First, it's important to know the difference between threads and queues and what GCD really does. When we use dispatch queues (through GCD), we're really queueing, not threading. The Dispatch framework was designed specifically to get us away from threading, as Apple admits that "implementing a correct threading solution [can] become extremely difficult, if not [sometimes] impossible to achieve." Therefore, to perform tasks concurrently (tasks that we don't want freezing the UI), all we need to do is create a queue of those tasks and hand it to GCD. And GCD handles all of the associated threading. Therefore, all we're really doing is queueing.
The second thing to know right away is what a task is. A task is all of the code within that queue block (not within the queue, because we can add things to a queue all of the time, but within the closure where we added it to the queue). A task is sometimes referred to as a block and a block is sometimes referred to as a task (but they are more commonly known as tasks, particularly in the Swift community). And no matter how much or little code, all of the code within the curly braces are considered a single task:
serialQueue.async {
// this is one task
// it can be any number of lines with any number of methods
}
serialQueue.async {
// this is another task added to the same queue
// this queue now has two tasks
}
And it's obvious mentioning that concurrent simply means at the same time with other things and serial means one after the other (never at the same time). To serialize something, or to put something in serial, just means to execute it from start to finish in its order from left to right, top to bottom, uninterrupted.
There are two types of queues, serial and concurrent, but all queues are concurrent relative to each other. The fact that you want to run any code "in the background" means that you want to run it concurrently with another thread (usually the main thread). Therefore, all dispatch queues, serial or concurrent, execute their tasks concurrently relative to other queues. Any serialization performed by queues (by serial queues), have only to do with the tasks within that single [serial] dispatch queue (like in the example above where there are two tasks within the same serial queue; those tasks will be executed one after the other, never simultaneously).
SERIAL QUEUES (often known as private dispatch queues) guarantee the execution of tasks one at a time from start to finish in the order that they were added to that specific queue. This is the only guarantee of serialization anywhere in the discussion of dispatch queues--that the specific tasks within a specific serial queue are executed in serial. Serial queues can, however, run simultaneously with other serial queues if they are separate queues because, again, all queues are concurrent relative to each other. All tasks run on distinct threads but not every task is guaranteed to run on the same thread (not important, but interesting to know). And the iOS framework does not come with any ready-to-use serial queues, you must make them. Private (non-global) queues are serial by default, so to create a serial queue:
let serialQueue = DispatchQueue(label: "serial")
You can make it concurrent through its attribute property:
let concurrentQueue = DispatchQueue(label: "concurrent", attributes: [.concurrent])
But at this point, if you aren't adding any other attributes to the private queue, Apple recommends that you just use one of their ready-to-go global queues (which are all concurrent). At the bottom of this answer, you'll see another way to create serial queues (using the target property), which is how Apple recommends doing it (for more efficient resource management). But for now, labeling it is sufficient.
CONCURRENT QUEUES (often known as global dispatch queues) can execute tasks simultaneously; the tasks are, however, guaranteed to initiate in the order that they were added to that specific queue, but unlike serial queues, the queue does not wait for the first task to finish before starting the second task. Tasks (as with serial queues) run on distinct threads and (as with serial queues) not every task is guaranteed to run on the same thread (not important, but interesting to know). And the iOS framework comes with four ready-to-use concurrent queues. You can create a concurrent queue using the above example or by using one of Apple's global queues (which is usually recommended):
let concurrentQueue = DispatchQueue.global(qos: .default)
RETAIN-CYCLE RESISTANT: Dispatch queues are reference-counted objects but you do not need to retain and release global queues because they
are global, and thus retain and release is ignored. You can access
global queues directly without having to assign them to a property.
There are two ways to dispatch queues: synchronously and asynchronously.
SYNC DISPATCHING means that the thread where the queue was dispatched (the calling thread) pauses after dispatching the queue and waits for the task in that queue block to finish executing before resuming. To dispatch synchronously:
DispatchQueue.global(qos: .default).sync {
// task goes in here
}
ASYNC DISPATCHING means that the calling thread continues to run after dispatching the queue and does not wait for the task in that queue block to finish executing. To dispatch asynchronously:
DispatchQueue.global(qos: .default).async {
// task goes in here
}
Now one might think that in order to execute a task in serial, a serial queue should be used, and that's not exactly right. In order to execute multiple tasks in serial, a serial queue should be used, but all tasks (isolated by themselves) are executed in serial. Consider this example:
whichQueueShouldIUse.syncOrAsync {
for i in 1...10 {
print(i)
}
for i in 1...10 {
print(i + 100)
}
for i in 1...10 {
print(i + 1000)
}
}
No matter how you configure (serial or concurrent) or dispatch (sync or async) this queue, this task will always be executed in serial. The third loop will never run before the second loop and the second loop will never run before the first loop. This is true in any queue using any dispatch. It's when you introduce multiple tasks and/or queues where serial and concurrency really come into play.
Consider these two queues, one serial and one concurrent:
let serialQueue = DispatchQueue(label: "serial")
let concurrentQueue = DispatchQueue.global(qos: .default)
Say we dispatch two concurrent queues in async:
concurrentQueue.async {
for i in 1...5 {
print(i)
}
}
concurrentQueue.async {
for i in 1...5 {
print(i + 100)
}
}
1
101
2
102
103
3
104
4
105
5
Their output is jumbled (as expected) but notice that each queue executed its own task in serial. This is the most basic example of concurrency--two tasks running at the same time in the background in the same queue. Now let's make the first one serial:
serialQueue.async {
for i in 1...5 {
print(i)
}
}
concurrentQueue.async {
for i in 1...5 {
print(i + 100)
}
}
101
1
2
102
3
103
4
104
5
105
Isn't the first queue supposed to be executed in serial? It was (and so was the second). Whatever else happened in the background is not of any concern to the queue. We told the serial queue to execute in serial and it did... but we only gave it one task. Now let's give it two tasks:
serialQueue.async {
for i in 1...5 {
print(i)
}
}
serialQueue.async {
for i in 1...5 {
print(i + 100)
}
}
1
2
3
4
5
101
102
103
104
105
And this is the most basic (and only possible) example of serialization--two tasks running in serial (one after the other) in the background (to the main thread) in the same queue. But if we made them two separate serial queues (because in the above example they are the same queue), their output is jumbled again:
serialQueue.async {
for i in 1...5 {
print(i)
}
}
serialQueue2.async {
for i in 1...5 {
print(i + 100)
}
}
1
101
2
102
3
103
4
104
5
105
And this is what I meant when I said all queues are concurrent relative to each other. These are two serial queues executing their tasks at the same time (because they are separate queues). A queue does not know or care about other queues. Now lets go back to two serial queues (of the same queue) and add a third queue, a concurrent one:
serialQueue.async {
for i in 1...5 {
print(i)
}
}
serialQueue.async {
for i in 1...5 {
print(i + 100)
}
}
concurrentQueue.async {
for i in 1...5 {
print(i + 1000)
}
}
1
2
3
4
5
101
102
103
104
105
1001
1002
1003
1004
1005
That's kind of unexpected, why did the concurrent queue wait for the serial queues to finish before it executed? That's not concurrency. Your playground may show a different output but mine showed this. And it showed this because my concurrent queue's priority wasn't high enough for GCD to execute its task sooner. So if I keep everything the same but change the global queue's QoS (its quality of service, which is simply the queue's priority level) let concurrentQueue = DispatchQueue.global(qos: .userInteractive), then the output is as expected:
1
1001
1002
1003
2
1004
1005
3
4
5
101
102
103
104
105
The two serial queues executed their tasks in serial (as expected) and the concurrent queue executed its task quicker because it was given a high priority level (a high QoS, or quality of service).
Two concurrent queues, like in our first print example, show a jumbled printout (as expected). To get them to print neatly in serial, we would have to make both of them the same serial queue (the same instance of that queue, as well, not just the same label). Then each task is executed in serial with respect to the other. Another way, however, to get them to print in serial is to keep them both concurrent but change their dispatch method:
concurrentQueue.sync {
for i in 1...5 {
print(i)
}
}
concurrentQueue.async {
for i in 1...5 {
print(i + 100)
}
}
1
2
3
4
5
101
102
103
104
105
Remember, sync dispatching only means that the calling thread waits until the task in the queue is completed before proceeding. The caveat here, obviously, is that the calling thread is frozen until the first task completes, which may or may not be how you want the UI to perform.
And it is for this reason that we cannot do the following:
DispatchQueue.main.sync { ... }
This is the only possible combination of queues and dispatching methods that we cannot perform—synchronous dispatching on the main queue. And that's because we are asking the main queue to freeze until we execute the task within the curly braces... which we dispatched to the main queue, which we just froze. This is called deadlock. To see it in action in a playground:
DispatchQueue.main.sync { // stop the main queue and wait for the following to finish
print("hello world") // this will never execute on the main queue because we just stopped it
}
// deadlock
One last thing to mention is resources. When we give a queue a task, GCD finds an available queue from its internally-managed pool. As far as the writing of this answer, there are 64 queues available per qos. That may seem like a lot but they can quickly be consumed, especially by third-party libraries, particularly database frameworks. For this reason, Apple has recommendations about queue management (mentioned in the links below); one being:
Instead of creating private concurrent queues, submit tasks to one of
the global concurrent dispatch queues. For serial tasks, set the
target of your serial queue to one of the global concurrent queues.
That way, you can maintain the serialized behavior of the queue while
minimizing the number of separate queues creating threads.
To do this, instead of creating them like we did before (which you still can), Apple recommends creating serial queues like this:
let serialQueue = DispatchQueue(label: "serialQueue", qos: .default, attributes: [], autoreleaseFrequency: .inherit, target: .global(qos: .default))
For further reading, I recommend the following:
https://developer.apple.com/library/archive/documentation/General/Conceptual/ConcurrencyProgrammingGuide/Introduction/Introduction.html#//apple_ref/doc/uid/TP40008091-CH1-SW1
https://developer.apple.com/documentation/dispatch/dispatchqueue
