iOS - Best practice to get async properties of collection in Swift - ios

Question

While working with PHAsset in Swift, I am facing a common problem which must have a good design/solution. For example, I have a collection of PHAsset, lets say assetCollection. Now I want to get total size of the assetCollection which is the sum of all assets in it.
I know there is a asynchronous API to get individual asset size https://stackoverflow.com/a/26551990/1084174 (in objective-c),
[[PHImageManager defaultManager] requestImageDataForAsset:asset options:nil resultHandler:^(NSData *imageData, NSString *dataUTI, UIImageOrientation orientation, NSDictionary *info) {
float imageSize = imageData.length;
//convert to Megabytes
imageSize = imageSize/(1024*1024);
NSLog(#"%f",imageSize);
}];
But when its a collection how do I design the solution?
What's in my mind is, I can run async call inside a loop for each asset in assetCollection summing sizes in total variable until when I get the last result (may be using global variable). total will be the final collection size. But I think there must be some better design/solution to such common problem.
It would be appreciated if anyone suggest.

Answer 1

After spending weeks i have come to a better solution. I am sharing my solution but still there might be some better design. Please share if you get any better.
Solution
I create a wrapper collection, let's say sizedAssetCollection with a property called size. I also implement add() and remove() methods. When I add or remove an item through my add/remove method i calculate update the size on the fly. Like,
void add (PHAsset *asset) {
[[PHImageManager defaultManager] requestImageDataForAsset:asset options:nil resultHandler:^(NSData *imageData, NSString *dataUTI, UIImageOrientation orientation, NSDictionary *info) {
float imageSize = imageData.length;
imageSize = imageSize/(1024*1024);
size += imageSize;
}];
}
void remove(PHAsset *asset) {
[[PHImageManager defaultManager] requestImageDataForAsset:asset options:nil resultHandler:^(NSData *imageData, NSString *dataUTI, UIImageOrientation orientation, NSDictionary *info) {
float imageSize = imageData.length;
imageSize = imageSize/(1024*1024);
size -= imageSize;
}];
}
Finally I get the expected result in size variable.