Grand Central Dispatch (GCD) provides a simple and robust mechanism for concurrent and asynchronous operations, primarily in Apple operating systems (e.g., iOS, macOS, watchOS, and tvOS), but also FreeBSD and MidnightBSD.
Grand Central Dispatch (GCD) provides a simple and robust mechanism for concurrent operations in Apple platforms including iOS, macOS, watchOS, and tvOS. This is described in Apple's Introducing Blocks and Grand Central Dispatch:
GCD combines an easy-to-use programming model with highly-efficient system services to radically simplify the code needed to make best use of multiple processors. ...
The central insight of GCD is shifting the responsibility for managing threads and their execution from applications to the operating system. As a result, programmers can write less code to deal with concurrent operations in their applications, and the system can perform more efficiently on single-processor machines, large multiprocessor servers, and everything in between. Without a pervasive approach such as GCD, even the best-written application cannot deliver the best possible performance, because it doesn’t have full insight into everything else happening in the system.
GCD is implemented as a set of extensions to the C language as well as a new API and runtime engine. While initially inspired by the challenge of multicore computing, these actually solve a more general problem: how to efficiently schedule multiple independent chunks of work.
The technology is developed by Apple, and the supporting runtime library is released as open-source.
Alternatives
There are a few native alternatives:
Operation queues: Operation queues are built upon GCD technology, but offer more elegant cancelation, priorities, dependencies, and the ability to wrap asynchronous tasks in operations.
Combine: The Combine framework provides a declarative Swift API for processing values over time. These values can represent many kinds of asynchronous events. Combine declares publishers to expose values that can change over time, and subscribers to receive those values from the publishers.
async-await: Swift concurrency eliminates the completion handler pattern and offers a simpler pattern for representing asynchronous patterns, error handling, etc.
