NSOperation

The secret to making apps snappy is to offload as much unnecessary work to the background as possible, and in this respect, the modern Cocoa developer has two options: Grand Central Dispatch and NSOperation. This article will primarily focus on the latter, though it’s important to note that the two are quite complementary (more on that later).

→ nshipster.com/nsoperation/

NSOperation is an abstract class for a single unit of work. It manages state, priority, dependencies, and cancellation in a thread-safe way. Use NSBlockOperation or NSInvocationOperation for simple cases without subclassing.

NSOperationQueue handles execution as a priority queue, running operations FIFO but with higher-priority ones first. It limits concurrent operations via maxConcurrentOperationCount.

State flows from ready to executing to finished, using KVO. Cancel operations early to avoid waste; set cancelled and finished to true, executing to false.

Priorities: VeryLow to VeryHigh for queue order.

Quality of Service: UserInteractive, UserInitiated, Utility, Background, or Default for resource allocation.

Asynchronous operations run in background; deprecated concurrent property. Dependencies ensure order: addDependency so one finishes before another starts. Avoid cycles. completionBlock runs once when done.

Use GCD for simple async; NSOperation for structured, repeatable tasks with state. They complement each other.