Pins a given scroll view’s width and height to its content size (and insets). Useful when you want to place a scroll view within another scroll view.

ScrollViewPinner.swift

//
//  ScrollViewPinner.swift
//
//  Created by Charles Joseph on 2017-11-01.
//

import Foundation
import UIKit

/// Pins a given scroll view’s width and height to its content size (and insets). Useful when you want to place a scroll view within another scroll view.
class ScrollViewPinner: NSObject {
    private(set) weak var scrollView: UIScrollView?

    var active: Bool = true {
        didSet {
            width.isActive = active
            height.isActive = active
            scrollView?.isScrollEnabled = !active
        }
    }

    private let width: NSLayoutConstraint
    private let height: NSLayoutConstraint

    init(scrollView: UIScrollView) {
        self.scrollView = scrollView
        scrollView.isScrollEnabled = false

        width = scrollView.widthAnchor.constraint(equalToConstant: 0)
        height = scrollView.heightAnchor.constraint(equalToConstant: 0)
        NSLayoutConstraint.activate([width, height])

        super.init()

        scrollView.addObserver(self, forKeyPath: "contentSize", options: [], context: nil)
        scrollView.addObserver(self, forKeyPath: "contentInset", options: [], context: nil)
    }

    deinit {
        scrollView?.removeObserver(self, forKeyPath: "contentSize")
        scrollView?.removeObserver(self, forKeyPath: "contentInset")
    }

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if (object as? UIScrollView) === scrollView && (keyPath == "contentSize" || keyPath == "contentInset") {
            updateConstraints()
        }
        else {
            super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
        }
    }

    private func updateConstraints() {
        guard let scrollView = scrollView else {
            return
        }

        width.constant = scrollView.contentSize.width + scrollView.contentInset.left + scrollView.contentInset.right
        height.constant = scrollView.contentSize.height + scrollView.contentInset.top + scrollView.contentInset.bottom
    }
}