A transition animator class which zooms into a specified UIView.

ZoomTransitionAnimator.swift

//
//  ZoomTransitionAnimator.swift
//  ScrapBeta
//
//  Created by Li-Heng Hsu on 06/12/2017.
//  Copyright © 2017 Li-Heng Hsu. All rights reserved.
//

import UIKit

protocol ZoomTransitionSource {
    
    func zoomTransitionAnimator(_ animator: ZoomTransitionAnimator, targetViewBeginningFrameFor operation: ZoomTransitionOperation) -> CGRect
    
}

protocol ZoomTransitionDestination {
    
    func zoomTransitionAnimator(_ animator: ZoomTransitionAnimator, targetViewFinalFrameFor operation: ZoomTransitionOperation) -> CGRect
	
}

enum ZoomTransitionOperation: Int {
    case zoomIn = 1, zoomOut = 2
}

class ZoomTransitionAnimator: NSObject, UIViewControllerAnimatedTransitioning {
    
    let duration: TimeInterval
    let damping: CGFloat
    let operation: ZoomTransitionOperation
    let source: ZoomTransitionSource
    let destination: ZoomTransitionDestination
    var targetViewBeginningFrame: CGRect {
        return source.zoomTransitionAnimator(self, targetViewBeginningFrameFor: operation)
    }
    var targetViewFinalFrame: CGRect {
        return destination.zoomTransitionAnimator(self, targetViewFinalFrameFor: operation)
    }
    
    init(duration: TimeInterval = 0.4, damping: CGFloat = 0.8, source: ZoomTransitionSource, destination: ZoomTransitionDestination, operation: ZoomTransitionOperation) {
        self.duration = duration
        self.damping = damping
        self.source = source
        self.destination = destination
        self.operation = operation
    }
    
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return duration
    }
    
    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        guard let fromView = transitionContext.view(forKey: .from) else { fatalError() }
        guard let toView = transitionContext.view(forKey: .to) else { fatalError() }
        let containerView = transitionContext.containerView
        
        let zoomInRatio = targetViewFinalFrame.width / targetViewBeginningFrame.width
        
        let toViewFirstFrame: CGRect = {
            let firstX = targetViewBeginningFrame.origin.x - (targetViewFinalFrame.origin.x / zoomInRatio)
            let firstY = targetViewBeginningFrame.origin.y - (targetViewFinalFrame.origin.y / zoomInRatio)
            let width = toView.frame.width / zoomInRatio
            let height = toView.frame.height / zoomInRatio
            return CGRect(x: firstX, y: firstY, width: width, height: height)
        }()
        
        let fromViewFinalFrame: CGRect = {
            let finalX = (fromView.frame.origin.x - targetViewBeginningFrame.origin.x) * zoomInRatio + targetViewFinalFrame.origin.x
            let finalY = (fromView.frame.origin.y - targetViewBeginningFrame.origin.y) * zoomInRatio + targetViewFinalFrame.origin.y
            let width = fromView.frame.width * zoomInRatio
            let height = fromView.frame.height * zoomInRatio
            return CGRect(x: finalX, y: finalY, width: width, height: height)
        }()
        
        let fromViewSnapshotView = fromView.snapshotView(afterScreenUpdates: false)!
        containerView.addSubview(fromViewSnapshotView)
        containerView.insertSubview(toView, at: 0)
        let backgroundView = UIView(frame: containerView.bounds)
        backgroundView.backgroundColor = fromView.backgroundColor
        containerView.insertSubview(backgroundView, aboveSubview: toView)
        let toViewSnapshotView = toView.snapshotView(afterScreenUpdates: true)!
        toViewSnapshotView.alpha = 0
        toViewSnapshotView.frame = toViewFirstFrame
        containerView.addSubview(toViewSnapshotView)
        
        fromView.isHidden = true
        
        containerView.backgroundColor = toView.backgroundColor
        
        UIView.animate(withDuration: duration, delay: 0, usingSpringWithDamping: damping, initialSpringVelocity: 0, options: .curveLinear, animations: {
            fromViewSnapshotView.frame = fromViewFinalFrame
            toViewSnapshotView.frame = toView.frame
            toViewSnapshotView.alpha = 1
        }, completion: { (completed) in
            fromView.isHidden = false
            containerView.backgroundColor = nil
            fromViewSnapshotView.removeFromSuperview()
            toViewSnapshotView.removeFromSuperview()
            backgroundView.removeFromSuperview()
            transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
        })
    }
    
}

Updated

1. One class for both zoom-in and zoom-out actions.
2. Modified the protocols so a view controller could be both a zoom-out source and a zoom-in source.