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This question already has an answer here:

In my app, I am trying to be able to download two objects, one 'article' and one image, from my server. The 'article'-object (translated from sql-query to a subsubclass of NSObject) has a pointer to where to download the image, which obviously takes longer to download than the article-query. When I 'download' the articles, they end up in an NSArray. When they are complete, then I am iterating through the array to start asynchronous downloads of each image, kind of like this:

for(Article *a in articles)
{
    NSString *url = a.imageURL;
    //Start async download of this image
}

I'm using Parse Framework to do this, so the block would probably look weird to people who haven't used it, so I'll just explain instead: In the block (when the async download is finished), I am assigning the returned NSData (the image) to another array, an NSMutableArray named articleImages, and I want the index of the image in this array to correspond to the index of the article in the other; the same chronology in both arrays.

For now, I'm doing this: [articleImages insertObject:data atIndex:i];(where i comes from the for-loop, not really using foreach as above) because the block is still inside the same for-loop. This is working for now, but I've only tested with three articles so far. If I'm not completely wrong here, this is a poor and flawed way of doing this, as for the scenarios that image2 is downloaded faster than image1, even though it was started a tiny bit later. In those cases, I imagine I would stumble upon an exception saying I can't insert objects at index:2 in an empty array.

I was thinking about filling an array with dummy data, with the size [articles count], and later replace the correct-index (i) dummy data with the downloaded images as they come in in random order. But I feel dirty by even thinking of this. There has to be a proper way to do this? I'm not completely sure what to search for either..

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marked as duplicate by Josh Caswell, Simon Goldeen, zhangyangyu, Brian Nickel♦, Cole Johnson Jul 30 '13 at 23:52

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
1  
Why don't you just add a property image of type UIImage to your Article class and then set this property to the downloaded image once the download is finished? –  s1m0n Jul 30 '13 at 18:57
    
@s1m0n Because the articles are editable, and later uploadable. With the current framework, it would mean that when I save and upload the object, it would include the NSData, and use double the space. I could 'disable' the image for upload in a workaround, I'm sure, but I wanted to do it this way. And I've always wondered about this too.. –  Sti Jul 30 '13 at 19:03
    
Fixed-sized array with empty values ? Why not use good old plain c arrays ? –  Nicolas Jul 30 '13 at 19:03

2 Answers 2

up vote 3 down vote accepted

Another idea: Make articleImages a NSMutableDictionary and use the image index (or some unique article id) as key:

articleImages[@(index)] = imageData;
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up vote 0 down vote

You need to put some dumy data first then replace it. I think there is no other way to do this.

I had a similar scenario before, where I did something like:

- (void)initiateArray:(int)count
{
   _myArray = [[NSMutableArray aloc] init];
   for (int loop = 0; loop < count; loop++)
   {
      [_myArray addObject:[NSNull null]];
   }
}

Later I replaced object using:

[_myArray replaceObjectAtIndex:index withObject:newObject];
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