App freezes on main thread on iPhone 5c but not on iPhone 6s

Question

I'm working on an app with a navigation controller. Swift + objective C

Important note: This freeze happens 100% when testing on iPhone 5c and never happens when testing on iPhone 6s

The scenario is:

1. Go to TableViewController A
2. Touch an item of the list to reach TableViewController B
3. Go back to TVController A by pressing the top left arrow
4. The app shows TableViewController A but freezes right away

The UI is frozen but the app is still running. If I press the Pause button (debug mode), I can see that the Thread 1 (Main thread) stack is:

0 semaphore_wait_trap
45 UIApplicationMain
46 main
47 start

I don't know where to start investigating so that I can find where the problem is.

Any ideas?

Answer 1

After a day looking into it, I finally found out that it was linked to MKMapView deinit.

Later I found this post about a very similar topic:

WARNING: Output of vertex shader 'v_gradient' not read by fragment shader

According to - the god - @mojuba , who found that the freeze was also related to the deinit phase offered a fix for the DEBUG mode. Because this issue seems to happen only in DEBUG...

I successfully applied his fix to my code as followed:

deinit {
    #if DEBUG
      // Xcode8/iOS10 MKMapView bug workaround
      if let mV = locationPickerView.mapView {
        VControllerB.unusedObjects.append(mV) // addObject:_mapView];
      }
    #endif
  }

NOTE that this produces a memory leak as mapView is never freed and kept within a static array of the VControllerB class.

NOTE 2: I reported the bug in Apple Bug Reporter while using Xcode 8.2.1 in Jan 27.

Answer 2

This is a deadlock. Looks like you use dispatch_sync on main thread. To fix problem - check other threads. In some thread you will find the code that is waiting when dispatched block ends ...