retain sent to deallocated instance with KVO (EXC_BREAKPOINT) - ios

Question

I have this very strange error happening when I'm changing view controllers in my iOs app.
First some background info.
I am retrieving an image from the web in a background NSOperation. This operation is started from another (background) operation that is instantiated in an collection view cell. The way this works is that the cell creates an object , then sets itself as an observer for that object, then creates an NSOperation with that object as a parameter. This first level operation will start the second operation that will get the image from the web and another NSOperation that will try to get the data from a file (if possible) and report it via delegation to the first operation. That first level operation will change a property on the observed object thus triggering the KVO. The collection/tableView cell will update from the - observeValueChange method.
Here is the problem:
Sometime the cell disappears (reused or deallocated) and when the background thread tries to set the value on the observed object it triggers an EXC_BREAKPOINT exception ([collectionViewCell message retain sent to deallocated instance]).
In order to prevent this I tried implementing -prepareForReuse and -dealloc on the cells. But the error keeps happening.
The flow seem like this:
-User loads VC that has collectionViewWithCells
-cell creates object and NSOperation 1
NSoperation 1 creates NSOperation2 (this is of two types get from web or get from file)
NSOpeartion 2 get image from internet or from a local file
NSoperation 2 sends data to NSOperation1
User has left this screen
NSOperation 1 tries to set data on observed object
-- CRASH
Here is the code inside the cell:
#interface CustomCollectionViewCell ()
#property (strong, nonatomic) NSOperationQueue *imagesOperationQueue;
#property (strong, nonatomic) ImageObject *imgObj;
#end
#implementation CustomCollectionViewCell
- (id)initWithFrame:(CGRect)frame
{
self = [super initWithFrame:frame];
if (self) {
// Initialization code
}
return self;
}
- (void)prepareForReuse{
[self clearDelegatesAndObservers];
[super prepareForReuse];
}
/*
// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect
{
// Drawing code
}
*/
- (void) getImage {
self.imgObj = [ImageObject newRefrenceWithId:obj_ref];
[self.imgObj addObserver:self forKeyPath:#"data" options:(NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld) context:NULL];
TaskImageReqCache *imgReq = [[TaskImageReqCache alloc] initWithUrl:imgUrl andImageObject:self.imgObj];
[self.imagesOperationQueue addOperation:imgReq];
}
#pragma mark - KVO
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
if (object == self.imgObj) {
UIImage *img = [UIImage imageWithData:self.imgObj.data];
self.thumbnailImage.image = img;
}
}
- (void)dealloc
{
[self clearDelegatesAndObservers];
}
- (void)clearDelegatesAndObservers
{
[self.imagesOperationQueue cancelAllOperations];
self.thumbnailImage.image = nil;
[self.imgObj removeObserver:self forKeyPath:#"data"];
[self.pageListAdapter removeDelegateAtIndex:self.myIndexInCollection];
self.imgObj = nil;
}
In the first Level NSOperation this is where the exception breakpoint shows the crash happening:
- (void)didLoadDataFromFile:(NSData *)data
{
if (self.isCancelled) {
[self.opQueue cancelAllOperations];
[self completeOperation];
return;
}
if (!fileDownloadedFromWeb) {
self.observedObject.data = data; // CRASH
}
dataFromDisk = data;
fileReadDone = YES;
if (debugLog) {
NSLog(#"image loaded from local cache (%#)",self.sUrl);
}
}
Any suggestion on how to prevent this crash?
Thanks.
Edited to add:
what I am trying to achieve is: When a tableView cell is displayed a nsoperation is activated to get an image from the net. If the user scrolls quickly and the operation has not finished I need to end it and deallocate any data, and when the cell is reused start a new operation to get the appropriate image from the internet...

Answer 1

Based on comments below, we know that:
- (void)didLoadDataFromFile:(NSData *)data
is called on a different thread to dealloc, so there is a race condition. You need to access self.observedObject on the same thread as the thread it is deallocated on. I'm presuming "observedObject" is a weak reference?
dispatch_sync(dispatch_get_main_queue(), ^{
if (!fileDownloadedFromWeb) {
// Get a strong reference. This will retain observedObject - we must do this
// on the same thread as observedObject:dealloc is called, to prevent retaining
// an object during (or after) dealloc.
ObservedObject *strongRef = self.observedObject;
// This will do nothing if strongRef is nil.
strongRef.data = data;
}
});
A more structured approach would be to have the cell fetch all its images from a singleton cache (it looks as though at the moment there is no caching). The cell would obviously need to register itself as an observer for a particular URL in the cache, and the cache would notify the cell when the URL had downloaded. The cache should post that notification on the main thread.
The cache itself would manage all downloads, and there would be no background deallocation problem because it would be a singleton.
If you don't want to cache, that's fine. Use the same architecture, but call the cache an image fetcher instead. You can always add caching later if you want to.
EDIT - if your objects may be reused, rather than deallocated, as is the case for UITableViewCells, then the cell needs to be careful to ignore notifications about images that relate to a previous fetch. Either of these models will work
a) The cell retains a reference to the NSOperation until the NSOperation calls it back, or until prepareForReuse is called. Any callback from an unrecognised NSOperation must be a previous fetch (that we tried to cancel), and should be ignored. I don't really recommend this model, having the cell know about the operation AND vice versa seems silly.
b) The NSOperation sends a notification when it completes (on the main thread), and in the user info specifies the url/path that was requested. The UITableViewCell remembers what url/path it was trying to fetch, and ignores notifications that relate to other images. It unobserved that path in dealloc/prepareForReuse.

Answer 2

This was getting to long to be a comment so I'll make it an answer.
The reason why it's crashing has to do with the fact that UICollectionViewCells get recycled and deallocated. ARC is has put a [cvcell retain] in the wrong place. So, there are a few options:
One way to fix this is to just not create a NSOperation from a UICollectionViewCell.
Force the users to stay on the UICollectionViewController / UICollectionView so that it stays in memory.
Keep a property / pointer to the UICollectionViewController / UICollectionView so that it stays in memory even when the user has left it. (Make sure you retain it as strong or retain).
NOTE: All of these solutions do the same thing, force ARC to put the retain call somewhere else or to remove it entirely.

Answer 3

Cells get reused and reassigned frequently without your control so you should avoid assigning pending requests or operations to them.
Instead handle operations in your collection view data source (the view controller), and keep track of the operations not per cell but per indexPath's in a dictionary.
Even better keep this as a good experience and use something trusted and tested such as SDwebImage.