How to let code in block execute first

Question

-(UIImage *)getImageFromURL:(NSURL *)imageURL{ __block UIImage *image = [UIImage new]; PHFetchResult *result = [PHAsset fetchAssetsWithALAssetURLs:@[imageURL] options:nil]; PHAsset *asset = result.firstObject; PHImageManager *manager = [PHImageManager defaultManager]; [manager requestImageForAsset:asset targetSize:CGSizeMake(100, 100) contentMode:PHImageContentModeAspectFit options:nil resultHandler:^(UIImage * _Nullable result, NSDictionary * _Nullable info) { image = result; }]; return image; }

the code in block "image = result" execute after code out of block "return image" , so it always return nil. How can i make the code in block execute first. Use thread or NSOperation or GCD?

Use completion block. You just can't synchronously return the result from asynchronous call (without blocking UI and without time travel). — Sulthan
– Sulthan, Commented May 5, 2016 at 10:39

Bhadresh Mulsaniya · Accepted Answer · 2016-05-05 09:11:13Z

-(UIImage *)getImageFromURL:(NSURL *)imageURL{ __block UIImage *image = [UIImage new]; dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ PHFetchResult *result = [PHAsset fetchAssetsWithALAssetURLs:@[imageURL] options:nil]; PHAsset *asset = result.firstObject; PHImageManager *manager = [PHImageManager defaultManager]; [manager requestImageForAsset:asset targetSize:CGSizeMake(100, 100) contentMode:PHImageContentModeAspectFit options:nil resultHandler:^(UIImage * _Nullable result, NSDictionary * _Nullable info) { image = result; }]; dispatch_async(dispatch_get_main_queue(), ^{ return image; }); }); }

you cannot 'return image' in a block when this block has no return . Like function , block can also return value , but block 'dispatch_async' has no return

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