UIViewController interactive transition - Presented view disappears when interactive dismiss is canceled

Question

I have a demo app where I'm trying to mimic the Mail app "new message" interactive transition - you can drag down to dismiss, but if you don't drag far enough the view pops back up and the transition is cancelled. I was able to duplicate the transition and interactivity in my demo app, but I noticed that when the dismiss transition is cancelled, the presented view controller is animated back up into place, then vanishes. Here's what it looks like:

My best guess is that the transition context's container view is being removed for some reason, since I added the presented view controller's view to it. Here is the presentation and dismiss code inside the UIViewControllerAnimatedTransitioning objects:

Show Transition

func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
    guard let fromVC = transitionContext.viewController(forKey: .from),
        let toVC = transitionContext.viewController(forKey: .to)
        else {
            return
    }

    let containerView = transitionContext.containerView
    let finalFrame = transitionContext.finalFrame(for: toVC)
    let duration = transitionDuration(using: transitionContext)
    let topSafeAreaSpace = fromVC.view.safeAreaInsets.top // using fromVC safe area since it's on screen and has correct insets
    let topGap: CGFloat = topSafeAreaSpace + 20

    containerView.addSubview(toVC.view)

    toVC.view.frame = CGRect(x: 0,
                             y: containerView.frame.height,
                             width: toVC.view.frame.width,
                             height: toVC.view.frame.height - 30)

    UIView.animate(withDuration: duration, animations: {
        toVC.view.frame = CGRect(x: finalFrame.minX,
                                 y: finalFrame.minY + topGap,
                                 width: finalFrame.width,
                                 height: finalFrame.height - topGap)

        let sideGap: CGFloat = 20
        fromVC.view.frame = CGRect(x: sideGap,
                                   y: topSafeAreaSpace,
                                   width: fromVC.view.frame.width - 2 * sideGap,
                                   height: fromVC.view.frame.height - 2 * topSafeAreaSpace)
        fromVC.view.layer.cornerRadius = 10
        fromVC.view.layoutIfNeeded()
    }) { _ in
        transitionContext.completeTransition(true)
    }
}

Dismiss Transition

func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
    guard let fromVC = transitionContext.viewController(forKey: .from),
        let toVC = transitionContext.viewController(forKey: .to)
        else {
            return
    }

    let finalFrame = transitionContext.finalFrame(for: toVC)
    let duration = transitionDuration(using: transitionContext)

    UIView.animate(withDuration: duration, animations: {
        toVC.view.frame = finalFrame
        fromVC.view.frame = CGRect(x: fromVC.view.frame.minX,
                                   y: finalFrame.height,
                                   width: fromVC.view.frame.width,
                                   height: fromVC.view.frame.height)

        toVC.view.layer.cornerRadius = 0

        toVC.view.layoutIfNeeded()
    }) { _ in
        transitionContext.completeTransition(true)
    }
}

And here is the code in the UIPercentDrivenInteractiveTransition object:

func handleGesture(_ gesture: UIPanGestureRecognizer) {
    #warning("need to use superview?")
    let translation = gesture.translation(in: gesture.view)
    var progress = translation.y / 400
    progress = min(1, max(0, progress)) // constraining value between 1 and 0

    switch gesture.state {
    case .began:
        interactionInProgress = true
        viewController.dismiss(animated: true, completion: nil)
    case .changed:
        shouldCompleteTransition = progress > 0.5
        update(progress)
    case .cancelled:
        interactionInProgress = false
        cancel()
    case .ended:
        interactionInProgress = false
        if shouldCompleteTransition {
            finish()
        } else {
            cancel()
        }
    default:
        break
    }
}

Any help would be greatly appreciated. It's worth noting I used this Ray Wenderlich tutorial as a reference - https://www.raywenderlich.com/322-custom-uiviewcontroller-transitions-getting-started However where they used image snapshots to animate the transition I'm using the view controller's views.

Answer 1

Thanks to a comment by @Dare, I realized all that was needed was a small update to the dismiss animation completion block:

// before - broken

transitionContext.completeTransition(true)

// after - WORKING!

transitionContext.completeTransition(!transitionContext.transitionWasCancelled)