Grand Central Dispatch

Concurrency & Parallelism

Queue Abstraction

Dispatch Queues

Dispatch Queues are objects that maintain a queue of tasks and execute these tasks
in their turn

Queue Type vs Task Type

Ghost of the Syntax Past

						// Get some queue to put our task into
						dispatch_async( dispatch_get_global_queue(
							Int(QOS_CLASS_USER_INITIATED.rawValue), 0) )
						{
							/* do some heavy work */
							dispatch_async(dispatch_get_main_queue()) {
								/* present results to the user */
							}
						}
						

Depricated since iOS 10.0 👌

Dispatch Queues since Swift 3

						// Get some queue to put our task into
						DispatchQueue.global(qos: .userInitiated).async {
							/* do some task */
							DispatchQueue.main.async {
								/* present results to the user */
							}
						}
						

Any UI operation should be performed on the main queue

Quality of Service

					.userInteractive  // highest priority
					.userInitiated
					.utility          // default
					.background       // lowest priority
					

System prioritizes and schedules queues
according to their QoS attribute

Creating Dispatch Queues

						let backgroundQueue = DispatchQueue(label: "com.app.queue",
								qos: .background,
								attributes: .concurrent)

						backgroundQueue.async {
							print("Dispatched to background queue")
						}
						

Created queue can be accessed by label

Dispatch Groups

							let group = DispatchGroup()
							DispatchQueue.global().async(group: group){
								/* do stuff */
							}
							group.wait() // wait for every task in group to complete

							group.notify(queue: DispatchQueue.main) {
								/* do this after every task in the group is completed */
							}
						

Allow to perform barrier based synchronisation

Dispatch Semaphores

							let semaphore = DispatchSemaphore(value: 10)
							semaphore.wait()
								/* access some shared data */
							semaphore.signal()
						

Implementation of a traditional counting semaphore

Operation Queues

Operation

						// Performs the receiver’s non-concurrent task
						func main()
						// The block to execute after the operation’s main task is completed
						var completionBlock: (() -> Void)?
						

An abstract class you use to encapsulate the code and data associated with a single task

Operation Queue

							let q = OperationQueue()
							q.qualityOfService = .utility
							q.maxConcurrentOperationCount = 10
							q.isSuspended = true
							let op = BlockOperation { /* do stuff */ }
							op.completionHandler = { /* do other stuff */}
							q.addOperation(op: op)
							q.isSuspended = false
							q.waitUntilAllOperationsAreFinished()
						

Operation Priority & Dependencies

							let op = BlockOperation { /* do stuff */ }
							op.queuePriority = .high
							let dependentOp = BlockOperation { /* do other stuff */ }
							dependantOP.addDependency(op) // requires 'op' to be completed
							q.addOperation(op)
						

Operation Queue schedules operations according to their dependencies and priority

Operation Status

							var isCancelled: Bool
							var isReady: Bool
							var isExecuting: Bool
							var isFinished: Bool
							var isConcurrent: Bool
						

Subclassing Operation

							class FileringOperation: Operation {
								var filter: CIFilter!
								weak var imageView: UIImageView?
								var result: UIImage?

								func main() {
									if isCanceled { return }
									if let source = imageView?.image {
										result = filter.apply(to: source)
									}
								}
							}
						

Key-Value Coding

KVC

Key-value coding is a mechanism that allows objects to provide indirect access to their properties

							// Returns the value for the property identified by a given key
							func value(forKey key: String) -> Any?
							// Sets the property of the receiver specified by a given key to a given value
							func setValue(_ value: Any?, forKey key: String)
						

Adopting KVC

							class KVCPerson: NSObject {
								@objc var name: String = ""
							}
							var kvc = KVCPerson()
							kvc.setValue("I'm a KVC Object", forKey: #keyPath(KVCPerson.name))
						

There is no explicit KVC protocol, but any subclass of NSObject is KVC compliant

KeyPaths in Swift 4

							class Person {
								var name: String
								var bestFriend: Person? = nil
							}
							var han = Person(name: "Han Solo")
							var luke = Person(name: "Luke Skywalker")
							luke.bestFriend = han

							luke[keyPath: \Person.name] = "Master Luke"
							luke[keyPath: \Person.bestFriend?.name]  // "Han Solo"
						

Dedicated KeyPath type allows to reference properties

Works with any Swift type
Would support subscripts in the future

Key-Value Observers

Key-value observing is a mechanism that allows objects to be notified of changes to specified properties of other objects

							func observeValue(forKeyPath keyPath: String?, of object: Any?,
								change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?)
						

							func addObserver(_ observer: NSObject, forKeyPath keyPath: String,
								options: NSKeyValueObservingOptions = [], context: UnsafeMutableRawPointer?)
							func removeObserver(_ observer: NSObject, forKeyPath keyPath: String)
						

Responsible Observation

							class MyObserver: NSObject {
								var myContext = 88888888
								override init() {
									super.init()
									objectToObserve.addObserver(self,
												forKeyPath: #keyPath(Observed.propertyName),
												options: .new, context: &myContext)
								}
								deinit {
									objectToObserve.removeObserver(self,
												forKeyPath: #keyPath(Observed.propertyName),
												context: &myContext)
								}
							}
							

Responding to KeyValue Changes

							override func observeValue(forKeyPath keyPath: String?, of object: Any?,
									change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
								if context == &myContext {
									if let newValue = change?[.newKey] {
										print("Value changed: \(newValue)")
									}
								} else {
									super.observeValue(forKeyPath: keyPath, of: object,
										change: change, context: context)
								}
							}
						

Considered Harmful!

• Comes through one method
• Requires you to handle superclasses yourself
• Obscures control flow
• Can crash while deregistering

Property Observers

							class Person {
								var name: String {
									willSet {
										print("name will be set to \(newValue)")
									}
									didSet {
										print("name has been changed from \(oldValue)")
									}
								}
							}
						

Notifications

Notification Center

Provides a mechanism for broadcasting information within a program

						let defaultCenter = NotificationCenter.default
						

Notification

						  name: String // unique name
						  object: AnyObject? // sender
						  userInfo: [:]? // additional data
						

An object distributed through Notification Centers

Custom Notifications

							extension Notification.Name {
								static let myNotification = Notification.Name("MyNotification")
								static let myNotificationWithPayload = Notification.Name("MyNotificationWithPayload")
							}
						

Any notification should have an unique name within the app

All default notification names are stored in
Notification.Name class

Send Notification

						defaultCenter.post(name: .myNotification, object: self)
						defaultCenter.post(name: .myNotificationWithPayload,
							object: self,
							userInfo: ["payload" : myPayload])
					

Observe

							defaultCenter.addObserver(self,
								selector: #selector(myNotificationHandler),
								name: "MyNotification", object: sender)
						

Calls self.myNotificationHandler
whenever MyNotification is posted

							defaultCenter.removeObserver(self,
								name: "MyNotification", object: sender)
						

Observers should be removed manually on deinit

Observe with Block

						defaultCenter.addObserver(forName: "MyNotification",
							object: sender, queue: nil) {
								[weak self] notification in
								/* handle notification */
							}
						

Performs closure in specified queue

Strong references to self in closures
can create reference cycles

Concurrency

						DispatchQueue.main.async {
							defaultCenter.post(name: "MyNotification", object: nil,
								userInfo: ["data":data])
						}
						

Notification centers deliver notifications on the thread
in which the notification was posted

Considered Harmful?

• Notifications make it harder to trace the flow of control
• Deinitialized observers crash the app
• Notification centers are not well suited for unit tests