No cancelation for disk query operation about sdwebimage HOT 5 CLOSED

Great catch! However, cancellation alone will not solve this issue. cancel is called on a different thread from the operation itself, and according to the docs:

-(void)cancel
If the operation has already finished executing, this method has no effect.

Issuing cancel would race with the completion of the operation, so the race condition would merely be less frequent.

The proper solution to this is to include a URL check to make sure the UIImageView is still trying to show the same URL as originally requested when the operation was queued.

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Hi,

In my opinion the best solution would be a combination of these two approaches, but you should definitely start with the one you've suggested.

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That could be tricky. Actually I've already implemented the URL checking solution like @adamjernst 's suggestion, which you can have a look at following commit.

jamztang@a5f9bb9

However, that's breaks multiple fetching of images using a single delegate object. (For example using SDWebImagePrefetcher or delegating the SDWebImageManagerDelegate to your viewController, the images arrives in unpredictable sequences, and the following logic isn't able to know whether that delegate is interested on the last response only or all responses, and discarded all responses not equal to the expected URL.

 // The disk query returned, but sometimes it was an older request for this delegate which is supposed to be cancelled. 
 // We compare the URL and the key and see if it was the actual request.
 if ( ! [[[urls objectAtIndex:idx] absoluteString] isEqualToString:key]) {
     return;
 }

So the main objective is to let SDWebImageManager to know whether the delegate only interested on the last response or all responses, we've to come up a solution for this use case.

Using NSNotificationCenter instead of the delegation pattern could probably solve this problem (which requires a huge refactoring), but I am sure @rs originally has reason to use delegation at the first point and needed to be take consider of, and I am thinking if there's any other ways to do.

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@mystcolor your solution is very similar to my original code (which never made it into trunk), but what's missing is that you need to also check that the URL is equal for that delegate index before removing the delegate from the array. You're removing all instances of the delegate when any one of them returns, but you should only remove the one that has a corresponding URL equal to the just-completed URL. This is a bit confusing, but see my code here:

adamjernst@04f96e7

That should do it. Let me know if I'm missing something (which I very well might be). Thanks!

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Thanks a lot Adam.

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