HardRubberbandView.swift

HardRubberbandView.swift

import UIKit

final class HardRubberbandView: UIView, UIScrollViewDelegate {
    var contentSize: CGSize = .zero {
        didSet {
            scrollView.contentSize = contentSize
            contentView.frame.size = contentSize
        }
    }
    @objc
    private(set) var contentOffset: CGPoint = .zero {
        willSet { willChangeValue(forKey: #keyPath(HardRubberbandView.contentOffset)) }
        didSet { didChangeValue(forKey: #keyPath(HardRubberbandView.contentOffset)) }
    }
    /// range 0 ~ 1
    var hardness: CGFloat = 0.84 {
        didSet { assert((0...1).contains(hardness), "`hardness` can only set range 0 ~ 1") }
    }
    let contentView = UIView()
    private let scrollView = UIScrollView()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        initialize()
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        initialize()
    }
    
    private func initialize() {
        scrollView.frame = bounds
        scrollView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        scrollView.delegate = self
        addSubview(scrollView)
        scrollView.addSubview(contentView)
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        contentView.frame.size = scrollView.contentSize
    }
    
    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        updateOffset(with: scrollView.contentOffset)
    }
    
    func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
        if !decelerate {
            updateOffset(with: scrollView.contentOffset)
        }
    }
    
    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        updateOffset(with: scrollView.contentOffset)
    }
    
    private func updateOffset(with newOffset: CGPoint) {
        let origin: CGPoint = {
            var p: CGPoint = .zero
            if newOffset.x < 0 {
                p.x = newOffset.x * hardness
            } else if (newOffset.x + scrollView.bounds.width) > scrollView.contentSize.width {
                p.x = (newOffset.x + scrollView.bounds.width - scrollView.contentSize.width) * hardness
            }
            if newOffset.y < 0 {
                p.y = newOffset.y * hardness
            } else if (newOffset.y + scrollView.bounds.height) > scrollView.contentSize.height {
                p.y = (newOffset.y + scrollView.bounds.height - scrollView.contentSize.height) * hardness
            }
            return p
        }()
        contentView.frame.origin = origin
        contentOffset = {
            var p = contentView.convert(CGPoint.zero, to: self)
            p.x = -p.x
            p.y = -p.y
            return p
        }()
    }
}

final class ViewController: UIViewController {
    private let scrollView = HardRubberbandView()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        view.addSubview(scrollView)
        scrollView.frame = CGRect(x: 0, y: 300, width: view.bounds.width, height: 100)
        scrollView.contentSize = CGSize(width: 1000, height: 100)
        scrollView.contentView.backgroundColor = .orange
        for color in [UIColor.blue, .red, .brown, .yellow] {
            let superview = scrollView.contentView
            let view = UIView()
            view.backgroundColor = color
            let prev = superview.subviews.last
            superview.addSubview(view)
            view.translatesAutoresizingMaskIntoConstraints = false
            view.leftAnchor.constraint(equalTo: prev?.rightAnchor ?? superview.leftAnchor, constant: 10).isActive = true
            view.heightAnchor.constraint(equalToConstant: 80).isActive = true
            view.topAnchor.constraint(equalTo: superview.topAnchor, constant: 10).isActive = true
            if let prev = prev {
                view.widthAnchor.constraint(equalTo: prev.widthAnchor, multiplier: 1).isActive = true
            }
        }
        scrollView.contentView.subviews.last?.rightAnchor.constraint(equalTo: scrollView.contentView.rightAnchor, constant: -10).isActive = true
    }
    
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}