Final Grand Central Dispatch tutorial in Swift

GCD concurrency tutorial for newbies

The Grand Central Dispatch (GCD, or simply Dispatch) framework relies on the underlying thread pool design sample. Which means there are a set variety of threads spawned by the system – primarily based on some components like CPU cores – they’re all the time out there ready for duties to be executed concurrently. 🚦

Creating threads on the run is an costly process so GCD organizes duties into particular queues, and afterward the duties ready on these queues are going to be executed on a correct and out there thread from the pool. This method results in nice efficiency and low execution latency. We are able to say that the Dispatch framework is a really quick and environment friendly concurrency framework designed for contemporary multicore hardwares and wishes.

Concurrency, multi-tasking, CPU cores, parallelism and threads

A processor can run duties made by you programmatically, that is normally known as coding, growing or programming. The code executed by a CPU core is a thread. So your app goes to create a course of that’s made up from threads. 🤓

Prior to now a processor had one single core, it might solely cope with one process at a time. Afterward time-slicing was launched, so CPU’s might execute threads concurrently utilizing context switching. As time handed by processors gained extra horse energy and cores so that they had been able to actual multi-tasking utilizing parallelism. ⏱

These days a CPU is a really highly effective unit, it is able to executing billions of duties (cycles) per second. Due to this excessive availability pace Intel launched a know-how known as hyperthreading. They divided CPU clock cycles between (normally two) processes working on the similar time, so the variety of out there threads basically doubled. 📈

As you may see concurrenct execution will be achieved with varied strategies, however you need not care about that a lot. It is as much as the CPU structure the way it solves concurrency, and it is the working system’s process how a lot thread goes to be spawned for the underlying thread pool. The GCD framework will disguise all of the complexity, but it surely’s all the time good to know the essential ideas. 👍

Synchronous and asynchronous execution

Every work merchandise will be executed both synchronously or asynchronously.

Have you ever ever heard of blocking and non-blocking code? This is similar situaton right here. With synchronous duties you may block the execution queue, however with async duties your name will immediately return and the queue can proceed the execution of the remaining duties (or work gadgets as Apple calls them). 🚧

Synchronous execution

When a piece merchandise is executed synchronously with the sync technique, this system waits till execution finishes earlier than the strategy name returns.

Your operate is most definitely synchronous if it has a return worth, so func load() -> String goes to most likely block the factor that runs on till the sources is totally loaded and returned again.

Asynchronous execution

When a piece merchandise is executed asynchronously with the async technique, the strategy name returns instantly.

Completion blocks are an excellent sing of async strategies, for instance in the event you have a look at this technique func load(completion: (String) -> Void) you may see that it has no return kind, however the results of the operate is handed again to the caller afterward via a block.

This can be a typical use case, if you must look ahead to one thing inside your technique like studying the contents of an enormous file from the disk, you do not need to block your CPU, simply due to the sluggish IO operation. There will be different duties that aren’t IO heavy in any respect (math operations, and many others.) these will be executed whereas the system is studying your file from the bodily exhausting drive. 💾

With dispatch queues you may execute your code synchronously or asynchronously. With syncronous execution the queue waits for the work, with async execution the code returns instantly with out ready for the duty to finish. ⚡️

Dispatch queues

As I discussed earlier than, GCD organizes process into queues, these are identical to the queues on the shopping center. On each dispatch queue, duties can be executed in the identical order as you add them to the queue – FIFO: the primary process within the line can be executed first – however it’s best to notice that the order of completion isn’t assured. Duties can be accomplished in accordance with the code complexity. So in the event you add two duties to the queue, a sluggish one first and a quick one later, the quick one can end earlier than the slower one. ⌛️

Serial and concurrent queues

There are two sorts of dispatch queues. Serial queues can execute one process at a time, these queues will be utilized to synchronize entry to a particular useful resource. Concurrent queues alternatively can execute a number of duties parallell in the identical time. Serial queue is rather like one line within the mall with one cashier, concurrent queue is like one single line that splits for 2 or extra cashiers. 💰

Principal, international and customized queues

The major queue is a serial one, each process on the primary queue runs on the primary thread.

International queues are system supplied concurrent queues shared via the working system. There are precisely 4 of them organized by excessive, default, low precedence plus an IO throttled background queue.

Customized queues will be created by the consumer. Customized concurrent queues all the time mapped into one of many international queues by specifying a High quality of Service property (QoS). In a lot of the instances if you wish to run duties in parallel it’s endorsed to make use of one of many international concurrent queues, it’s best to solely create customized serial queues.

System supplied queues

• Serial major queue
• Concurrent international queues
• excessive precedence international queue
• default precedence international queue
• low precedence international queue
• international background queue (io throttled)

Customized queues by high quality of service

• userInteractive (UI updates) -> serial major queue
• userInitiated (async UI associated duties) -> excessive precedence international queue
• default -> default precedence international queue
• utility -> low precedence international queue
• background -> international background queue
• unspecified (lowest) -> low precedence international queue

Sufficient from the speculation, let’s examine how you can use the Dispatch framework in motion! 🎬

The way to use the DispatchQueue class in Swift?

Right here is how one can get all of the queues from above utilizing the model new GCD syntax out there from Swift 3. Please notice that it’s best to all the time use a worldwide concurrent queue as an alternative of making your personal one, besides if you’re going to use the concurrent queue for locking with boundaries to realize thread security, extra on that later. 😳

The way to get a queue?

import Dispatch

DispatchQueue.major
DispatchQueue.international(qos: .userInitiated)
DispatchQueue.international(qos: .userInteractive)
DispatchQueue.international(qos: .background)
DispatchQueue.international(qos: .default)
DispatchQueue.international(qos: .utility)
DispatchQueue.international(qos: .unspecified)
DispatchQueue(label: "com.theswiftdev.queues.serial")
DispatchQueue(label: "com.theswiftdev.queues.concurrent", attributes: .concurrent)

So executing a process on a background queue and updating the UI on the primary queue after the duty completed is a reasonably simple one utilizing Dispatch queues.

DispatchQueue.international(qos: .background).async {
    

    DispatchQueue.major.async {
        
    }
}

Sync and async calls on queues

There isn’t any massive distinction between sync and async strategies on a queue. Sync is simply an async name with a semaphore (defined later) that waits for the return worth. A sync name will block, alternatively an async name will instantly return. 🎉

let q = DispatchQueue.international()

let textual content = q.sync {
    return "this can block"
}
print(textual content)

q.async {
    print("this can return immediately")
}

Principally in the event you want a return worth use sync, however in each different case simply go together with async. DEADLOCK WARNING: it’s best to by no means name sync on the primary queue, as a result of it will trigger a impasse and a crash. You need to use this snippet if you’re in search of a secure option to do sync calls on the primary queue / thread. 👌

Do not name sync on a serial queue from the serial queue’s thread!

Delay execution

You may merely delay code execution utilizing the Dispatch framework.

DispatchQueue.major.asyncAfter(deadline: .now() + .seconds(2)) {
    
}

Carry out concurrent loop

Dispatch queue merely lets you carry out iterations concurrently.

DispatchQueue.concurrentPerform(iterations: 5) { (i) in
    print(i)
}

Debugging

Oh, by the best way it is only for debugging function, however you may return the title of the present queue by utilizing this little extension. Don’t use in manufacturing code!!!

extension DispatchQueue {
    static var currentLabel: String {
        return String(validatingUTF8: __dispatch_queue_get_label(nil))!
    }
}

Utilizing DispatchWorkItem in Swift

DispatchWorkItem encapsulates work that may be carried out. A piece merchandise will be dispatched onto a DispatchQueue and inside a DispatchGroup. A DispatchWorkItem will also be set as a DispatchSource occasion, registration, or cancel handler.

So that you identical to with operations by utilizing a piece merchandise you may cancel a working process. Additionally work gadgets can notify a queue when their process is accomplished.

var workItem: DispatchWorkItem?
workItem = DispatchWorkItem {
    for i in 1..<6 {
        guard let merchandise = workItem, !merchandise.isCancelled else {
            print("cancelled")
            break
        }
        sleep(1)
        print(String(i))
    }
}

workItem?.notify(queue: .major) {
    print("carried out")
}

DispatchQueue.international().asyncAfter(deadline: .now() + .seconds(2)) {
    workItem?.cancel()
}
DispatchQueue.major.async(execute: workItem!)

Concurrent duties with DispatchGroups

So that you must carry out a number of community calls as a way to assemble the information required by a view controller? That is the place DispatchGroup may also help you. Your whole lengthy working background process will be executed concurrently, when the whole lot is prepared you may obtain a notification. Simply watch out you must use thread-safe knowledge constructions, so all the time modify arrays for instance on the identical thread! 😅

func load(delay: UInt32, completion: () -> Void) {
    sleep(delay)
    completion()
}

let group = DispatchGroup()

group.enter()
load(delay: 1) {
    print("1")
    group.go away()
}

group.enter()
load(delay: 2) {
    print("2")
    group.go away()
}

group.enter()
load(delay: 3) {
    print("3")
    group.go away()
}

group.notify(queue: .major) {
    print("carried out")
}

Word that you simply all the time must stability out the enter and go away calls on the group. The dispatch group additionally permits us to trace the completion of various work gadgets, even when they run on completely different queues.

let group = DispatchGroup()
let queue = DispatchQueue(label: "com.theswiftdev.queues.serial")
let workItem = DispatchWorkItem {
    print("begin")
    sleep(1)
    print("finish")
}

queue.async(group: group) {
    print("group begin")
    sleep(2)
    print("group finish")
}
DispatchQueue.international().async(group: group, execute: workItem)



group.notify(queue: .major) {
    print("carried out")
}

Yet another factor that you should utilize dispatch teams for: think about that you simply’re displaying a properly animated loading indicator whilst you do some precise work. It would occurs that the work is completed sooner than you’d count on and the indicator animation couldn’t end. To resolve this example you may add a small delay process so the group will wait till each of the duties end. 😎

let queue = DispatchQueue.international()
let group = DispatchGroup()
let n = 9
for i in 0..<n {
    queue.async(group: group) {
        print("(i): Working async process...")
        sleep(3)
        print("(i): Async process accomplished")
    }
}
group.wait()
print("carried out")

Semaphores

A semaphore) is just a variable used to deal with useful resource sharing in a concurrent system. It is a actually highly effective object, listed below are a couple of necessary examples in Swift.

The way to make an async process to synchronous?

The reply is easy, you should utilize a semaphore (bonus level for timeouts)!

enum DispatchError: Error {
    case timeout
}

func asyncMethod(completion: (String) -> Void) {
    sleep(2)
    completion("carried out")
}

func syncMethod() throws -> String {

    let semaphore = DispatchSemaphore(worth: 0)
    let queue = DispatchQueue.international()

    var response: String?
    queue.async {
        asyncMethod { r in
            response = r
            semaphore.sign()
        }
    }
    semaphore.wait(timeout: .now() + 5)
    guard let consequence = response else {
        throw DispatchError.timeout
    }
    return consequence
}

let response = strive? syncMethod()
print(response)

Lock / single entry to a useful resource

If you wish to keep away from race situation you’re most likely going to make use of mutual exclusion. This might be achieved utilizing a semaphore object, but when your object wants heavy studying functionality it’s best to think about a dispatch barrier primarily based answer. 😜

class LockedNumbers {

    let semaphore = DispatchSemaphore(worth: 1)
    var parts: [Int] = []

    func append(_ num: Int) {
        self.semaphore.wait(timeout: DispatchTime.distantFuture)
        print("appended: (num)")
        self.parts.append(num)
        self.semaphore.sign()
    }

    func removeLast() {
        self.semaphore.wait(timeout: DispatchTime.distantFuture)
        defer {
            self.semaphore.sign()
        }
        guard !self.parts.isEmpty else {
            return
        }
        let num = self.parts.removeLast()
        print("eliminated: (num)")
    }
}

let gadgets = LockedNumbers()
gadgets.append(1)
gadgets.append(2)
gadgets.append(5)
gadgets.append(3)
gadgets.removeLast()
gadgets.removeLast()
gadgets.append(3)
print(gadgets.parts)

Await a number of duties to finish

Similar to with dispatch teams, it’s also possible to use a semaphore object to get notified if a number of duties are completed. You simply have to attend for it…

let semaphore = DispatchSemaphore(worth: 0)
let queue = DispatchQueue.international()
let n = 9
for i in 0..<n {
    queue.async {
        print("run (i)")
        sleep(3)
        semaphore.sign()
    }
}
print("wait")
for i in 0..<n {
    semaphore.wait()
    print("accomplished (i)")
}
print("carried out")

Batch execution utilizing a semaphore

You may create a thread pool like conduct to simulate restricted sources utilizing a dispatch semaphore. So for instance if you wish to obtain a number of photographs from a server you may run a batch of x each time. Fairly helpful. 🖐

print("begin")
let sem = DispatchSemaphore(worth: 5)
for i in 0..<10 {
    DispatchQueue.international().async {
        sem.wait()
        sleep(2)
        print(i)
        sem.sign()
    }
}
print("finish")

The DispatchSource object

A dispatch supply is a elementary knowledge kind that coordinates the processing of particular low-level system occasions.

Alerts, descriptors, processes, ports, timers and plenty of extra. Every thing is dealt with via the dispatch supply object. I actually do not need to get into the small print, it is fairly low-level stuff. You may monitor recordsdata, ports, indicators with dispatch sources. Please simply learn the offical Apple docs. 📄

I would wish to make just one instance right here utilizing a dispatch supply timer.

let timer = DispatchSource.makeTimerSource()
timer.schedule(deadline: .now(), repeating: .seconds(1))
timer.setEventHandler {
    print("hiya")
}
timer.resume()

Thread-safety utilizing the dispatch framework

Thread security is an inevitable subject if it involves multi-threaded code. At first I discussed that there’s a thread pool below the hood of GCD. Each thread has a run loop object related to it, you may even run them by hand. If you happen to create a thread manually a run loop can be added to that thread robotically.

let t = Thread {
    print(Thread.present.title ?? "")
     let timer = Timer(timeInterval: 1, repeats: true) { t in
         print("tick")
     }
     RunLoop.present.add(timer, forMode: .defaultRunLoopMode)

    RunLoop.present.run()
    RunLoop.present.run(mode: .commonModes, earlier than: Date.distantPast)
}
t.title = "my-thread"
t.begin()

You shouldn’t do that, demo functions solely, all the time use GCD queues!

Queue != Thread

A GCD queue isn’t a thread, in the event you run a number of async operations on a concurrent queue your code can run on any out there thread that matches the wants.

Thread security is all about avoiding tousled variable states

Think about a mutable array in Swift. It may be modified from any thread. That is not good, as a result of finally the values inside it are going to be tousled like hell if the array isn’t thread secure. For instance a number of threads are attempting to insert values to the array. What occurs? In the event that they run in parallell which ingredient goes to be added first? Now because of this you want generally to create thread secure sources.

Serial queues

You need to use a serial queue to implement mutual exclusivity. All of the duties on the queue will run serially (in a FIFO order), just one course of runs at a time and duties have to attend for one another. One massive draw back of the answer is pace. 🐌

let q = DispatchQueue(label: "com.theswiftdev.queues.serial")

q.async() {
  
}

q.sync() {
  
}

Concurrent queues utilizing boundaries

You may ship a barrier process to a queue in the event you present an additional flag to the async technique. If a process like this arrives to the queue it will be sure that nothing else can be executed till the barrier process have completed. To sum this up, barrier duties are sync (factors) duties for concurrent queues. Use async boundaries for writes, sync blocks for reads. 😎

let q = DispatchQueue(label: "com.theswiftdev.queues.concurrent", attributes: .concurrent)

q.async(flags: .barrier) {
  
}

q.sync() {
  
}

This technique will end in extraordinarily quick reads in a thread secure atmosphere. It’s also possible to use serial queues, semaphores, locks all of it is dependent upon your present scenario, but it surely’s good to know all of the out there choices is not it? 🤐

A number of anti-patterns

You must be very cautious with deadlocks, race situations and the readers writers downside. Often calling the sync technique on a serial queue will trigger you a lot of the troubles. One other challenge is thread security, however we have already coated that half. 😉

let queue = DispatchQueue(label: "com.theswiftdev.queues.serial")

queue.sync {
    
    queue.sync {
        
    }
}


DispatchQueue.international(qos: .utility).sync {
    
    DispatchQueue.major.sync {
        
    }
}

The Dispatch framework (aka. GCD) is an incredible one, it has such a possible and it actually takes a while to grasp it. The true query is that what path goes to take Apple as a way to embrace concurrent programming into an entire new degree? Guarantees or await, perhaps one thing totally new, let’s hope that we’ll see one thing in Swift 6.