Snapshot-based transitions in UIKit

/// Snapshot-based keyframe-animated views transition
/// Allows to perform transition form one view to another, optionally animating child views
public final class SnapshotTransition {

    /// Create snapshot transition
    ///
    /// - Parameters:
    ///   - from: source view
    ///   - to: target view
    ///   - container: container view in which transition occurs
    ///   - clipToBounds: set to false if you don't want bounds clipping (default value is true)
    ///   - childTransitions: array of (source childview, target childview) tuples that should be
    ///     animated separately alongside transition (default value is an empty array)
    public init(from: UIView,
                to: UIView,
                in container: UIView,
                clipToBounds: Bool = default,
                childTransitions: [(from: UIView, to: UIView)] = default)

    /// Call when you are ready to perform transition (when your views are layed out etc.)
    public func prepare()

    /// Call inside UIView.animateKeyframes animations closure to add transition animation keyframes
    public func addKeyframes()

    /// Call when transition completes, to clean it up and remove from container view
    public func cleanUp()
}

// MARK: UIViewControllerAnimatedTransitioning

func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
    let sourceView: UIView = ... // provide a view from presenting view controller
    let targetView: UIView = ... // provide the presented view controller's view

    // preapre transition context, add target view to container if needed
    if targetView.superview == nil {
        transitionContext.containerView.addSubview(targetView)
        targetView.frame = transitionContext.containerView.bounds
    }

    let transition = SnapshotTransition(
        from: sourceView, 
        to: targetView,
        in: transitionContext.containerView,
        childTransitions: [
            // optionally provide child views pairs that should be 
            // animated separately alongside the transition:
            (from: sourceView.subviewA, to: targetView.subviewA),
            (from: sourceView.subviewB, to: targetView.subviewB),
            (from: sourceView.subviewC, to: targetView.subviewC)
        ]
    )

    transition.prepare()

    UIView.animateKeyframes(
        withDuration: transitionDuration(using: transitionContext),
        delay: 0,
        options: [],
        animations: { transition.addKeyframes() },
        completion: { finished in
            transition.cleanUp()
            let completed = finished && !transitionContext.transitionWasCancelled
            transitionContext.completeTransition(completed)
        }
    )
}