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Final Grand Central Dispatch tutorial in Swift


Study the rules of multi-threading with the GCD framework in Swift. Queues, duties, teams all the pieces you will ever want I promise.

iOS

GCD concurrency tutorial for newbies

The Grand Central Dispatch (GCD, or simply Dispatch) framework is predicated on the underlying thread pool design sample. Which means that there are a hard and fast variety of threads spawned by the system – based mostly on some components like CPU cores – they’re all the time out there ready for duties to be executed concurrently. ๐Ÿšฆ

Creating threads on the run is an costly job 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 desires.

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

A processor can run duties made by you programmatically, that is normally known as coding, creating 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. ๐Ÿค“

Up to now a processor had one single core, it might solely cope with one jobย at a time. In a while 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. โฑ

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 expertise known as hyperthreading. They divided CPU clock cycles between (normally two) processes operating on the similar time, so the variety of out there threads basically doubled. ๐Ÿ“ˆ

As you’ll be able to see concurrenct execution could be achieved with numerous methods, 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 job how a lot thread goes to be spawned for the underlying thread pool. The GCD framework will conceal all of the complexity, nevertheless it’s all the time good to grasp the essential rules. ๐Ÿ‘


Synchronous and asynchronous execution

Every work merchandise could be executed both synchronously or asynchronously.

Have you ever ever heard of blocking and non-blocking code? This is identical situaton right here. With synchronous duties you will 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). ๐Ÿšง

Synchronous execution

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

Your operate is most certainly 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 tactic name returns instantly.

Completion blocks are an excellent sing of async strategies, for instance in case you take a look at this technique func load(completion: (String) -> Void) you’ll be able to see that it has no return sort, however the results of the operate is handed again to the caller afterward by means of a block.

This can be a typical use case, if it’s a must to await one thing inside your technique like studying the contents of an enormous file from the disk, you do not wish to block your CPU, simply due to the gradual IO operation. There could be different duties that aren’t IO heavy in any respect (math operations, and so forth.) these could be executed whereas the system is studying your file from the bodily arduous drive. ๐Ÿ’พ

With dispatch queues you’ll be able to 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. โšก๏ธ


Dispatch queues

As I discussed earlier than, GCD organizes job into queues, these are identical to the queues on the shopping center. On each dispatch queue, duties will probably be executed in the identical order as you add them to the queue – FIFO: the primary job within the line will probably be executed first – however it is best to word that the order of completion is just not assured. Duties will probably be accomplished in keeping with the code complexity. So in case you add two duties to the queue, a gradual one first and a quick one later, the quick one can end earlier than the slower one.ย โŒ›๏ธ

Serial and concurrent queues

There are two varieties of dispatch queues. Serial queues can execute one job at a time, these queues could be utilized to synchronize entry to a particular useful resource. Concurrent queues however 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. ๐Ÿ’ฐ

Principal, international and customized queues

The foremost queue is a serial one, each job on the principle queue runs on the principle thread.

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

Customized queues could be created by the person. 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 is best to solely create customized serial queues.

System supplied queues

  • Serial foremost 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 foremost 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)ย -> low precedence international queue

Sufficient from the idea, let’s examine how one can use the Dispatch framework in motion! ๐ŸŽฌ


Learn how 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 word that it is best to all the time use a world concurrent queue as an alternative of making your personal one, besides if you’ll use the concurrent queue for locking with boundaries to attain thread security, extra on that later. ๐Ÿ˜ณ

Learn how to get a queue?

import Dispatch

DispatchQueue.foremost
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 job on a background queue and updating the UI on the principle queue after the duty completed is a reasonably simple one utilizing Dispatch queues.

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

    DispatchQueue.foremost.async {
        
    }
}

Sync and async calls on queues

There is no such thing as a large distinction between sync and async strategies on a queue.ย Sync is simply an async name with a semaphore (defined later) that waits for the return worth. A sync name will block, however an async name will instantly return. ๐ŸŽ‰

let q = DispatchQueue.international()

let textual content = q.sync {
    return "it will block"
}
print(textual content)

q.async {
    print("it will return immediately")
}

Principally in case you want a return worth use sync, however in each different case simply go together with async. DEADLOCK WARNING: it is best to by no means name sync on the principle queue, as a result of it’s going to trigger a impasse and a crash. You need to use this snippet if you’re searching for a secure approach to do sync calls on the principle queue / thread. ๐Ÿ‘Œ

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.foremost.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’ll be able to return the identify 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 could 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’ll be able to cancel a operating job. Additionally work gadgets can notify a queue when their job 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: .foremost) {
    print("accomplished")
}

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

Concurrent duties with DispatchGroups

So you have to 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 operating background job could be executed concurrently, when all the pieces is prepared you will obtain a notification. Simply watch out it’s a must to use thread-safe information buildings, so all the time modify arrays for instance on the identical thread! ๐Ÿ˜…

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: .foremost) {
    print("accomplished")
}

Be aware that you simply all the time need to steadiness 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 totally 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: .foremost) {
    print("accomplished")
}

Yet one more factor that you should use dispatch teams for: think about that you simply’re displaying a properly animated loading indicator when you do some precise work. It would occurs that the work is finished quicker than you’d anticipate and the indicator animation couldn’t end. To resolve this case you’ll be able to add a small delay job so the group will wait till each of the duties end. ๐Ÿ˜Ž

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

Semaphores

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

Learn how to make an async job to synchronous?

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

enum DispatchError: Error {
    case timeout
}

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

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 outcome = response else {
        throw DispatchError.timeout
    }
    return outcome
}

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 could possibly be achieved utilizing a semaphore object, but when your object wants heavy studying functionality it is best to take into account a dispatch barrier based mostly resolution. ๐Ÿ˜œ

class LockedNumbers {

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

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

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

Anticipate a number of duties to finish

Similar to with dispatch teams, you can even 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("accomplished")

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 plenty of photographs from a server you’ll be able to run a batch of x each time. Fairly helpful. ๐Ÿ–

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 information sort that coordinates the processing of particular low-level system occasions.

Indicators, descriptors, processes, ports, timers and lots of extra. All the pieces is dealt with by means of the dispatch supply object. I actually do not wish to get into the main points, it is fairly low-level stuff. You may monitor information, ports, alerts with dispatch sources. Please simply learn the offical Apple docs. ๐Ÿ“„

I might 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("hi there")
}
timer.resume()

Thread-safety utilizing the dispatch framework

Thread security is an inevitable subject if it involves multi-threaded code. To start with I discussed that there’s a thread pool beneath the hood of GCD. Each thread has a run loop object related to it, you’ll be able to even run them by hand. If you happen to create a thread manually a run loop will probably be added to that thread routinely.

let t = Thread {
    print(Thread.present.identify ?? "")
     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.identify = "my-thread"
t.begin()

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

Queue != Thread

A GCD queue is just not a thread, in case 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 is just not thread secure. For instance a number of threads try to insert values to the array. What occurs? In the event that they run in parallell which component goes to be added first? Now that is why 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 large draw back of the answer is pace. ๐ŸŒ

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

q.async() {
  
}

q.sync() {
  
}

Concurrent queues utilizing boundaries

You may ship a barrier job to a queue in case you present an additional flag to the async technique. If a job like this arrives to the queue it’s going to be certain that nothing else will probably be executed till the barrier job have completed. To sum this up, barrier duties are sync (factors) duties for concurrent queues. Use async boundaries for writes, sync blocks for reads. ๐Ÿ˜Ž

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 setting. You may as well use serial queues, semaphores, locks all of it relies on your present scenario, nevertheless it’s good to know all of the out there choices is not it? ๐Ÿค


A couple 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 problem is thread security, however we have already coated that half. ๐Ÿ˜‰

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

queue.sync {
    
    queue.sync {
        
    }
}


DispatchQueue.international(qos: .utility).sync {
    
    DispatchQueue.foremost.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 actual query is that what path goes to take Apple as a way to embrace concurrent programming into a complete new stage? Guarantees or await, perhaps one thing completely new, let’s hope that we’ll see one thing in Swift 6.

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