Using Pan gesture recognizer to bring down a view controller

Question

I want to be able to swipe down on my main view controller and have my second view controller be brought down from above. I want this transition to be interactive in the sense that you can go back and forth between VCs without having to complete the whole transition (following the finger of the user like a scrollview with paging does).

This is effect can be seen when sliding between View Controllers on the new snapchat update. When you first open the app you are on the camera screen. If you swipe in any direction (up, down, left or right) the corresponding view comes in and covers the camera screen. When you swipe between views they are all essentially covering the main view (not moving it of to the side like in many tutorials i find online which use scrollview to achieve this effect).

Essentially it would look like a vertical page view controller, but instead of moving the current view controller off the screen as the new one comes in, the new one would cover the current one until it is completely on top of it.

From what I understand, a pan gesture recognizer could do the trick. Does anyone know how I can go about doing this. I have been searching forever and can not seem to figure it out.

Answer 1

I want to be able to swipe down on my main view controller and have my second view controller be brought down from above. I want this transition to be interactive

Okay, so you are describing a custom interactive transition animation. The exact details for how you implement this depend on whether you want this to be a push transition or a present (modal) transition. I assume it is to be a presentation (modal) transition.

So you will start by setting the presented view controller's transitioningDelegate. Everything takes off from there. The pan gesture recognizer will call present, and the transitioning delegate's delegate methods will be called:

• animationController(forPresented:presenting:source:)
• interactionControllerForPresentation(using:)

The interaction controller that you return from the second method is responsible to responding to each change in the pan gesture by updating the "frame" of the animation (as well as the transition coordinator).

If this is for iOS 10 only, this is very easy because you can use the UIViewPropertyAnimator. It has the remarkable ability to "hurry" to the end or start of the animation when the gesture ends and you decide to complete or cancel the animation. Otherwise, you're probably best off using a UIPercentDrivenInteractiveTransition object to help you.