My top concurrency posts

My notes on queuing tasks up and making things concurrent in swift



  • Call items async or sync
  • Create dependencies with callbacks
  • Call things on the main thread or in background thread


  • Launch many operations async or serial and attach a completionBlock when they all complete
  • If one operation fail, you have to call .leave. or else complete is never called, you can use .wait for group timeout


  • Ability to cancel many tasks in the queue
  • Simpler than NSOperationQueue
  • Has ability to wait (side effect is that it block the thread)
  • Can be built for Search trotting functionality for instance


  • More complex than DispatchWorkItem (can be a good idea to start with DispatchWorkItem before advancing to NSOperation)
  • Ability to cancel single or all operations in a queue
  • Ability to make one operation dependant on the completion of another
  • Ability to override and create subclasses
  • More control over different states the items has
  • Can set priorities and assign different queues
  • Can nest Operation queues in a multi-dimensional structure
  • Can run one task after the previous finished etc (or all at the same time)

Concurrent perform:

  • Great for managing many small cpu intensive tasks, and dividing tasks efficiently to threads
  • Can be used for async network calls if semaphore is used
  • Can be used with AtomicValue (thread-safe variable) (see ParallelLoop project)
  • Can do Any combination of: sync, async, concurrent, serial


  • Semaphores has the ability to convert-a-callback based method into a returning-style-method. ✨
  • Semaphores also has the ability to timeout similarly to dispatchgroup


  • write about async UnitTesting in xCode (or ref to the one you wrote before)