Fix that completion block and set image block are called asynchronously for UIView+WebCache

[dreampiggy]

Add a more common macro to do current queue check

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This merge request fixes / reffers to the following issues: #2092

Pull Request Description

Our use relay on the behavior that completion and set-image block should happend during one run loop without add it to the end of main queue. This means we should use dispatch_main_async_safe to dispatch queue when current queue is main queue.

This behavior is changed during #2047. At that time I didn't realized that completion and set-image block should immediately call when current queue is main queue. So we then do code refactor about this logic.

dispatch_queue_t targetQueue = shouldUseGlobalQueue ? dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0) : dispatch_get_main_queue();

dispatch_async(targetQueue, ^{});

In fact, we should do like this:

if (shouldUseGlobalQueue) {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        [sself sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock];
        dispatch_main_async_safe(callCompletedBlockClojure);
    });
} else {
  // If current queue is already main queue, this will become all immediately block call instead of adding block to queue
    dispatch_main_async_safe(^{
        [sself sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock];
        dispatch_main_async_safe(callCompletedBlockClojure);
    });
}

This can work, but not looks well. I think we can create a more common usage macro, like this, to solve this kind of use case:

#define dispatch_queue_async_safe(queue, block)\
    if (strcmp(dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL), dispatch_queue_get_label(queue)) == 0) {\
        block();\
    } else {\
        dispatch_async(queue, block);\
    }

#define dispatch_main_async_safe(block) dispatch_queue_async_safe(dispatch_get_main_queue(), block)

Then we just need to change it to this:

dispatch_queue_async_safe(targetQueue, ^{});

I don't think this is a break API change. But if someone think this change should not happened on 4.2.2 patch release. I can change back to that first one in description to avoid modify any header file.

Any idea ?

[codecov-io]

Codecov Report

Merging #2093 into master will decrease coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2093      +/-   ##
==========================================
- Coverage    78.3%   78.22%   -0.09%     
==========================================
  Files          35       35              
  Lines        3383     3384       +1     
==========================================
- Hits         2649     2647       -2     
- Misses        734      737       +3

Impacted Files	Coverage Δ
SDWebImage/UIView+WebCache.m	59.37% <100%> (ø)	⬆️
SDWebImage/SDWebImageCompat.h	100% <100%> (ø)	⬆️
SDWebImage/SDWebImageWebPCoder.m	84.84% <0%> (-1.14%)	⬇️

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[bpoplauschi]

Looks good @dreampiggy. I will merge it.