Grand Central Dispatch (GCD) is Apple's implementation of multithreading on iOS and macOS. It uses DispatchQueues to schedule work on threads and provides classes like DispatchWorkItem and DispatchGroup to help manage asynchronous tasks. GCD also defines quality of service levels like userInteractive, userInitiated, and background to indicate the relative priority of tasks.

1. GCD
Grand Central Dispatch
Johnson

2. Queue
參參考資料
https://www.raywenderlich.com/79149/grand-central-dispatch-tutorial-swift-part-1

3. • DispatchQueue
• DispatchTime
• DIspatchQos
• DispatchGroup
• DispatchWorkItem

4. Dispatch Qos
• userInteractive
• userInitiated
• default
• utility
• background
• unspecified