objective c - for loop to fetch image from iCloud sequentially - objective-c

Question

on IOS, I need to get metadata for a selected set of images. But since the images are backed up to iCloud, sometimes it may immediately return (cached) and sometimes it may take a second or two.
The for loop runs through quickly, I am able to wait for all of the images to be processed before I move forward. But they still are being fetched in parallel. How do I make the for loop run sequentially by waiting for the block to finish before moving on to next image.
// Step 4: Fetch Details like Metadata for this batch
-(void) getDetailsForThisBatchOfNewAssets:(NSMutableArray*) mArrBatchOfNewAssets
withCompletionHandler:(blockReturnsMArrAndMArr) blockReturns{
NSLog(#"%s with arraySize of %lu",__PRETTY_FUNCTION__, (unsigned long)[mArrBatchOfNewAssets count] );
long assetCount = [mArrBatchOfNewAssets count];
NSMutableArray *mArrNewAssetsAndDetails = [[NSMutableArray alloc] init];
NSMutableArray *mArrNewAssetFailed = [[NSMutableArray alloc] init];
if(assetCount == 0){
NSLog(#" Looks like there are no NEW media files on the device.");
return;
}
else
NSLog(#"found %ld assets in all that need to be backed up", assetCount);
dispatch_group_t groupForLoopGetDetails = dispatch_group_create();
for(long i = 0 ; i < assetCount; i++){
PHAsset *currentAsset = [[mArrBatchOfNewAssets objectAtIndex:i] objectForKey:#"asset"];
NSString *mediaIdentifier = [[[currentAsset localIdentifier] componentsSeparatedByString:#"/"] firstObject];
[mArrIdentifiersInThisBatch addObject:mediaIdentifier];
dispatch_group_enter(groupForLoopGetDetails);
[mediaManager getDetailedRecordForAsset:currentAsset
withCompletionHandler:^(NSMutableDictionary *mDicDetailedRecord, NSMutableDictionary *mDicRecordForError)
{
if(mDicRecordForError[#"error"]){
[mArrNewAssetFailed addObject:mDicRecordForError];
NSLog(#"Position %ld - Failed to fetch Asset with LocalIdentifier: %#, adding it to Failed Table. Record: %#",i,[currentAsset localIdentifier], mDicRecordForError);
} else {
[mArrNewAssetsAndDetails addObject:mDicDetailedRecord ];
NSLog(#"Position %ld - added asset with LocalIdentifier to mArrNewAssetsAndDetails %#",i,[currentAsset localIdentifier]);
}
dispatch_group_leave(groupForLoopGetDetails);
}];
} // end of for loop that iterates through each asset.
dispatch_group_notify(groupForLoopGetDetails, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSLog(#"Completed gathering details for this batch of assets for backup. Count : %lu and failed Media count: %lu",(unsigned long)[mArrNewAssetsAndDetails count], (unsigned long)[mArrNewAssetFailed count]);
blockReturns(mArrNewAssetsAndDetails,mArrNewAssetFailed);
});
}
I have looked through several questions on SO on this topic but still have not figured out how to make this run sequentially.
I don't want to do a "self call" for this method, because I'm already doing "self call" at another place before I reach this method and my code is now growing into too many notifications and catches because of that.

Answer 1

Assuming the completion handler of getDetailedRecordForAsset is called on a different thread, you can use a semaphore to block execution (Note: DO NOT DO this on the main thread) inside the loop while waiting for the completion handler.
Remove the dispatch group stuff, then, inside the loop:
create a semaphore right before calling getDetailedRecordForAsset like so: dispatch_semaphore_t semaphore = dispatch_semaphore_create( 0);
as the last statement of the completion handler call dispatch_semaphore_signal( semaphore);
immediately after calling getDetailedRecordForAsset, wait for the end of the completion handler with dispatch_semaphore_wait( semaphore, DISPATCH_TIME_FOREVER);
So the structure of the loop will look like:
for (assets)
{
... // get current asset, media identifier as above
dispatch_semaphore_t semaphore = dispatch_semaphore_create( 0);
[mediaManager getDetailedRecordForAsset:currentAsset
withCompletionHandler:^(NSMutableDictionary *mDicDetailedRecord, NSMutableDictionary *mDicRecordForError)
{
... // handle error or add asset details as above
dispatch_semaphore_signal( semaphore);
}
dispatch_semaphore_wait( semaphore, DISPATCH_TIME_FOREVER);
}