I'm trying to synchronize the following code in iOS5:
- an object has a method which makes an HTTP request from which it
gets some data, including an URL to an image - once the data arrives, the textual data is used to populate a
CoreData model - at the same time, a second thread is dispatched async to download
the image; this thread will signal via KVO to a viewController when
the image is already cached and available in the CoreData model. - since the image download will take a while, we immediately return
the CoreData object which has all attributes but for the image to
the caller. - Also, when the second thread is done downloading, the CoreData model
can be saved.
This is the (simplified) code:
- (void)insideSomeMethod
{
[SomeHTTPRequest withCompletionHandler:
^(id retrievedData)
{
if(!retrievedData)
{
handler(nil);
}
// Populate CoreData model with retrieved Data...
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSURL* userImageURL = [NSURL URLWithString:[retrievedData valueForKey:@"imageURL"]];
aCoreDataNSManagedObject.profileImage = [NSData dataWithContentsOfURL:userImageURL];
});
handler(aCoreDataNSManagedObject);
[self shouldCommitChangesToModel];
}];
}
- (void)shouldCommitChangesToModel
{
dispatch_barrier_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSError *error = nil;
if(![managedObjectContext save:&error])
{
// Handle error
}
});
}
But what's going on is that the barrier-based save-block is always executed before the the image-loading block. That is,
dispatch_barrier_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSError *error = nil;
if(![managedObjectContext save:&error])
{
// Handle error
}
});
Executes before:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSURL* userImageURL = [NSURL URLWithString:[retrievedData valueForKey:@"imageURL"]];
aCoreDataNSManagedObject.profileImage = [NSData dataWithContentsOfURL:userImageURL];
});
So obviously I'm not really dispatching the image-loading block before the barrier, or the barrier would wait until the image-loading block is done before executing (which was my intention).
What am I doing wrong? how do I make sure the image-loading block is enqueued before the barrier block?
Best Answer
At first glance the issue may be that you are dispatching the barrier block on a global concurrent queue. You can only use barrier blocks on your own custom concurrent queue. Per the GCD docs on dispatch_barrier_async, if you dispatch a block to a global queue, it will behave like a normal dispatch_async call.
Mike Ash has a good blog post on GCD barrier blocks: http://www.mikeash.com/pyblog/friday-qa-2011-10-14-whats-new-in-gcd.html
Good luck
T