Crash in [UIAXDelegateClearer dealloc]

I got the same question...

I check my code and find that there is an object dealloc twice in my dealloc function, not sure that is the reason.

In case anyone else runs into this, it looks like a UIKit bug.

Basically it'll happen if

- Object X has a reference to a UIScrollview SV.

- X is SV's delegate

- When X deallocs, it has the last live reference to SV

Plus it only happens on particular devices, in particular setups - I can only get it to happen in the simulator with the accessibility inspector turned on, although I've had plenty of crash reports from real devices.

Basically, UIScrollView will attach an associated 'UIAXDelegateClearer' object to its delegate, so that when the delegate is freed the associated object is too, and it's destructor can zero out the scrollview's delegate (since it's dead now).

Normally when the scrollview deallocs, it removes this UIAXDelegateClearer from it's delegate. But if the delegate is in the process of deallocing when the SV deallocs, for some reason it doesn't remove it, and it's now left pointing at a dead scrollview. Then the UIAXDelegateClearer gets freed, tries to zero the delegate on the dead scrollview, and my app crashes and makes my users sad.

edit: rdar://22812548

edit2: Oooooh, it's because they made the delegate weak in iOS9. At the start of [ReadView dealloc] it zeros out weak references to itself, which include the scrollview's delegate property. But this bypasses the scrollview's mechanism for managing the UIAXDelegateClearer because that uses setDelegate, which is not called.

thanks for your analyse, I want to know what's your solution to this problem..

If this is the bug you're hitting, the solution is to make sure the UIScrollView isn't freed inside it's delegate's dealloc.

I do something like this:

@implementation ReadView
- (void)dealloc
{
   UIScrollView *sv = self.scrollview;
   dispatch_async( dispatch_get_main_queue(), ^{ sv.tag = 123; } );
}

This keeps the scrollview alive until a little later, when it's safe to free.

Thanks!

Thanks for the solution. We also started experiencing these crashes on iOS 9 iPads when voice over was enabled. It didn't always crash with voice over - but it did often enough to be a huge pain for users.