message sent to deallocated instance error

Question

Im constantly being given an error that reads *** -[NSKeyValueObservance retain]: message sent to deallocated instance 0x86c75f10. I have tried running the Zombies template and here is the screenshot of what it provides.

It points to a managedObject, and I'm having trouble figuring out where the object is being deallocated. Here is the block of code that the compiler takes me to after each crash.

- (void)setIsFavourite:(BOOL)isFavourite shouldPostToAnalytics:(BOOL)shouldPostToAnalytics;
{
    // check whether we need to generate preferences objects just in time
    if(!self.preferences && !self.series.preferences /*&& isFavourite*/)
    {
        if(self.series)
        {
            [self.series addPreferencesObject];
        }
        else
        {
            [self addPreferencesObject];
        }
    }

    //Crash In here
    self.preferences.isFavourite = @(isFavourite);
    self.series.preferences.isFavourite = @(isFavourite);

EDIT: If you need to see a larger size of the image here is a larger resolution link.

Answer 1

OK, I hit something similar and found a way to debug this kind of issue with NSKeyValueObservance. To debug, do the following:

1. In Xcode, open the "Breakpoint Navigator".
2. Add a new symbolic breakpoint with: -[NSKeyValueObservance _initWithObserver:property:options:context:originalObservable:]
3. To that breakpoint, add an action and set it to "Debugger Command".
4. Set the following command: expr (void)NSLog(@"observer <0x%p>: %@ <%p>, property: %@", $arg1, (id)NSStringFromClass((id)[(id)$arg3 class]), $arg3, (id)$arg4)
5. Click the "Automatically continue after evaluating expression".

Now you can run your application and take the steps necessary to reproduce your crash. And yes, you'll want NSZombies enabled. Note: it's going to run slow and you're going to get a ton of debug output, but just be patient. It'll get there eventually.

When you hit the crash when trying to message a deallocated NSKeyValueObservance, you'll be presented with the address of the original object. Highlight the address and hit cmd-e to enter the text in the search buffer. Then hit cmd-g find the next occurrence of the string in the debugger output. You're going to potentially find the address a couple of times, so look for the address that follows the observer <0x?????> output. The output on that line should tell you what object is being observed and for which property.

In my case, when I figured this all out, it turned out that I was observing a synthesized property that depended on an object in array and during a certain operation, the order of the objects in the array changed without doing the correct KVO notifications, and that caused my crash.