SKTaskManager

Now its easy to manage asynchronous task whether its is sequential or parallel. Create SKTask as many as your async task you have (i.e. if you want to download 5 images asynchronously, then create 5 SKTasks)

SKTask *aTask=[SKTask taskWithBlock:^(id result, BlockTaskCompletion completion) {
    // your async task goes here i.e. image downloading in background
    // once task is completed call this block

    //completion(nil); //this is important otherwise next task will not be exexute
}];

Now add all SKTasks to array and call this method to perform task parallel. (i.e. All 5 images should start downloading at same time)

[SKTaskManager parallerOperations:arrTask completion:^{
    NSLog(@"all task completed");
}];

Call this method to perform task sequentially. (i.e. only one image should be downloaded at a time. once its finish then next should start)

[SKTaskManager sequenceOperations:arrTask completion:^{
    NSLog(@"all task completed");
}];