scrollViewWillBeginZooming(_ scrollView: UIScrollView, with view: UIView?) {
        behaviors.forEach { $0.beforeZoomingBegins(scrollView, view: view) }
    }

    func scrollViewDidEndZooming(_ scrollView: UIScrollView, with view: UIView?, atScale scale: CGFloat) {
        behaviors.forEach { $0.afterZoomingEnds(scrollView, view: view, scale: scale) }
    }

    func scrollViewDidScrollToTop(_ scrollView: UIScrollView) {
        behaviors.forEach { $0.afterScrollingToTop(scrollView) }
    }

    func scrollViewDidChangeAdjustedContentInset(_ scrollView: UIScrollView) {
        behaviors.forEach { $0.afterChangingAdjustedContentInset(scrollView) }
    }
}

extension UIScrollView {
    // MARK: Objective-C Runtime

    private enum Keys {
        static var behaviorContainer = "behaviorContainer"
    }

    private var behaviorContainer: ScrollViewBehaviorContainer {
        if let container = objc_getAssociatedObject(self, &Keys.behaviorContainer) as? ScrollViewBehaviorContainer {
            return container
        }
        let container = ScrollViewBehaviorContainer()
        objc_setAssociatedObject(self, &Keys.behaviorContainer, container, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        return container
    }

    // MARK: Public methods
    
    func addBehavior(_ behavior: ScrollViewBehavior) {
        behaviorContainer.behaviors.append(behavior)
    }
    
    func addBehaviors(_ behaviors: [ScrollViewBehavior]) {
        behaviorContainer.behaviors.append(contentsOf: behaviors)
    }
}