What is `OperationQueue` and how is it different from GCD?

Answer: OperationQueue is a higher-level abstraction over GCD. It adds features like dependencies between operations, cancellation, and maximum concurrency limits.

	OperationQueue	GCD
Dependencies	✅ Yes	❌ No
Cancellation	✅ Yes	Limited
Max concurrency	✅ Yes	Limited
KVO observable	✅ Yes	❌ No
Overhead	Higher	Lower

Code Example:

let queue = OperationQueue()
queue.maxConcurrentOperationCount = 3

let op1 = BlockOperation { print("Download image") }
let op2 = BlockOperation { print("Process image") }
let op3 = BlockOperation { print("Save image") }

// op2 and op3 won't start until op1 finishes
op2.addDependency(op1)
op3.addDependency(op2)

queue.addOperations([op1, op2, op3], waitUntilFinished: false)

// Cancel all
queue.cancelAllOperations()