Thinking Out Loud

Timing Matters

Thu, Nov 03, 2016
  1. ios-dev
  2. presentation

Just a quick heads up, since this just bit me again.

Let’s try a little experiment. Feel free to follow along in a Swift Playground.

Given the following code, solve for vc.presentationController.

let vc = UIViewController()
vc.modalPresentationStyle = .popover
vc.presentationController

If you guessed UIPopoverPresentationController you are correct. Let’s try another one.

let vc = UIViewController()
vc.presentationController

This one is a tiny trick question. The real answer is _UIFullscreenPresentationController which is a private class, but I would have accepted “some type of full screen presentation controller” given that .fullScreen is the default modalPresentationStyle.

Ok, one last one — and the point of this blog post.

let vc = UIViewController()
vc.presentationController // Obviously _UIFullscreenPresentationController
vc.modalPresentationStyle = .popover
vc.presentationController // ???

If you guessed UIPopoverPresentationController you would be dead wrong. The correct answer is _UIFullscreenPresentationController. presentationController is cached when created and the only way I know of to invalidate that is to present the view controller and then dismiss it. This is just a warning that timing matters when it comes to these things.

My workaround is to first make sure the view controller in question is currently presented — by verifying its presentingViewController is non-nil — before asking for its presentationController.

By the way, I have filed this as a radar. Works as intended, which is fine but I just thought more people might want to know how it works.