How to isolate the cause of strange iOS app crash

Question

There's a crash in the app that I'm working on that I'm having a difficult time tracking down the cause of. Here is the only set of events which causes the crash:

The app opens with the main view controller (VCmain) being presented. A button is triggered which opens a modal view controller (VCmodalA) via a segue. VCmodalA simply displays some information gathered about a core data object. VCmodalA is then dismissed by a button press. At some future point another view controller is presented modally (VCmodalB), which has some fields for the creation of a new core data object. After the object is created, if VCmodalB is dismissed, the app crashes with the following error:

*** -[UILabel _supportsContentDimensionVariables]: message sent to deallocated instance 0x818e200

If VCmodalA is not displayed prior to VCmodalB, or a new core data object is not created, or VCmodalB is not dismissed, then no crash occurs. I am at a loss what '_supportsContentDimensionVariables' means or who is sending it, although it appears to be sent to a label which was on VCmodalB (therefore the crash must be caused because a call is being made to a label that was deallocated when the view controller was dismissed).

I've spent hours poking around in Instruments looking at the Zombie left behind and trying to isolate the offending code by commenting it out, but I have been totally unsuccessful. At this point any hints would be welcome!!

Thanks so much!

Answer 1

There are two top causes of these kinds of errors: failure to use ARC, and direct access of ivars (particularly if you're not using ARC). Fixing those two issues is the best way to avoid these kinds of crashes.

As for how to debug it, first, you want to audit your accesses to UILabel objects. If you have any ivars that point to UILabel they should be strong or weak , never assign .

You should make sure that view controllers don't run code when they're not onscreen. This crash makes me think that this is a likely problem. For example, do not setup timers in viewDidLoad or initWithFrame: . Set them up in viewDidAppear: and tear them down in viewWillDisappear: . Similarly for KVO or delegation. View controllers manage views; if they're doing something when their view is not onscreen, your design is incorrect.