In this Grand Central Dispatch tutorial, you’ll delve into basic GCD concepts, including:

• Multithreading
• Dispatch queues
• Concurrency

Multi-core devices, on the other hand, execute multiple threads at the same time via parallelism.

GCD is built on top of threads. Under the hood, it manages a shared thread pool. With GCD, you add blocks of code or work items to dispatch queues and GCD decides which thread to execute them on.

You submit units of work to this queue, and GCD executes them in a FIFO order (first in, first out), guaranteeing that the first task submitted is the first one started.

Dispatch queues are thread-safe, meaning you can simultaneously access them from multiple threads.

Concurrent queues allow multiple tasks to run at the same time. The queue guarantees tasks start in the order you add them. Tasks can finish in any order, and you have no knowledge of the time it will take for the next task to start, nor the number of tasks running at any given time.

Queue Types

GCD provides three main types of queues:

• Main queue: Runs on the main thread and is a serial queue.
• Global queues: Concurrent queues shared by the whole system. Four such queues exist, each with different priorities: high, default, low and background. The background priority queue has the lowest priority and is throttled in any I/O activity to minimize negative system impact.
• Custom queues: Queues you create that can be serial or concurrent. Requests in these queues end up in one of the global queues.

QoS

When sending tasks to the global concurrent queues, you don’t specify the priority directly. Instead, you specify a quality of service (QoS) class property. This indicates the task’s importance and guides GCD in determining the priority to give to the task.

The QoS classes are:

• User-interactive: This represents tasks that must complete immediately to provide a nice user experience. Use it for UI updates, event handling and small workloads that require low latency. The total amount of work done in this class during the execution of your app should be small. This should run on the main thread.
• User-initiated: The user initiates these asynchronous tasks from the UI. Use them when the user is waiting for immediate results and for tasks required to continue user interaction. They execute in the high-priority global queue.
• Utility: This represents long-running tasks, typically with a user-visible progress indicator. Use it for computations, I/O, networking, continuous data feeds and similar tasks. This class is designed to be energy efficient. This gets mapped into the low-priority global queue.
• Background: This represents tasks the user isn’t directly aware of. Use it for prefetching, maintenance and other tasks that don’t require user interaction and aren’t time-sensitive. This gets mapped into the background priority global queue.

In general, you want to use async when you need to perform a network-based or CPU-intensive task in the background without blocking the current thread.

Delaying Tasks Execution

Why not use Timer? You could consider using it if you have repeated tasks that are easier to schedule with Timer. Here are two reasons to stick with dispatch queue’s asyncAfter():

// 1
let delayInSeconds = 2.0

// 2
DispatchQueue.main.asyncAfter(deadline: .now() + delayInSeconds) { [weak self] in
  guard let self = self else {
    return
  }

  if !PhotoManager.shared.photos.isEmpty {
    self.navigationItem.prompt = nil
  } else {
    self.navigationItem.prompt = "Add photos with faces to Googlyify them!"
  }

  // 3
  self.navigationController?.viewIfLoaded?.setNeedsLayout()
}

• One is readability. To use Timer, you have to define a method, then create the timer with a selector or invocation to the defined method. With DispatchQueue.main.asyncAfter(), you simply add a closure.
• Timer is scheduled on run loops, so you’d also have to make sure you scheduled it on the correct run loop — and in some cases for the correct run loop modes. In this regard, working with dispatch queues is easier.

Readers-Writers Problem

The Swift collection types like Array and Dictionary aren’t thread-safe when declared mutable.

private var unsafePhotos: [Photo] = []
  
var photos: [Photo] {
  return unsafePhotos
}

It may look like there’s a lot of copying in your code when passing collections back and forth. Don’t worry about the memory usage implications of this. The Swift collection types are optimized to make copies only when necessary, for instance, when your app modifies an array passed by value for the first time.

The getter for this property is termed a read method, as it’s reading the mutable array. The caller gets a copy of the array and is protected against inappropriately mutating the original array. However, this doesn’t provide any protection against one thread calling the write method addPhoto(_:) while another thread simultaneously calls the getter for the photos property.

GCD provides an elegant solution of creating a read/write lock using dispatch barriers. Dispatch barriers are a group of functions acting as a serial-style bottleneck when working with concurrent queues.

When you submit a DispatchWorkItem to a dispatch queue, you can set flags to indicate that it should be the only item executed on the specified queue for that particular time. This means all items submitted to the queue prior to the dispatch barrier must complete before DispatchWorkItem executes.

Barrier Flow

Notice how in normal operation, the queue acts just like a normal concurrent queue. But when the barrier is executing, it essentially acts as a serial queue. That is, the barrier is the only thing executing. After the barrier finishes, the queue goes back to being a normal concurrent queue.

private let concurrentPhotoQueue =
  DispatchQueue(
    label: "com.raywenderlich.GooglyPuff.photoQueue",
    attributes: .concurrent)

func addPhoto(_ photo: Photo) {
  // 1
  concurrentPhotoQueue.async(flags: .barrier) { [weak self] in
    guard let self = self else {
      return
    }

    // 2
    self.unsafePhotos.append(photo)

    // 3
    DispatchQueue.main.async { [weak self] in
      self?.postContentAddedNotification()
    }
  }
}

To ensure thread safety with your writes, you need to perform reads on concurrentPhotoQueue. You need return data from the function call, so an asynchronous dispatch won’t cut it. In this case, sync would be an excellent candidate.

You need to be careful, though. Imagine if you call sync and target the current queue you’re already running on. This would result in a deadlock situation.

Deadlocks

In your case, the sync call will wait until the closure finishes, but the closure can’t finish — or even start! — until the currently executing closure finishes, which it can’t! This should force you to be conscious of which queue you’re calling from — as well as which queue you’re passing in.

Here’s a quick overview of when and where to use sync:

• Main queue: Be very careful for the same reasons as above. This situation also has potential for a deadlock condition, which is especially bad on the main queue because the whole app will become unresponsive.
• Global queue: This is a good candidate to sync work through dispatch barriers or when waiting for a task to complete so you can perform further processing.
• Custom serial queue: Be very careful in this situation. If you’re running in a queue and call sync targeting the same queue, you’ll definitely create a deadlock.

var photos: [Photo] {
  var photosCopy: [Photo] = []

  // 1
  concurrentPhotoQueue.sync {
    // 2
    photosCopy = self.unsafePhotos
  }
  return photosCopy
}

Using Dispatch Groups

How can you monitor these concurrent asynchronous events to achieve this?

With dispatch groups, you can group together multiple tasks. Then, you can either wait for them to complete or receive a notification once they finish. Tasks can be asynchronous or synchronous and can even run on different queues.

DispatchGroup manages dispatch groups. You’ll first look at its wait method. This synchronous method blocks your current thread until all the group’s enqueued tasks finish.

// 1
DispatchQueue.global(qos: .userInitiated).async {
  var storedError: NSError?

  // 2
  let downloadGroup = DispatchGroup()
  for address in [
    PhotoURLString.overlyAttachedGirlfriend,
    PhotoURLString.successKid,
    PhotoURLString.lotsOfFaces
  ] {
    guard let url = URL(string: address) else { return }

    // 3
    downloadGroup.enter()
    let photo = DownloadPhoto(url: url) { _, error in
      storedError = error

      // 4
      downloadGroup.leave()
    }   
    PhotoManager.shared.addPhoto(photo)
  }   

  // 5      
  downloadGroup.wait()

  // 6
  DispatchQueue.main.async {
    completion?(storedError)
  }   
}

Call enter() to manually notify the group that a task has started. You must balance out the number of enter() calls with the number of leave() calls, or your app will crash.

You can use wait(timeout:) to specify a timeout and bail out on waiting after a specified time.

Dispatch Groups Notify

// 2    
downloadGroup.notify(queue: DispatchQueue.main) {
  completion?(storedError)
}

notify(queue:work:) serves as the asynchronous completion closure. It runs when there are no more items left in the group. You also specify that you want to schedule the completion work to run on the main queue.

let _ = DispatchQueue.global(qos: .userInitiated)
DispatchQueue.concurrentPerform(iterations: addresses.count) { index in
  let address = addresses[index]
  guard let url = URL(string: address) else { return }
  downloadGroup.enter()
  let photo = DownloadPhoto(url: url) { _, error in
    storedError = error
    downloadGroup.leave()
  }
  PhotoManager.shared.addPhoto(photo)
}

This implementation includes a curious line of code: let _ = DispatchQueue.global(qos: .userInitiated). This causes GCD to use a queue with a .userInitiated quality of service for the concurrent calls.

When is it appropriate to use DispatchQueue.concurrentPerform(iterations:execute:)? You can rule out serial queues because there’s no benefit there – you may as well use a normal for loop. It’s a good choice for concurrent queues that contain looping, though, especially if you need to keep track of progress.

Canceling Dispatch Blocks

Be aware that you can only cancel a DispatchWorkItem before it reaches the head of a queue and starts executing.

// 1
addresses += addresses + addresses

// 2
var blocks: [DispatchWorkItem] = []

for index in 0..<addresses.count {
  downloadGroup.enter()

  // 3
  let block = DispatchWorkItem(flags: .inheritQoS) {
    let address = addresses[index]
    guard let url = URL(string: address) else {
      downloadGroup.leave()
      return
    }
    let photo = DownloadPhoto(url: url) { _, error in
      storedError = error
      downloadGroup.leave()
    }
    PhotoManager.shared.addPhoto(photo)
  }
  blocks.append(block)

  // 4
  DispatchQueue.main.async(execute: block)
}

// 5
for block in blocks[3..<blocks.count] {

  // 6
  let cancel = Bool.random()
  if cancel {

    // 7
    block.cancel()

    // 8
    downloadGroup.leave()
  }
}

• You dispatch the block asynchronously to the main queue. For this example, using the main queue makes it easier to cancel select blocks since it's a serial queue. The code that sets up the dispatch blocks is already executing on the main queue. Thus, you know that the download blocks will execute at some later time.

• If the random value is true, you cancel the block. This can only cancel blocks that are still in a queue and haven't began executing. You can't cancel a block in the middle of execution.

• Here, you remember to remove the canceled block from the dispatch group.

Using Semaphores

Take a brief look at how you can use semaphores to test asynchronous code.

let url = try XCTUnwrap(URL(string: urlString))

// 1
let semaphore = DispatchSemaphore(value: 0)
_ = DownloadPhoto(url: url) { _, error in
  if let error = error {
    XCTFail("\(urlString) failed. \(error.localizedDescription)")
  }

  // 2
  semaphore.signal()
}
let timeout = DispatchTime.now() + .seconds(defaultTimeoutLengthInSeconds)

// 3
if semaphore.wait(timeout: timeout) == .timedOut {
  XCTFail("\(urlString) timed out")
} 

• You create a semaphore and set its start value. This represents the number of things that can access the semaphore without needing to increment it. Another name for incrementing a semaphore is signaling it.
• You signal the semaphore in the completion closure. This increments its count and signals that the semaphore is available to other resources.
• You wait on the semaphore with a given timeout. This call blocks the current thread until you signal the semaphore. A non-zero return code from this function means that the timeout period expired. In this case, the test fails because the network should not take more than 10 seconds to return — a fair point!