GCD concurrency tutorial for freshmen
The Grand Central Dispatch (GCD, or simply Dispatch) framework relies on the underlying thread pool design sample. Because of this there are a hard and fast variety of threads spawned by the system – primarily based on some components like CPU cores – they’re all the time accessible ready for duties to be executed concurrently. 🚦
Creating threads on the run is an costly activity so GCD organizes duties into particular queues, and afterward the duties ready on these queues are going to be executed on a correct and accessible 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 multi-core {hardware} and desires.
Concurrency, multi-tasking, CPU cores, parallelism and threads
A processor can run duties made by you programmatically, that is normally referred to 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 may solely take care of one activity at a time. Afterward time-slicing was launched, so CPU’s may 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 expertise referred to as hyper-threading. They divided CPU clock cycles between (normally two) processes operating on the identical time, so the variety of accessible threads primarily doubled. 📈
As you may see concurrent execution will be achieved with numerous strategies, however you needn’t 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 conceal all of the complexity, nevertheless 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 methodology, this system waits till execution finishes earlier than the tactic name returns.
Your perform is more than likely synchronous if it has a return worth, so func load() -> String goes to most likely block the factor that runs on till the assets is totally loaded and returned again.
Asynchronous execution
When a piece merchandise is executed asynchronously with the async methodology, the tactic name returns instantly.
Completion blocks are sing of async strategies, for instance in the event you take a look at this methodology 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 afterward by means of a block.
It is a typical use case, if it’s a must to look forward to one thing inside your methodology 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 will be different duties that aren’t IO heavy in any respect (math operations, and so forth.) these will be executed whereas the system is studying your file from the bodily arduous 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 shall be executed in the identical order as you add them to the queue – FIFO: the primary activity within the line shall be executed first – however it’s best to observe that the order of completion is just not assured. Duties shall be accomplished in response to the code complexity. So in the event 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. ⌛️
Serial and concurrent queues
There are two forms of dispatch queues. Serial queues can execute one activity at a time, these queues will be utilized to synchronize entry to a particular useful resource. Concurrent queues however 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. 💰
Major, international and customized queues
The principle queue is a serial one, each activity on the primary queue runs on the primary 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 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 many of the instances if you wish to run duties in parallel it is suggested to make use of one of many international concurrent queues, it’s best to solely create customized serial queues.
System supplied queues
- Serial essential 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 essential 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 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 accessible from Swift 3. Please observe that it’s best to all the time use a world concurrent queue as an alternative of making your individual one, besides if you’ll 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.essential
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 primary queue after the duty completed is a reasonably simple one utilizing Dispatch queues.
DispatchQueue.international(qos: .background).async {
DispatchQueue.essential.async {
}
}Sync and async calls on queues
There isn’t a large 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, however 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 in the event you want a return worth use sync, however in each different case simply go along 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 should use this snippet in case you are searching for a secure method 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’ll be able to merely delay code execution utilizing the Dispatch framework.
DispatchQueue.essential.asyncAfter(deadline: .now() + .seconds(2)) {
}Carry out concurrent loop
Dispatch queue merely permits you to carry out iterations concurrently.
DispatchQueue.concurrentPerform(iterations: 5) { (i) in
print(i)
}Debugging
Oh, by the best way it is only for debugging objective, however you may return the identify of the present queue through the use of 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 will also be set as a DispatchSource occasion, registration, or cancel handler.
So that you similar to with operations through the use of a piece merchandise you may cancel a operating 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: .essential) {
print("accomplished")
}
DispatchQueue.international().asyncAfter(
deadline: .now() + .seconds(2)
) {
workItem?.cancel()
}
DispatchQueue.essential.async(execute: workItem!)
Concurrent duties with DispatchGroups
So it’s essential to carry out a number of community calls to be able to assemble the info required by a view controller? That is the place DispatchGroup may help you. Your whole lengthy operating background activity will be executed concurrently, when every little thing is prepared you may obtain a notification. Simply watch out it’s a must to use thread-safe information 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: .essential) {
print("accomplished")
}Observe that you just 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: .essential) {
print("accomplished")
}Yet one more factor that you need to use dispatch teams for: think about that you just’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 anticipate 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("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 a number of essential examples in Swift.
The way to make an async activity to synchronous?
The reply is easy, you need to 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 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 might be 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 contemplate a dispatch barrier primarily based answer. 😜
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)Watch for a number of duties to finish
Identical to with dispatch teams, you can too 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’ll be able to create a thread pool like habits to simulate restricted assets utilizing a dispatch semaphore. So for instance if you wish to obtain plenty of photographs 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 basic information kind that coordinates the processing of particular low-level system occasions.
Alerts, descriptors, processes, ports, timers and lots of extra. Every part 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’ll be able to monitor recordsdata, ports, alerts with dispatch sources. Please simply learn the official Apple docs. 📄
I might prefer 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("whats up")
}
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 may even run them by hand. If you happen to create a thread manually a run loop shall be added to that thread robotically.
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 the event you run a number of async operations on a concurrent queue your code can run on any accessible 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 within it are going to be tousled like hell if the array is just not 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 parallel which aspect goes to be added first? Now that is why you want typically to create thread secure assets.
Serial queues
You should 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. 🐌
let q = DispatchQueue(label: "com.theswiftdev.queues.serial")
q.async() {
}
q.sync() {
}Concurrent queues utilizing obstacles
You’ll be able to ship a barrier activity to a queue in the event you present an additional flag to the async methodology. If a activity like this arrives to the queue it’s going to be certain that nothing else shall be executed till the barrier activity 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 methodology will lead to extraordinarily quick reads in a thread secure surroundings. You may as well use serial queues, semaphores, locks all of it depends upon your present state of affairs, nevertheless it’s good to know all of the accessible choices is not it? 🤐
A couple of anti-patterns
It’s a must to be very cautious with deadlocks, race circumstances and the readers writers downside. Normally calling the sync methodology on a serial queue will trigger you many of the troubles. One other situation is thread security, however we have already lined that half. 😉
let queue = DispatchQueue(label: "com.theswiftdev.queues.serial")
queue.sync {
queue.sync {
}
}
DispatchQueue.international(qos: .utility).sync {
DispatchQueue.essential.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 to be able to embrace concurrent programming into an entire new degree? Guarantees or async / await, perhaps one thing fully new, let’s hope that we’ll see one thing in Swift 6.
