iOS - gcd queues

Threads vs Processes

Threads

• Threads are used for smaller tasks
• Threads within the same process share the same address space

Processes

• Processes are used for more heavier tasks – basically the execution of applications.
• Processes within processes do not share the same address space

Grand Central Dispatch

• An abstracton layer built on top of the original Grand Central Dispatch queue that makes it easier for developers to understand and create more efficient applications
• A dispatch queue performs tasks asynchronously and concurrently in the order they that are added to the queue
• A serial dispatch queue only performs tasks synchronously. The former task must execute and complete before the latter.
  • Use serial queues when you need to synchronize values

Which one will execute first?

let q = DispatchQueue.global(qos: .userInteractive)

q.async { () -> Void in print(“tic”) }

print(“tac”)

prints “tac” first because the queue will immediately return and execute the next line print("tac") and then sometime in the future print("tact") will execute.

How about this one?

let q1 = DispatchQueue(label: “queue1”) let q2 = DispatchQueue(label: “queue2”) let q3 = DispatchQueue(label: “queue3”)

q1.async { () -> Void in print(1) }

q2.async { () -> Void in print(2) }

q3.async { () -> Void in print(3) }

print(“end”)

Who knows? Because each of the print statements are in three different, concurrent queues, the ordering is unknown.

Beware of UIKit and CoreData

• UIKit and CoreData are examples of two iOS frameworks that are not thread safe.
• UIKit can only be execute on the Main thread
• CoreData objects can only be executed and accessed on the thread that they are created.