PhotoAlbumTransition.swift

func start(with context: TransitionContext) {
  guard let contextView = backDelegate.backContextView(for: self,
                                                       with: context.foreViewController) else {
                                                        return
  }
  guard let foreImageView = foreDelegate.foreContextView(for: self) else {
    return
  }

  let snapshotter = TransitionViewSnapshotter(containerView: context.containerView)
  context.defer {
    snapshotter.removeAllSnapshots()
  }

  foreImageView.isHidden = true
  context.defer {
    foreImageView.isHidden = false
  }

  let imageSize = foreImageView.image!.size

  let fitScale = min(foreImageView.bounds.width / imageSize.width,
                     foreImageView.bounds.height / imageSize.height)
  let fitSize = CGSize(width: fitScale * imageSize.width, height: fitScale * imageSize.height)

  let snapshotContextView = snapshotter.snapshot(of: contextView,
                                                 isAppearing: context.direction == .backward)

  context.compose(with: FadeTransition(target: .foreView, style: .fadeIn))
  context.compose(with: SpringFrameTransition(target: .target(snapshotContextView),
                                              size: fitSize))

  if let toolbar = foreDelegate.toolbar(for: self) {
    context.compose(with: SlideUpTransition(target: .target(toolbar)))
  }

  // This transition doesn't directly produce any animations, so we inform the context that it is
  // complete here, otherwise the transition would never complete:
  context.transitionDidEnd()
}