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    <p>Dealing with asynchronous methods is a pain ;)</p> <p>In your case its guaranteed that the completion block will execute on the specified queue. However, you need to ensure that the queue has a max concurrent operations count of 1, otherwise concurrent access to shared resources is not safe. That's a classic race <a href="http://en.wikipedia.org/wiki/Race_condition" rel="nofollow">http://en.wikipedia.org/wiki/Race_condition</a>. The max concurrent operations of a NSOperationQueue can be set with a property.</p> <p>In general, completion handlers may execute on any thread, unless otherwise specified.</p> <p>Dealing with asynchronous methods gets a lot easier when using a concept called "Promises" <a href="http://en.wikipedia.org/wiki/Promise_(programming)" rel="nofollow">http://en.wikipedia.org/wiki/Promise_(programming)</a>. Basically, "Promises" represent a result that will be evaluated in the future - nonetheless the promise itself is immediately available. Similar concepts are named "futures" or "deferred".</p> <p>There is an implementation of a promise in Objective-C on GitHub: RXPromise. When using it you also get safe access from within the handler blocks to shared resources. An implementation would look as follows:</p> <pre><code>-(RXPromise*) fetchImageFromURL:(NSString*)urlString queue:(NSOperationQueue*) queue { @autoreleasepool { RXPromise* promise = [[RXPromise alloc] init]; NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:urlString]]; [NSURLConnection sendAsynchronousRequest:request queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) { if (data != nil) { [promise fulfillWithValue:data]; } else { // There was an error [promise rejectWithReason:error]; }; }]; return promise; } } </code></pre> <p>Then call it:</p> <pre><code>- (void) fetchImages { ... for (NSUInteger index = 0; index &lt; N; ++index) { NSString* urlString = ... [self fetchImageFromURL:urlString, self.queue] .then(^id(id data){ [self.allImages replaceObjectAtIndex:index withObject:[UIImage imageWithData:data]]; return @"OK"; }, ^id(NSError* error) { [self.allImages replaceObjectAtIndex:index withObject:[UIImage imageNamed:@"error.png"]]; return error; }); } } </code></pre>
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    1. POTracking Asynchronous Connections with Blocks
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    1. COMany thanks, I will give it a try today...
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    2. COIt does not recognise the type RXPromise, do I need to import something?
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    3. COIt's a single class (header and implementation file) that must be incorporated into your project. It requires ARC enabled. You can "download" the project via "git clone <git url>" or simple clicking the "ZIP" button.
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