iOS Q & A


What is Grand Central Dispatch?

Grand Central Dispatch (GCD) is a low-level API provided by Apple for managing concurrent and asynchronous execution in macOS, iOS, watchOS, and tvOS apps. GCD simplifies the implementation of multithreading by providing a set of APIs for scheduling tasks, managing execution queues, and synchronizing access to shared resources. Key concepts of GCD include:


  • Dispatch Queues: Dispatch queues are lightweight data structures used to execute tasks concurrently or serially. GCD provides two types of queues: serial queues, which execute tasks in a FIFO (first-in, first-out) order on a single thread, and concurrent queues, which execute tasks concurrently on multiple threads.
  • Global Dispatch Queues: GCD provides a set of global dispatch queues with different quality-of-service (QoS) levels, such as user-initiated, default, utility, and background. These global queues are managed by the system and can be used to perform tasks with different priorities and resource requirements.
  • Dispatch Work Items: Work items are blocks of code or functions that can be scheduled for execution on dispatch queues. GCD provides APIs for creating and dispatching work items asynchronously or synchronously, allowing developers to perform tasks concurrently or serially as needed.
  • Dispatch Groups: Dispatch groups are used to synchronize the execution of multiple tasks and track when they complete. Tasks added to a dispatch group can wait synchronously or asynchronously for all tasks to complete before continuing execution.
  • Dispatch Semaphores: Dispatch semaphores are synchronization primitives used to control access to shared resources or coordinate the execution of critical sections of code. Semaphores can be used to implement thread-safe access to data structures or limit the maximum number of concurrent tasks.


Grand Central Dispatch simplifies the implementation of concurrency and parallelism in iOS apps by providing a high-performance and efficient API for managing tasks and executing code concurrently.

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