As I understand there are 3 types of DispatchQueue in swift:
- Main (serial) (Main Thread)
- Global (Concurrent) (Background Threads working in parallel)
- Custom (Concurrent or serial)
And each one maybe work (asynch or synch)
First question:
Is it main queue working on UI thread only and not working on another thread?
If the answer yes , how DispatchQueue.Main.async not blocking UI thread.
If the answer No , what is the benefit of using DispatchQueue.global as long as DispatchQueue.Main.async work in another thread.
Second question:
what is deference between DispatchQueue.global (async) and DispatchQueue.global (sync) as long as this queue working Concurrent and where to use each one?
Third question:
what is defference between
- (serial and sync)
- (Concurrent and async)
As I understand:
Queue is not Thread
Main and global queue may work in same thread
Dispatched: means put task in queue
If Global queue dispatched in Main queue as sync , the dispatched task will work on same thread of Main queue and dispatched task added to Global queue , And this task will freezing the thread
If Global queue dispatched in Main queue as async , the dispatched task will work on other thread of Main queue and dispatched task added to Global queue , And this task will not freezing the thread
If Main queue dispatched in Main queue as async , the dispatched task will work on same thread of Main queue
If Main queue dispatched in Main queue as sync will make exception because make deadlock
Dispatch.sync: put task in queue and wait it until finish
Dispatch.async: put task in queue and not wait it until finish (The task may work in same thread or in another thread)
If task dispatched on Global queue and this accord from Main thread then the task will added to Global queue , and new thread will be create and the task will start working immediately in the new thread
If task dispatched on Main queue and this accord from Main thread then the task will added to Main queue , and will not work immediately until older tasks in queue finish working (because Main queue is sequential )
DispatchQueue's do not correspond to a single thread directly. The only restriction is that you are only allowed to access the UI from the main thread, which can be done through DispatchQueue.main. However, there's no guarantee that the system will dispatch your execution block to a specific thread if you call it on a specific queue.
DispatchQueue.async is a non-blocking operation, so you can execute several code blocks asynchronously on the same queue without blocking a specific thread, this is why you should always dispatch operations to the main queue asynchronously, to avoid blocking UI updates, since the main queue is solely responsible for UI related tasks. Calling async on any queue, does not guarantee that the execute will happen on a specific thread (be it background or main), it only guarantees that the operation will be executed in a non-blocking manner.
DispatchQueue.sync is a blocking operation, meaning that while a single sync code block is being executed, no other piece of code can be executed on the specific DispatchQueue, so if you dispatch a code block to the main queue synchronously, you will block UI updates and hence your app will freeze.
来源：https://stackoverflow.com/questions/52605016/difference-between-dispatchqueue-types-in-swift