Grand Central Dispatch - Features

Features

The dispatch framework declares several data types and functions to create and manipulate them:

• Dispatch Queues are objects that maintain a queue of tasks, either anonymous code blocks or functions, and execute these tasks in their turn. The library automatically creates several queues with different priority levels that execute several tasks concurrently, selecting the optimal number of tasks to run based on the operating environment. A client to the library may also create any number of serial queues, which execute tasks in the order they are submitted, one at a time. Because a serial queue can only run one task at a time, each task submitted to the queue is critical with regard to the other tasks on the queue, and thus a serial queue can be used instead of a lock on a contended resource.
• Dispatch Sources are objects that allow the client to register blocks or functions to execute asynchronously upon system events, such as a socket or file descriptor being ready for reading or writing, or a POSIX signal.
• Dispatch Groups are objects that allow several tasks to be grouped for later joining. Tasks can be added to a queue as a member of a group, and then the client can use the group object to wait until all of the tasks in that group have completed.
• Dispatch Semaphores are objects that allow a client to permit only a certain number of tasks to execute concurrently.