Question

I’m using AFNetworking to pull images from a URL, resize, store to disk and log the path in Core Data, then load to a table view and store . When the code executes it freezes my UI. I’m not sure if it’s the download or the manipulation that’s causing my troubles.

The code I’m using is below

- (void)getPhoto:(NSInteger)type forManagedObject:(MyManagedObject*)object {

    // download the photo
    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:object.photoUrl]];
    AFImageRequestOperation *operation = [AFImageRequestOperation imageRequestOperationWithRequest:request success:^(UIImage *image) {


        // MyManagedObject has a custom setters (setPhoto:,setThumb:) that save the
        // images to disk and store the file path in the database 
        object.photo = image;
        object.thumb = [image imageByScalingAndCroppingForSize:CGSizeMake(PhotoBlockCellButtonWidth, PhotoBlockCellButtonHeight)];

        NSError *nerror;
        if (![[DataStore sharedDataStore].managedObjectContext save:&nerror]) {
            NSLog(@"Whoops, couldn't save: %@", [nerror localizedDescription]);
            return;
        }

        // notify the table view to reload the table
        [[NSNotificationCenter defaultCenter] postNotificationName:@"ReloadTableView" object:nil];

    }];
    [operation start];
}

And here is a sample code relevant to the setter from my managed object

- (NSString*)uniquePath{

    // prepare the directory string
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];

    // acquire a list of all files within the directory and loop creating a unique file name
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSArray *existingFiles = [fileManager contentsOfDirectoryAtPath:documentsDirectory error:nil];
    NSString *uniquePath;
    do {
        CFUUIDRef newUniqueId = CFUUIDCreate(kCFAllocatorDefault);
        CFStringRef newUniqueIdString = CFUUIDCreateString(kCFAllocatorDefault, newUniqueId);

        uniquePath = [[documentsDirectory stringByAppendingPathComponent:(__bridge NSString *)newUniqueIdString] stringByAppendingPathExtension:@"png"];

        CFRelease(newUniqueId);
        CFRelease(newUniqueIdString);
    } while ([existingFiles containsObject:uniquePath]);

    return uniquePath;
}

- (NSString*)saveImage:(UIImage*)image{
    NSString *path = [self uniquePath];
    NSData *data = UIImagePNGRepresentation(image);
    [data writeToFile:path atomically:YES];
    return [NSString stringWithFormat:@"file://%@",path];
}

- (void) setPhoto:(UIImage *)image {
    self.photoUrl = [self saveImage:image];
}

I would like to push this to a background thread, but I’m not sure what the implications are with AFNetworking, Core Data, and Messaging in terms of thread safety. Any thought?

Answer 1

Based on Matt’s suggestion, I improved the UI by reworking my call as follows.

- (void)getPhoto:(NSInteger)type forManagedObject:(MyManagedObject*)object {

    // download the photo
    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:object.photoUrl]];
    AFImageRequestOperation *operation = [AFImageRequestOperation 
        imageRequestOperationWithRequest:request 
        imageProcessingBlock:^UIImage *(UIImage *image) {
            return [image imageByScalingAndCroppingForSize:CGSizeMake(PhotoBlockCellButtonWidth, PhotoBlockCellButtonHeight)];
        } 
        cacheName:nil 
        success:^(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image) {

            // MyManagedObject has a custom setters (setPhoto:,setThumb:) that save the
            // images to disk and store the file path in the database 
            object.photo = image;
            object.thumb = image;

            NSError *nerror;
            if (![[DataStore sharedDataStore].managedObjectContext save:&nerror]) {
                NSLog(@"Whoops, couldn't save: %@", [nerror localizedDescription]);
                return;
            }

            // notify the table view to reload the table
            [[NSNotificationCenter defaultCenter] postNotificationName:@"ReloadTableView" object:nil];                                                                 
        } 
        failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error) {
            NSLog(@"Error getting photo");
        }];
    [operation start];
}