# Concurrency

# Obtaining a Grand Central Dispatch (GCD) queue

Grand Central Dispatch works on the concept of "Dispatch Queues". A dispatch queue executes tasks you designate in the order which they are passed. There are three types of dispatch queues:

  • Serial Dispatch Queues (aka private dispatch queues) execute one task at a time, in order. They are frequently used to synchronize access to a resource.
  • Concurrent Dispatch Queues (aka global dispatch queues) execute one or more tasks concurrently.
  • The Main Dispatch Queue executes tasks on the main thread.

To access the main queue:

let mainQueue = DispatchQueue.main

let mainQueue = dispatch_get_main_queue()

The system provides concurrent global dispatch queues (global to your application), with varying priorities. You can access these queues using the DispatchQueue class in Swift 3:

let globalConcurrentQueue = DispatchQueue.global(qos: .default)

equivalent to

let globalConcurrentQueue = DispatchQueue.global()

let globalConcurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)

In iOS 8 or later, the possible quality of service values which may be passed are .userInteractive, .userInitiated, .default, .utility, and .background. These replace the DISPATCH_QUEUE_PRIORITY_ constants.

You can also create your own queues with varying priorities:

let myConcurrentQueue = DispatchQueue(label: "my-concurrent-queue", qos: .userInitiated, attributes: [.concurrent], autoreleaseFrequency: .workItem, target: nil)
let mySerialQueue = DispatchQueue(label: "my-serial-queue", qos: .background, attributes: [], autoreleaseFrequency: .workItem, target: nil)

let myConcurrentQueue = dispatch_queue_create("my-concurrent-queue", DISPATCH_QUEUE_CONCURRENT)
let mySerialQueue = dispatch_queue_create("my-serial-queue", DISPATCH_QUEUE_SERIAL)

In Swift 3, queues created with this initializer are serial by default, and passing .workItem for autorelease frequency ensures an autorelease pool is created and drained for each work item. There is also .never, which means you will be managing your own autorelease pools yourself, or .inherit which inherits the setting from the environment. In most cases you probably won't use .never except in cases of extreme customization.

# Concurrent Loops

GCD provides mechanism for performing a loop, whereby the loops happen concurrently with respect to each other. This is very useful when performing a series of computationally expensive calculations.

Consider this loop:

for index in 0 ..< iterations {
    // Do something computationally expensive here

You can perform those calculations concurrently using concurrentPerform (in Swift 3) or dispatch_apply (in Swift 2):

DispatchQueue.concurrentPerform(iterations: iterations) { index in
    // Do something computationally expensive here

dispatch_apply(iterations, queue) { index in
    // Do something computationally expensive here

The loop closure will be invoked for each index from 0 to, but not including, iterations. These iterations will be run concurrently with respect to each other, and thus the order that they run is not guaranteed. The actual number of iterations that happen concurrently at any given time is generally dictated by the capabilities of the device in question (e.g. how many cores does the device have).

A couple of special considerations:

  • The `concurrentPerform`/`dispatch_apply` may run the loops concurrently with respect to each other, but this all happens synchronously with respect to the thread from which you call it. So, do not call this from the main thread, as this will block that thread until the loop is done.
  • Because these loops happen concurrently with respect to each other, you are responsible for ensuring the thread-safety of the results. For example, if updating some dictionary with the results of these computationally expensive calculations, make sure to synchronize those updates yourself.
  • Note, there is some overhead associated in running concurrent loops. Thus, if the calculations being performed inside the loop are not sufficiently computationally intensive, you may find that any performance gained by using concurrent loops may be diminished, if not be completely offset, by the overhead associated with the synchronizing all of these concurrent threads. So, you are responsible determining the correct amount of work to be performed in each iteration of the loop. If the calculations are too simple, you may employ "striding" to include more work per loop. For example, rather than doing a concurrent loop with 1 million trivial calculations, you may do 100 iterations in your loop, doing 10,000 calculations per loop. That way there is enough work being performed on each thread, so the overhead associated with managing these concurrent loops becomes less significant.
  • # Running tasks in a Grand Central Dispatch (GCD) queue

    To run tasks on a dispatch queue, use the sync, async, and after methods.

    To dispatch a task to a queue asynchronously:

    let queue = DispatchQueue(label: "myQueueName")
    queue.async {
        //do something
        DispatchQueue.main.async {
            //this will be called in main thread
            //any UI updates should be placed here
    // ... code here will execute immediately, before the task finished

    To dispatch a task to a queue synchronously:

    queue.sync {
        // Do some task
    // ... code here will not execute until the task is finished

    To dispatch a task to a queue after a certain number of seconds:

    queue.asyncAfter(deadline: .now() + 3) {
        //this will be executed in a background-thread after 3 seconds
    // ... code here will execute immediately, before the task finished

    NOTE: Any updates of the user-interface should be called on the main thread! Make sure, that you put the code for UI updates inside DispatchQueue.main.async { ... }

    Types of queue:

    let mainQueue = dispatch_get_main_queue()
    let highQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)
    let backgroundQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0)

    To dispatch a task to a queue asynchronously:

    dispatch_async(queue) {
        // Your code run run asynchronously. Code is queued and executed 
        // at some point in the future.
    // Code after the async block will execute immediately

    To dispatch a task to a queue synchronously:

    dispatch_sync(queue) {
        // Your sync code
    // Code after the sync block will wait until the sync task finished

    To dispatch a task to after a time interval (use NSEC_PER_SEC to convert seconds to nanoseconds):

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(2.5 * Double(NSEC_PER_SEC))), dispatch_get_main_queue()) {
        // Code to be performed in 2.5 seconds here

    To execute a task asynchronously and than update the UI:

    dispatch_async(queue) {
        // Your time consuming code here
        dispatch_async(dispatch_get_main_queue()) {
            // Update the UI code 

    NOTE: Any updates of the user-interface should be called on the main thread! Make sure, that you put the code for UI updates inside dispatch_async(dispatch_get_main_queue()) { ... }

    # Running Tasks in an OperationQueue

    You can think of an OperationQueue as a line of tasks waiting to be executed. Unlike dispatch queues in GCD, operation queues are not FIFO (first-in-first-out). Instead, they execute tasks as soon as they are ready to be executed, as long as there are enough system resources to allow for it.

    Get the main OperationQueue:

    let mainQueue = OperationQueue.main

    Create a custom OperationQueue:

    let queue = OperationQueue()
    queue.name = "My Queue"
    queue.qualityOfService = .default

    Quality of Service specifies the importance of the work, or how much the user is likely to be counting on immediate results from the task.

    Add an Operation to an OperationQueue:

    // An instance of some Operation subclass
    let operation = BlockOperation {
        // perform task here

    Add a block to an OperationQueue:

    myQueue.addOperation {
        // some task

    Add multiple Operations to an OperationQueue:

    let operations = [Operation]()
    // Fill array with Operations

    Adjust how many Operations may be run concurrently within the queue:

    myQueue.maxConcurrentOperationCount = 3 // 3 operations may execute at once
    // Sets number of concurrent operations based on current system conditions
    myQueue.maxConcurrentOperationCount = NSOperationQueueDefaultMaxConcurrentOperationCount

    Suspending a queue will prevent it from starting the execution of any existing, unstarted operations or of any new operations added to the queue. The way to resume this queue is to set the isSuspended back to false:

    myQueue.isSuspended = true
    // Re-enable execution
    myQueue.isSuspended = false

    Suspending an OperationQueue does not stop or cancel operations that are already executing. One should only attempt suspending a queue that you created, not global queues or the main queue.

    # Creating High-Level Operations

    The Foundation framework provides the Operation type, which represents a high-level object that encapsulates a portion of work that may be executed on a queue. Not only does the queue coordinate the performance of those operations, but you can also establish dependencies between operations, create cancelable operations, constrain the degree of concurrency employed by the operation queue, etc.

    Operations become ready to execute when all of its dependencies are finished executing. The isReady property then changes to true.

    Create a simple non-concurrent Operation subclass:

    class MyOperation: Operation {
        init(<parameters>) {
            // Do any setup work here
        override func main() {
            // Perform the task
    class MyOperation: NSOperation {
        init(<parameters>) {
            // Do any setup work here
        override func main() {
            // Perform the task

    Add an operation to an OperationQueue:


    This will execute the operation concurrently on the queue.

    Manage dependencies on an Operation.

    Dependencies define other Operations that must execute on a queue before that Operation is considered ready to execute.


    Run an Operation without a queue:


    Dependencies will be ignored. If this is a concurrent operation, the task may still be executed concurrently if its start method offloads work to background queues.

    Concurrent Operations.

    If the task that an Operation is to perform is, itself, asynchronous, (e.g. a URLSession data task), you should implement the Operation as a concurrent operation. In this case, your isAsynchronous implementation should return true, you'd generally have start method that performs some setup, then calls its main method which actually executes the task.

    When implementing an asynchronous Operation begins you must implement isExecuting, isFinished methods and KVO. So, when execution starts, isExecuting property changes to true. When an Operation finishes its task, isExecuting is set to false, and isFinished is set to true. If the operation it is cancelled both isCancelled and isFinished change to true. All of these properties are key-value observable.

    Cancel an Operation.

    Calling cancel simply changes the isCancelled property to true. To respond to cancellation from within your own Operation subclass, you should check the value of isCancelled at least periodically within main and respond appropriately.


    # Syntax

  • Swift 3.0
  • DispatchQueue.main // Get the main queue
  • DispatchQueue(label: "my-serial-queue", attributes: [.serial, .qosBackground]) // Create your own private serial queue
  • DispatchQueue.global(attributes: [.qosDefault]) // Access one of the global concurrent queues
  • DispatchQueue.main.async { ... } // Dispatch a task asynchronously to the main thread
  • DispatchQueue.main.sync { ... } // Dispatch a task synchronously to the main thread
  • DispatchQueue.main.asyncAfter(deadline: .now() + 3) { ... } // Dispatch a task asynchronously to the main thread to be executed after x seconds
  • Swift < 3.0
  • dispatch_get_main_queue() // Get the main queue running on the main thread
  • dispatch_get_global_queue(dispatch_queue_priority_t, 0) // Get global queue with specified priority dispatch_queue_priority_t
  • dispatch_async(dispatch_queue_t) { () -> Void in ... } // Dispatch a task asynchronously on the specified dispatch_queue_t
  • dispatch_sync(dispatch_queue_t) { () -> Void in ... } // Dispatch a task synchronously on the specified dispatch_queue_t
  • dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(nanoseconds)), dispatch_queue_t, { ... }); // Dispatch a task on the specified dispatch_queue_t after nanoseconds