UIPageViewController crashes when triggering page change during animation

Originator:futuretap
Number:rdar://24607949 Date Originated:11-Feb-2016 04:19 PM
Status:Duplicate/24607701/Closed Resolved:
Product:iOS SDK Product Version:9.2
Classification:Other Bug Reproducible:Always
 
[This is a duplicate of rdar://24607701. We're affected in our Where To? app as well where I can reproduce the crash 100%. Maybe also related to rdar://16002125]

Summary:
When triggering a page change through `setViewControllers:​animated:​completion:​` on a UIPageViewController while the controller is animating (e.g. showing a bounce animation)​, the controller crashes (most of the time)​.

Steps to Reproduce:
Steps to Reproduce:​
0. Open the example project
1. Run it on an actual device
2. Scroll to the previous page (there is no, so the page view controller starts bouncing)​
3. Try to go as far into the bounce as possible for a higher success rate
4. Lift your finger so that the bounce animation starts
5. Immediately (while the bounce animation is playing)​ tap the 'Next Page' button

Expected Results:
The next page is shown

Actual Results:
The app crashes

Version:
iPhone 6 iOS 9.2 /​ iPhone 6s iOS 9.0.2

Notes:
Sample Project: https://github.com/michaelochs/radars/tree/master/24607701%20-%20PageViewControllerCrash

Configuration:
iPhone 6 & iPhone 6s

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