Threading issue with UICollectionview loading images from CloudKit - multithreading

Question

I'm having a threading issue loading images in a collectionview where the data is coming from cloudkit. I know this is a threading/blocking issue because before I implemented CK, I dumped some images in a folder on my desktop and read/parsed them from there and had no issue. With CK, I just created a handful of records via the dashboard and I'm successfully getting the expected records returned and use the images from those results to populate the CV cells. I store the CK query results in an array and use the size of that array to set the numberOfItemsInSection delegate.
Here's the issue...in the numberOfItemsInSection delegate method, I'm calling the model class, which executes the CK query. Since that is obviously a network call, I put that in a background thread. From logging, I can see the query execute and the results come back very quickly - within 2-3 seconds. However, the CV cells never display and I don't see the custom cell get initialized (via logging). But if I tap the camera button and take a photo, which I've implemented, I take the resulting image and add it to the array, then call reloadData on the CV and all the cells (and images) appear, including the new image just taken with the camera.
By accident, I found out a hack that somewhat works, which is calling reloadData on the CV inside the background thread of the numberOfItemsInSection delegate method. As a result, I thought I might have stumbled on to the solution by switching back to the main thread when calling reloadData, but that put it in a sort of endless loop of continuously calling the numberOfItemsInSection method and cellForItemAtIndexPath and made it to where it lagged to a point that you could barely scroll and tapping on any of the cells wouldn't do anything.
At this point, after trying many, many various things, I'm at a complete loss on how to fix this. I know this is probably a pretty easy solution as it's very common to load images asynchronously to populate a collectionview or tableview. Can someone please provide some guidance? Thanks in advance!!!
#property (nonatomic) NSInteger numberOfItemsInSection;
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
NSLog(#"***numberOfItemsInSection***");
dispatch_queue_t fetchQ = dispatch_queue_create("load image data", NULL);
dispatch_async(fetchQ, ^{
self.numberOfItemsInSection = [self.imageLoadManager.imageDataArray count];
[self.myCollectionView reloadData]; // should be done on main thread!
});
NSLog(#"numberOfItemsInSection: %ld", (long)self.numberOfItemsInSection);
return self.numberOfItemsInSection;
}
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
static NSString *CellIdentifier = #"Cell"; // string value identifier for cell reuse
ImageViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:CellIdentifier forIndexPath:indexPath];
NSLog(#"cellForItemAtIndexPath: section:%ld row:%ld", (long)indexPath.section, (long)indexPath.row);
cell.layer.borderWidth = 1.0;
cell.layer.borderColor = [UIColor grayColor].CGColor;
cell.imageView.contentMode = UIViewContentModeScaleAspectFit;
ImageData *imageData = [self.imageLoadManager imageDataForCell:indexPath.row]; // maps the model to the UI
dispatch_async(dispatch_get_main_queue(), ^{
if (imageData.imageURL.path) {
cell.imageView.image = [UIImage imageWithContentsOfFile:imageData.imageURL.path];
[cell setNeedsLayout];
} else {
// if imageURL is nil, then image is coming in from the camera as opposed to the cloud
cell.imageView.image = imageData.image;
[cell setNeedsLayout];
}
});
return cell;
}

Answer 1

before returning self.numberOfItemsInSection you should wait until the async call is finished. You can do that using semaphores. But then why are you doing this async? You are just getting the count of an array. And then you shouldn't reloadData there. Where do you start your CloudKit query? are you doing that onViewDidLoad? That is also an async operation. When that completes just doe a reloadData of your collectionView. Besides that doing this would be enough:
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
return [self.imageLoadManager.imageDataArray count];
}
If you really want to use async there, then you do have to wait for the result. You could change your code to something like:
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
NSLog(#"***numberOfItemsInSection***");
dispatch_semaphore_t sema = dispatch_semaphore_create(0);
dispatch_queue_t fetchQ = dispatch_queue_create("load image data", NULL);
dispatch_async(fetchQ, ^{
self.numberOfItemsInSection = [self.imageLoadManager.imageDataArray count];
[self.myCollectionView reloadData]; // should be done on main thread!
dispatch_semaphore_signal(sema);
});
NSLog(#"numberOfItemsInSection: %ld", (long)self.numberOfItemsInSection);
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
return self.numberOfItemsInSection;
}
And then why do you go to the main queue in cellForItemAtIndexPath? It's already executed on the main queue.