Threads are killing my app - objective-c

Question

Looking for some advice on stablizing my app. First some requirements - files with PII (personally identifying information) must be encrypted when on disk. Tumbnails and logos are in the custom TableViewCells (if available) and must be decrypted before display.
There are several layers of threading going on. There is a central function getFileData that checks to see if files are on the device or if the files need to be obtained from the network. I desire to keep the UI responsive and (I think) therein lies my problem.
Here is some code:
This is the workhorse method for processing files in my application. It decides where the file is decrypts it and hands it back to a callback:
-(void)fetchFileData:(UserSession *) session
onComplete: (void(^)(NSData* data)) onComplete
{
NSURL *url = [File urlForMail:self.fileId andSession:session];
//NSLog(#"File id: %#", self.fileId);
NSString *encryptionKey = session.encryptionKey;
dispatch_queue_t cryptoQ = dispatch_queue_create(FILE_CRYPTOGRAPHY_QUEUE, NULL);
dispatch_async(cryptoQ, ^(void){
// Get the file and d/encrypt it
NSError *error = nil;
if ([File fileExistsAtUrl:url] == YES) {
NSLog(#"file is on disk.");
NSData *localEncryptedFile = [File getDataForFile:url];
NSData *decryptedFile = [RNDecryptor decryptData:localEncryptedFile
withPassword:encryptionKey
error:&error];
onComplete(decryptedFile);
dispatch_release(cryptoQ);
} else {
//NSLog(#"File is not on disk");
NSDictionary *remoteFile = [session.apiFetcher getFileContent:self.fileId
andToken:session.token];
if (remoteFile && [[remoteFile objectForKey:#"success"] isEqualToString:#"true"]) {
NSData *remoteFileData = [remoteFile objectForKey:#"data"];
NSString *mimeType = [remoteFile objectForKey:#"mimeType"];
self.mimeType = mimeType;
NSData *encryptedData = [RNEncryptor encryptData:remoteFileData
withSettings:kRNCryptorAES256Settings
password:encryptionKey
error:&error];
[encryptedData writeToURL:url atomically:YES];
onComplete(remoteFileData);
dispatch_release(cryptoQ);
}
}
});
Here is an example of a getFileData caller:
+(void)loadThumbnailForMail: (NSNumber*)thumbnailId
session: (UserSession*)session
callback: (void(^)(NSData* data))callback
{
File *file = [File findFile:thumbnailId inContext:session.mailDatabase.managedObjectContext];
dispatch_queue_t fetchQ = dispatch_queue_create(FILE_FETCHER_QUEUE_LABEL, NULL);
dispatch_async(fetchQ, ^(void) {
if (file) {
[file fetchFileData:session onComplete:^(NSData *data) {
if (data && file.mimeType) {
callback(data);
}
}];
}
});
dispatch_release(fetchQ);
}
Here is an example of the TableViewCell that is calling loadThumbnailForMail:
-(void)loadAndShowThumbnailImage:(Mail*) mail
{
UIImage *placeHolder = [UIImage imageNamed:#"thumbnail_placeholder.png"];
[self.thumbnailImageForMail setImage:placeHolder];
dispatch_queue_t loaderQ = dispatch_queue_create(THUMBNAIL_FETCHER, NULL);
dispatch_async(loaderQ, ^ {
[File loadThumbnailForMail: mail.thumbnailId
session: [UserSession instance]
callback: ^(NSData *data) {
dispatch_async(dispatch_get_main_queue(), ^{
UIImage *thumbnailImage = [UIImage imageWithData:data];
[self.thumbnailImageForMail setImage:thumbnailImage];
});
}];
});
dispatch_release(loaderQ);
}
I think that my issue here is the callback in my loadThumbnailImage. If the user scrolls fast enough I suspect that there could be two threads trying to access the same TableViewCell
(MyCell *cell = (MyCell*)[tableView dequeueReusableCellWithIdentifier:CellTableIdentifier];)
It doesn't always happen right away but eventually, after some scrolling the tableView list of cells the app crashes with this: * Terminating app due to uncaught exception 'NSGenericException', reason: '* Collection <__NSCFSet: 0xde6a650> was mutated while being enumerated.'
I need to have the decrypted images in the cells and the first solution (above) does this for me when the images are available but causes the app to crash. I am wondering if some sort of in memory cache would help improve this if I put images in memory there when they were decrypted and checked that cache in loadAndShowThumbnailImage before I kick off all the threads to get and decrypt them.
Thoughts? I have been banging on this for a week now trying different things and would appreciate some fresh perspective.
Thanks.

Answer 1

Based on Justins link and subsequent research I ended up going in this direction: In a UITableView, best method to cancel GCD operations for cells that have gone off screen?