Concurrency101

Gio + Iván holiday special: Concurrency

Playground Pages:

• Sync vs Async
• Serial vs Concurrent
• DispatchGroup
• DispatchQueue
• Thread-Safe Data Structure
• OperationQueue
• Semaphore
• Deadlock

Resources:

• 📺  | WWDC Modernizing Grand Central Dispatch Usage 🚨Must Watch🚨
• 🔗  | Concurrency Programming Guide
• 🔗 Khanlou | The GCD Handbook
• 🔗 Objc.io | Concurrent Programming: APIs and Challenges
• 🔗 libdispatch efficiency tips
• 🔗 Swift Lee | Asynchronous operations
• 🔗 Donny Wals | DispatchGroup

General Advice:

• Avoid using concurrent queues at all cost.
  • Instead use a serial queue per sub system and use a target queue if you need subsystems to interact.
  • Reader/writer locks tend to be a source of great complexity and hard to get right, specially in Swift which is bad at atomic access. If needed it’s better to use os_unfair_lock
• Combine helps handling concurrency with its stream of events API.
• Don’t use GlobalQueues.
  • Quote from Swift Evolution: Fixing race conditions in async/await example: dispatch_get_global_queue() is in practice on of the worst thing that the dispatch API provides
• Shouldn’t need to specify QoS unless for example you start a process in the main queue and want it to go in the background specifically. Otherwise let the system determine the QoS for you.