GCD (Grand Central Dispatch) is a low-level API for controlling several operations at the same time. This notion is employed to aid in the enhancement of application performance. This procedure is used to handle numerous jobs at once. The most relevant API for multitasking with Async and Sync programming in iOS is Grand Central Dispatch (GCD).
Dispatch Queue : The task is managed in FIFO (First In First Out) order by the Dispatch Queue. Dispatch queues are thread-safe because they can be accessed by several threads at the same time.
Concurrent : This process has started numerous tasks at the same time but does not know when they will finish. It can be completed in whatever sequence you want. They perform one or more things concurrently at the same time. The work is finished in the order in which it was queued, not in the order in which it was completed.
Serial : A single task will be executed at a time. It is possible to use it to synchronize access to a certain resource.
Sync : After the work is completed, a synchronous function returns control to the caller.
Asynchronous : An asynchronous function returns instantly after ordering a job to begin but does not wait for it to finish.