Final Grand Central Dispatch tutorial in Swift

0/5 No votes

Report this app

Description

[ad_1]

Be taught the ideas of multi-threading with the GCD framework in Swift. Queues, duties, teams all the pieces you may ever want I promise.

iOS

GCD concurrency tutorial for novices

The Grand Central Dispatch (GCD, or simply Dispatch) framework is predicated 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 at all times out there ready for duties to be executed concurrently. 🚦

Creating threads on the run is an costly job so GCD organizes duties into particular queues, and in a while 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, 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. πŸ€“

Prior to now a processor had one single core, it might solely take care of one jobΒ 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 have 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 identical time, so the variety of out there threads primarily doubled. πŸ“ˆ

As you’ll be able to see concurrenct execution may be achieved with varied 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, however it’s at all times good to know the essential ideas. πŸ‘


Synchronous and asynchronous execution

Every work merchandise may 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 objects 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 almost definitely synchronous if it has a return worth, so func load() -> String goes to in all probability 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 a very good sing of async strategies, for instance for those who 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 in a while via a block.

It is a typical use case, if it’s important 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 sluggish IO operation. There may be different duties that aren’t IO heavy in any respect (math operations, and so forth.) these may be executed whereas the system is studying your file from the bodily onerous drive. πŸ’Ύ

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. ⚑️


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 likely be executed in the identical order as you add them to the queue – FIFO: the primary job within the line will likely be executed first – however it’s best to word that the order of completion shouldn’t be assured. Duties will likely be accomplished in keeping with the code complexity. So for those who 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.Β βŒ›οΈ

Serial and concurrent queues

There are two forms of dispatch queues. Serial queues can execute one job at a time, these queues may 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. πŸ’°

Foremost, world and customized queues

The principal queue is a serial one, each job on the primary queue runs on the primary thread.

International queues are system offered 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 may be created by the consumer. Customized concurrent queues at all times mapped into one of many world 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 is suggested to make use of one of many world concurrent queues, it’s best to solely create customized serial queues.

System offered queues

  • Serial principal queue
  • Concurrent world queues
  • excessive precedence world queue
  • default precedence world queue
  • low precedence world queue
  • world background queue (io throttled)

Customized queues by high quality of service

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

Sufficient from the speculation, let’s examine the best way to 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 word that it’s best to at all times use a world concurrent queue as an alternative of making your individual one, besides if you’re going to use the concurrent queue for locking with obstacles to realize thread security, extra on that later. 😳

The way to get a queue?

import Dispatch

DispatchQueue.principal
DispatchQueue.world(qos: .userInitiated)
DispatchQueue.world(qos: .userInteractive)
DispatchQueue.world(qos: .background)
DispatchQueue.world(qos: .default)
DispatchQueue.world(qos: .utility)
DispatchQueue.world(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 primary queue after the duty completed is a fairly simple one utilizing Dispatch queues.

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

    DispatchQueue.principal.async {
        
    }
}

Sync and async calls on queues

There is no such thing as a huge distinction between sync and async strategies on a queue.Β 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. πŸŽ‰

let q = DispatchQueue.world()

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

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

Mainly for those who 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’s going to trigger a impasse and a crash. You need to use this snippet in case you are in search of a secure method to do sync calls on the primary queue / thread. πŸ‘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.principal.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 goal, however you’ll be able to return the title of the present queue through the use of 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 may be dispatched onto a DispatchQueue and inside a DispatchGroup. A DispatchWorkItem can be set as a DispatchSource occasion, registration, or cancel handler.

So that you identical to with operations through the use of a piece merchandise you’ll be able to cancel a working job. Additionally work objects 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: .principal) {
    print("finished")
}

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

Concurrent duties with DispatchGroups

So you could carry out a number of community calls with a view to assemble the info required by a view controller? That is the place DispatchGroup can assist you. Your entire lengthy working background job may be executed concurrently, when all the pieces is prepared you may obtain a notification. Simply watch out it’s important to use thread-safe information buildings, so at all times modify arrays for instance on the identical thread! πŸ˜…

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: .principal) {
    print("finished")
}

Observe that you simply at all times should stability out the enter and go away calls on the group. The dispatch group additionally permits us to trace the completion of various work objects, 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.world().async(group: group, execute: workItem)



group.notify(queue: .principal) {
    print("finished")
}

Yet another 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 count on and the indicator animation couldn’t end. To resolve this example you’ll be able to add a small delay job so the group will wait till each of the duties end. 😎

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

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 here are just a few necessary examples in Swift.

The way to make an async job to synchronous?

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

enum DispatchError: Error {
    case timeout
}

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

func syncMethod() throws -> String {

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

    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 in all probability going to make use of mutual exclusion. This may very well be achieved utilizing a semaphore object, but when your object wants heavy studying functionality it’s best to take into account a dispatch barrier primarily based resolution. 😜

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

Look ahead to a number of duties to finish

Identical 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.world()
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("finished")

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 numerous photos from a server you’ll be able to run a batch of x each time. Fairly useful. πŸ–

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

The DispatchSource object

A dispatch supply is a basic information sort that coordinates the processing of particular low-level system occasions.

Alerts, descriptors, processes, ports, timers and plenty of extra. All the things is dealt with via the dispatch supply object. I actually do not wish to get into the small print, it is fairly low-level stuff. You may monitor information, ports, indicators with dispatch sources. Please simply learn the offical Apple docs. πŸ“„

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("good day")
}
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 below the hood of GCD. Each thread has a run loop object related to it, you’ll be able to even run them by hand. Should you create a thread manually a run loop will likely be added to that thread routinely.

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, at all times use GCD queues!

Queue != Thread

A GCD queue shouldn’t be a thread, for those who 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 ultimately the values within it are going to be tousled like hell if the array shouldn’t be 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 aspect goes to be added first? Now that is why you want typically 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 huge draw back of the answer is pace. 🐌

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

q.async() {
  
}

q.sync() {
  
}

Concurrent queues utilizing obstacles

You may ship a barrier job to a queue for those who present an additional flag to the async technique. If a job like this arrives to the queue it’s going to make sure that nothing else will likely be executed till the barrier job have completed. To sum this up, barrier duties are sync (factors) duties for concurrent queues. Use async obstacles 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 lead to extraordinarily quick reads in a thread secure atmosphere. You can even use serial queues, semaphores, locks all of it is dependent upon your present scenario, however it’s good to know all of the out there choices is not it? 🀐


A couple of anti-patterns

It’s a must to be very cautious with deadlocks, race situations and the readers writers downside. Normally 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 lined that half. πŸ˜‰

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

queue.sync {
    
    queue.sync {
        
    }
}


DispatchQueue.world(qos: .utility).sync {
    
    DispatchQueue.principal.sync {
        
    }
}

The Dispatch framework (aka. GCD) is a tremendous 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 with a view to embrace concurrent programming into a complete new stage? Guarantees or await, possibly one thing totally new, let’s hope that we’ll see one thing in Swift 6.

[ad_2]

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.