ios - How to determine when an asynchronous operation (loop) has completely ended?

Question

Requirement: After obtaining the album information, get the first picture and assign a value to self.editImageView for display.
Problem: But now I want to getImageForCollectionViewafter it is completely completed. ##self.editImageView assignment, then the question is, how can I judge that the getImageForCollectionView function has been completed?

 - (void)getImageForCollectionView{
    _library = [[ALAssetsLibrary alloc] init];
    self.photos = [NSMutableDictionary dictionary];
    [_library enumerateGroupsWithTypes:ALAssetsGroupAll usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
        if (group) {
            NSMutableArray *array = [NSMutableArray array];
            [group enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) {
                if (result) {
                    [array addObject:result];
                }
            }];
            [self.photos setValue:array forKey:[group valueForProperty:@"ALAssetsGroupPropertyName"]];
        }
    } failureBlock:^(NSError *error) {
       
    }];
}

Answer 1

Place time-consuming operations in non-main threads, and those that require UI updates in the main thread.

__weak typeof(self) weakSelf = self;
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // not in main block task
    [weakSelf getImageForCollectionView];
    dispatch_async(dispatch_get_main_queue(), ^{
        // main block. change ui
        NSLog(@"%@", weakSelf.photos);
    });
});

EDIT:

- (void)getImageForCollectionView:(void(^)(void))callback {
    _library = [[ALAssetsLibrary alloc] init];
    self.photos = [NSMutableDictionary dictionary];
    
    dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        [_library enumerateGroupsWithTypes:ALAssetsGroupAll usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
            if (group) {
                NSMutableArray *array = [NSMutableArray array];
                [group enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) {
                    if (result) {
                        [array addObject:result];
                    }
                }];
                [self.photos setValue:array forKey:[group valueForProperty:@"ALAssetsGroupPropertyName"]];
            }
        } failureBlock:^(NSError *error) {
            
        }];
        dispatch_async(dispatch_get_main_queue(), ^{
            callback();
        });
    });
}

[self getImageForCollectionView:^{
    // ...
    NSLog(@"%@", self.photos);
}];