Wait unitil a loop with asynchronous tasks finishes executing

A completion Handler in iOS takes a thread to asynchronously get it done. So it means that we can’t know an exact point which the tasks finish. For example, if we use a for loop to execute several tasks in the background, how could we know all the task are done?

In this case you can use dispatch_group.

1. Create a dispatch group which graps all the tasks in a for loop with dispatch_group_create().
2. You point out a position which your code enters the dispatch group with dispatch_group_enter(groupname).
3. You point out a position which your code leave the dispatch group with dispatch_group_leave(groupname).
4. Lastly you can get a notification when all the tasks you assigned to a dispatch group is done with dispatch_group_notify.

If you want to see a concrete example for it, please refer this.