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 that there are a hard and fast variety of threads spawned by the system – based mostly on some elements 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 activity 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 strategy results in nice efficiency and low execution latency. We will say that the Dispatch framework is a really quick and environment friendly concurrency framework designed for contemporary multi-core {hardware} and wishes.
Concurrency, multi-tasking, CPU cores, parallelism and threads
A processor can run duties made by you programmatically, that is often 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. 🤓
Previously a processor had one single core, it might solely take care of one activity 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 in order 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 velocity Intel launched a know-how known as hyper-threading. They divided CPU clock cycles between (often two) processes working on the identical time, so the variety of out there threads basically doubled. 📈
As you may see concurrent execution will be achieved with numerous 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 activity how a lot thread goes to be spawned for the underlying thread pool. The GCD framework will cover all of the complexity, however it’s all the time good to grasp the essential rules. 👍
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 state of affairs 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 tactic name returns.
Your perform is most probably 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 a very good sing of async strategies, for instance for those who 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 perform is handed again to the caller in a while via a block.
This can be a typical use case, if it’s important to look forward to 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 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 synchronous 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 activity into queues, these are similar 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 activity within the line can be executed first – however you need to word that the order of completion just isn’t assured. Duties can be accomplished in line 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. ⌛️
Serial and concurrent queues
There are two sorts of dispatch queues. Serial queues can execute one activity at a time, these queues will be utilized to synchronize entry to a selected useful resource. Concurrent queues then again can execute a number of duties parallel 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. 💰
Important, international and customized queues
The primary queue is a serial one, each activity on the principle queue runs on the principle thread.
World 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 person. Customized concurrent queues all the time mapped into one of many international queues by specifying a High quality of Service property (QoS). In many of the circumstances if you wish to run duties in parallel it is suggested to make use of one of many international concurrent queues, you need 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! 🎬
Tips on 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 you need 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. 😳
Tips on how 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 activity on a background queue and updating the UI on the principle queue after the duty completed is a fairly simple one utilizing Dispatch queues.
DispatchQueue.international(qos: .background).async {
DispatchQueue.major.async {
}
}Sync and async calls on queues
There isn’t a huge 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, then again 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")
}Mainly for those who want a return worth use sync, however in each different case simply go along with async. DEADLOCK WARNING: you need to by no means name sync on the principle queue, as a result of it will trigger a impasse and a crash. You should utilize this snippet if you’re searching for a protected strategy to do sync calls on the principle queue / thread. 👌
Do not name sync on a serial queue from the serial queue’s thread!
Delay execution
You’ll be able to 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 {
.init(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 can be set as a DispatchSource occasion, registration, or cancel handler.
So that you similar to with operations by utilizing a piece merchandise you may cancel a working activity. Additionally work gadgets can notify a queue when their activity 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("finished")
}
DispatchQueue.international().asyncAfter(
deadline: .now() + .seconds(2)
) {
workItem?.cancel()
}
DispatchQueue.major.async(execute: workItem!)
Concurrent duties with DispatchGroups
So you could carry out a number of community calls with a purpose to assemble the information required by a view controller? That is the place DispatchGroup will help you. Your whole lengthy working background activity will 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 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.depart()
}
group.enter()
load(delay: 2) {
print("2")
group.depart()
}
group.enter()
load(delay: 3) {
print("3")
group.depart()
}
group.notify(queue: .major) {
print("finished")
}Notice that you simply all the time should stability out the enter and depart 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: .major) {
print("finished")
}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’d occurs that the work is completed quicker than you’d count on and the indicator animation couldn’t end. To resolve this case you may add a small delay activity 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): Operating async activity...")
sleep(3)
print("(i): Async activity accomplished")
}
}
group.wait()
print("finished")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 below are just a few essential examples in Swift.
Tips on how to make an async activity 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("finished")
}
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 end result = response else {
throw DispatchError.timeout
}
return end result
}
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 you need to think about a dispatch barrier based mostly 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 gadgets = LockedNumbers()
gadgets.append(1)
gadgets.append(2)
gadgets.append(5)
gadgets.append(3)
gadgets.removeLast()
gadgets.removeLast()
gadgets.append(3)
print(gadgets.parts)Look forward 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.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("finished")Batch execution utilizing a semaphore
You’ll be able to create a thread pool like habits to simulate restricted sources utilizing a dispatch semaphore. So for instance if you wish to obtain plenty of photos from a server you may run a batch of x each time. Fairly useful. 🖐
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 kind that coordinates the processing of particular low-level system occasions.
Indicators, descriptors, processes, ports, timers and plenty of extra. Every part is dealt with via the dispatch supply object. I actually do not wish to get into the main points, it is fairly low-level stuff. You’ll be able to monitor information, ports, indicators with dispatch sources. Please simply learn the official 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("hi there")
}
timer.resume()Thread-safety utilizing the dispatch framework
Thread security is an inevitable matter if it involves multi-threaded code. At first I discussed that there’s a thread pool beneath the hood of GCD. Each thread has a run loop object related to it, you may even run them by hand. In case you 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 just isn’t 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 inside it are going to be tousled like hell if the array just isn’t thread protected. For instance a number of threads try to insert values to the array. What occurs? In the event that they run in parallel which aspect goes to be added first? Now because of this you want typically to create thread protected sources.
Serial queues
You should utilize 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 velocity. 🐌
let q = DispatchQueue(label: "com.theswiftdev.queues.serial")
q.async() {
}
q.sync() {
}Concurrent queues utilizing boundaries
You’ll be able to ship a barrier activity to a queue for those who present an additional flag to the async technique. If a activity like this arrives to the queue it will be sure that nothing else can be executed till the barrier activity 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 lead to extraordinarily quick reads in a thread protected surroundings. You can even use serial queues, semaphores, locks all of it is dependent upon your present state of affairs, however it’s good to know all of the out there choices is not it? 🤐
A couple of anti-patterns
You need to be very cautious with deadlocks, race circumstances and the readers writers drawback. Often calling the sync technique on a serial queue will trigger you many of the troubles. One other difficulty 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 a tremendous 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 with a purpose to embrace concurrent programming into a complete new degree? Guarantees or async / await, perhaps one thing solely new, let’s hope that we’ll see one thing in Swift 6.
