enter()
Explicitly indicates that a block has entered the group.
Declaration
func enter()Discussion
Calling this function in Objective-C increments the current count of outstanding tasks in the group. Using this function (with leave()) allows your application to properly manage the task reference count if it explicitly adds and removes tasks from the group by a means other than using the dispatch_group_async function. A call to this function must be balanced with a call to leave(). You can use this function to associate a block with more than one group at the same time.