Multi-threading using SDWebImage causes NSManagedObjects to appear twice - ios

Question

I have the following problem. I'm currently building a recipe-based app and use a standard tableview to display the recipes. On the app launch, a file is downloaded from a server which includes recipe data. The data is then entered into CoreData making sure, that it's unique. At the same time, I use SDWebImage in cellForRowAtIndexPath in order to load async. After downloading the images I save the images and save their paths in an Recipe object (it's an NSManagedObject). I also use UIRefreshControl in order to refresh the tableview in case there had been changes on the server (it uses the same mechanism as on app launch).
The issue with the code below is the saveImage function. The line recipe.thumbImage = [NSString...] causes issues with CoreData in that when I refresh the app using the pull-down gesture (thus activating UIRefreshControl), the recipes appear twice in the tableView. If I delete the line, the problems go away.
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
...
NSString *imageURL = [#"http://www.example.com" stringByAppendingString:recipe.externalThumbImageURL];
[cell.thumbImageView setImageWithURL:[NSURL URLWithString:imageURL]
placeholderImage:[UIImage imageNamed:#"blankTableviewImage"]
completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType) {
if(error) NSLog(#"Error when downloading thumbImage, error: %#", error);
else {
[self saveImage:image forRecipeID:recipe.objectID];
}
}];
}
- (void)saveImage:(UIImage *)image forRecipeID:(NSManagedObjectID *)recipeID
{
if(image) {
ImageHandler *imageHandler = [[ImageHandler alloc] init];
NSError *error;
Recipe *recipe = (Recipe *)[self.recipeDatabase.managedObjectContext existingObjectWithID:recipeID error:&error];
NSString *fileName = [imageHandler getImageName:recipe.externalThumbImageURL];
NSString *localImageDirectory = [imageHandler imageDirectoryPathFromRecipe:recipe];
if (![[NSFileManager defaultManager] fileExistsAtPath:localImageDirectory]) {
[[NSFileManager defaultManager] createDirectoryAtPath:localImageDirectory withIntermediateDirectories:NO attributes:nil error:&error];
}
[imageHandler saveImage:image withFileName:fileName ofType:#"png" inDirectory:localImageDirectory];
recipe.thumbImage = [localImageDirectory stringByAppendingString:[NSString stringWithFormat:#"%#.png", fileName]];
if(error) NSLog(#"Error in saveImage – error: %#", error);
}
}
The function [cell.thumbImageView setImageWithURL:...] is part of SDWebImage and works async as far as I know. And I think that's part of the problem. And I have tried wrapping recipe.thumbImage in a [recipe.managedObjectContext performBlock:^{}] block, but it doesn't help either.
Does anyone have a hint as to what is the cause of the problem? I know that threading with CoreData is tricky and I've tried several things in order to make it work, but nothing has worked so far. Any hints are very much appreciated!

Answer 1

After some additional trial and error and reading I found out that it makes sense to create a child context when you do that kind of async updating. So in the saveImage method I now simply create a child context and wrap the recipe.thumbImage in a [context performBlock:^{}]. As I use UIManagedDocument with CoreData, you have to make sure to write the changes to the file.
Here's the code
- (void)saveImage:(UIImage *)image forRecipeID:(NSManagedObjectID *)recipeID
{
if(image) {
NSError *error;
NSManagedObjectContext *context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
context.parentContext = self.document.managedObjectContext;
Recipe *recipe = (Recipe *)[context existingObjectWithID:recipeID error:&error];
...
[context performBlock:^{
recipe.thumbImage = [localImageDirectory stringByAppendingString:[NSString stringWithFormat:#"%#.png", fileName]];
[context save:nil];
[self.document saveToURL:document.fileURL forSaveOperation:UIDocumentSaveForOverwriting completionHandler:nil];
}];
}
}
Here's also a good thread with more comments on multi-threading and core data and here an in-depth post