Assign objects to variables outside asynchronous blocks - ios

Question

I am curious if it is possible to easily assign and retain __block objects inside asynchronous blocks in MRC and ARC? I obtained the following code while rewriting a piece of code with minimal changes. I got stuck when trying to return the image from an asynchronous block. A concern is that the code would one day be converted to ARC. I do not want to have hidden memory crash after the conversion. My options were
Using a GCD object holder if such a thing exists
Using a homebrew object holder, an array or others
Directly adding the image to the array (which I am using)
Rewriting the code to another structure
Basically the code load multiple images in a background thread. Searching // image deallocated :( will give you the location.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
ALAssetsLibrary *assetsLibrary = nil;
for (int i = 0; i < sources.count; ++i) {
__block UIImage *image = nil;
id source = sources[i];
if ([source isKindOfClass:[NSString class]]) {
image = something
} else if ([source isKindOfClass:[NSURL class]]) {
NSURL *url = source;
if ([url.scheme isEqualToString:#"assets-library"]) {
dispatch_group_t group = dispatch_group_create();
if (!assetsLibrary)
assetsLibrary = [[[ALAssetsLibrary alloc] init] autorelease];
dispatch_group_enter(group);
[assetsLibrary assetForURL:url resultBlock:^(ALAsset *asset) {
image = something
dispatch_group_leave(group);
} failureBlock:^(NSError *error) {
dispatch_group_leave(group);
}];
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
// image deallocated :(
} else if (url.isFileURL) {
image = something
}
}
// add image to an array
}
dispatch_async(dispatch_get_main_queue(), ^{
// notify
});
});

Answer 1

I think moving the image variable declaration would be the first right step to think about this. You can't expect to have one image variable for multiple concurrent operations.
so to get started, let's do this-
ALAssetsLibrary *assetsLibrary = nil;
for (int i = 0; i < sources.count; ++i) {
__block UIImage *image = nil;
After that, you are currently keeping UIImage instances in an array? This is not recommended. Please find out a way to write these images to documents directory and later read from there when needed. Keeping UIImage instances in memory will lead to severe memory overuse issues.
You need to come up with a naming convention to write your images into documents directory. Whenever an image load completes, just save it to your local folder and later read from there.
You could also call the notify completion part as soon as one image is loaded and keep notifying about others as you get them. Not sure how your design is working.
Also, I noticed you are creating multiple groups. One new group with each iteration inside for loop here-
dispatch_group_t group = dispatch_group_create();
Moving it outside for loop would be a good call.
Hope this helps.