A UIViewPropertyAnimator subclass that adds a preparation phase

HSViewPropertyAnimator.swift

import UIKit

final class HSViewPropertyAnimator: UIViewPropertyAnimator {
    
    private var preparations: [(UIViewAnimatingPosition) -> ()] = []
    
    override func startAnimation() {
        runPreparations()
        super.startAnimation()
    }
    
    func addPreparation(_ preparation: @escaping (UIViewAnimatingPosition) -> ()) {
        preparations.append(preparation)
    }
    
    private func runPreparations() {
        let position: UIViewAnimatingPosition = {
            switch self.fractionComplete {
            case 0: return .start
            case 1: return .end
            default: return .current
            }
        }()
        
        preparations.forEach { $0(position) }
    }
    
}