ios App crashes when deallocating an UIView subclass instance - ios

Question

My app built natively with CocoaTouch and ARC crashes when deallocating a UIView subclass instance.
Here is the crash log.
OS Version: iOS 6.1.3 (10B329)
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x00000008
Crashed Thread: 0
0: 0x39de65b0 libobjc.A.dylib objc_msgSend + 16
1: 0x31edb694 CoreFoundation -[NSArray makeObjectsPerformSelector:] + 300
2: 0x33d8c57a UIKit -[UIView(UIViewGestures) removeAllGestureRecognizers] + 146
3: 0x33d8c144 UIKit -[UIView dealloc] + 440
4: 0x00240b36 MyApp -[StandardPanelView .cxx_destruct](self=0x20acba30, _cmd=0x00240985) + 434 at StandardPanelView.m:139
5: 0x39deaf3c libobjc.A.dylib object_cxxDestructFromClass(objc_object*, objc_class*) + 56
6: 0x39de80d2 libobjc.A.dylib objc_destructInstance + 34
7: 0x39de83a6 libobjc.A.dylib object_dispose + 14
8: 0x33d8c26a UIKit -[UIView dealloc] + 734
9: 0x0022aa14 MyApp -[StandardPanelView dealloc](self=0x20acba30, _cmd=0x379f1a66) + 156 at StandardPanelView.m:205
10: 0x39de8488 libobjc.A.dylib (anonymous namespace)::AutoreleasePoolPage::pop(void*) + 168
11: 0x31e98440 CoreFoundation _CFAutoreleasePoolPop + 16
12: 0x31f28f40 CoreFoundation __CFRunLoopRun + 1296
13: 0x31e9bebc CoreFoundation CFRunLoopRunSpecific + 356
14: 0x31e9bd48 CoreFoundation CFRunLoopRunInMode + 104
15: 0x35a502ea GraphicsServices GSEventRunModal + 74
16: 0x33db1300 UIKit UIApplicationMain + 1120
17: 0x00113c50 MyApp main(argc=1, argv=0x2fd3ed30) + 140 at main.m:23
The questions are:
what could be set wrong that makes the internal call to [UIView(UIViewGestures) removeAllGestureRecognizers] crash. One theory is that some gesture in the gestures array is deallocated already somewhere else.
When a UIView contains subviews, how is the sequence of deallocation process?
Some extra background info:
The crash happens, but there is no exact way to reproduce it.
The StandardPanelView instance works as delegate for gestures belongs to its subviews.
We are using flyweight on StandardPanelView instances, i.e., caching and recycling.
Thanks in advance for any hint about how this crash could happen.

Answer 1

My first impression is that you might be trying to access the StandardPanelView, which has just gotten deallocated.
What could be set wrong that makes the internal call to [UIView(UIViewGestures) removeAllGestureRecognizers] crash. One theory is that some gesture in the gestures array is deallocated already somewhere else.
It will not be because a UIGestureRecognizer got deallocated. The UIView strongly holds the UIGestureRecognizers in an NSArray. They will not be deallocated while they are still in the array.
However, the UIGestureRecognizer's delegate may have gotten deallocated. That is only an (assign) property, meaning that it is -not- strongly held, and if the delegate is deallocated, it will be a dangling pointer. So, if in [NSArray makeObjectsPerformSelector:] the delegate is used, this might happen.
When a UIView contains subviews, how is the sequence of deallocation process?
Objects are deallocated from 'parent' to 'child', ie. the superview is deallocated, then the subview, then the gesture recognizers. (Although whether the subviews are dealloced before the gesture recognizers is an implementation detail, so you probably shouldn't depend on it).
We can see this in a simple sample controller:
// The UIView that will be used as the main view
// This is the superview
#interface MyView : UIView
#end
#implementation MyView
- (void)dealloc {
NSLog(#"Dealloc MyView");
}
#end
// This view will be put inside MyView to be used as a subview
#interface MySubview : UIView
#end
#implementation MySubview
- (void)dealloc {
NSLog(#"Dealloc MySubview");
}
#end
// This is the gesture recognizer that we will use
// We will give one to each view, and see when it is deallocated
#interface MyGestureRecognizer : UIGestureRecognizer
#property (nonatomic, copy) NSString *tag;
#end
#implementation MyGestureRecognizer
#synthesize tag;
-(void)dealloc {
NSLog(#"Dealloc MyGestureRecognizer tag: %#", tag);
}
#end
// Just a test view controller that we will push on/pop off the screen to take a look at the deallocations
#interface TestViewController : UIViewController
#end
#implementation TestViewController
- (void)loadView {
self.view = [[MyView alloc] init];
MyGestureRecognizer *recognizer = [[MyGestureRecognizer alloc] initWithTarget:self action:#selector(doStuff)];
recognizer.tag = #"MyViewGestureRecognizer";
recognizer.delegate = self;
[self.view addGestureRecognizer:recognizer];
}
- (void)viewDidLoad {
[super viewDidLoad];
MySubview *subview = [[MySubview alloc] init];
MyGestureRecognizer *recognizer = [[MyGestureRecognizer alloc] initWithTarget:self action:#selector(doStuff)];
recognizer.tag = #"MySubviewGestureRecognizer";
recognizer.delegate = self;
[subview addGestureRecognizer:recognizer];
[self.view addSubview:subview];
}
- (void)doStuff {
// we don't actually care what it does
}
#end
All we are doing is adding MyView to be the main view for TestViewController, and then adding a MySubview inside MyView. We also attach a MyGestureRecognizer to each of the views.
When we push it off the screen, our log output presents:
Dealloc TestViewController
Dealloc MyView
Dealloc MySubview
Dealloc MyGestureRecognizer tag: MySubviewGestureRecognizer
Dealloc MyGestureRecognizer tag: MyViewGestureRecognizer
Sorry for the long answer... It has been about 3 months since you posted it, so maybe you already solved the problem, but in case anyone else stumbles across this answer, I hope it helps out.