What is Grand Central Dispatch (GCD)?
Why Interviewers Ask This
This is a classic screening question for Swift & iOS roles. Hiring managers ask it early in interviews to gauge your baseline understanding and determine if you can communicate technical concepts clearly.
Answer
Grand Central Dispatch (GCD) is Apple's low-level C API (also available in Swift via Dispatch) for managing concurrent operations across a thread pool. It abstracts thread management — you submit work to queues, GCD decides which thread to use. Dispatch queues: Main queue: serial, runs on the main thread — all UI updates MUST happen here: DispatchQueue.main.async { self.tableView.reloadData() // UI update on main thread }. Global queues: concurrent, different quality-of-service (QoS) levels: DispatchQueue.global(qos: .userInitiated).async { let data = self.fetchData() // Background work DispatchQueue.main.async { self.updateUI(data) // Back to main } }. Custom queues: let serialQueue = DispatchQueue(label: "com.app.serial") let concurrentQueue = DispatchQueue(label: "com.app.concurrent", attributes: .concurrent). QoS levels (priority): .userInteractive (highest — animations), .userInitiated (user-triggered actions), .default, .utility (background tasks), .background (lowest — indexing, sync). sync vs async: sync blocks the calling thread until work completes; async returns immediately, work runs in background. Never call sync on the main queue from the main thread — deadlock! DispatchGroup: wait for multiple async tasks: let group = DispatchGroup() group.enter(); asyncTask1 { group.leave() } group.enter(); asyncTask2 { group.leave() } group.notify(queue: .main) { self.allDone() }. DispatchWorkItem: encapsulate work that can be cancelled. DispatchBarrier: read/write synchronization on concurrent queues.
Pro Tip
Demonstrate both theoretical understanding and practical experience. Say what it is, then give an example of how you actually used it in a Swift & iOS codebase.