Difference between DispatchQueue.main.async and DispatchQueue.main.sync

user_8023423 0
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I have been using DispatchQueue.main.async for a long time to perform UI related operations.



Swift provides both DispatchQueue.main.async and DispatchQueue.main.sync, and both are performed on the main queue.



Can anyone tell me the difference between them? 

When should I use each?



DispatchQueue.main.async {
    self.imageView.image = imageView
    self.lbltitle.text = ""

}

DispatchQueue.main.sync {
    self.imageView.image = imageView
    self.lbltitle.text = ""
}
swift3 swift concurrency multithreading ios
4 Answers

GCD allows you to execute a task synchronously or asynchronously[About]

synchronous(block and wait) function returns a control when the task will be completed

asynchronous(dispatch and proceed) function returns a control immediately, dispatching the task to start to an appropriate queue but not waiting for it to complete.

[DispatchQueue]

user_47708770

sync or async methods have no effect on the queue on which they are called.

sync will block the thread from which it is called and not the queue on which it is called. It is the property of DispatchQueue which decides whether the DispatchQueue will wait for the task execution (serial queue) or can run the next task before current task gets finished (concurrent queue).

So even when DispatchQueue.main.async is an async call, a heavy duty operation added in it can freeze the UI as its operations are serially executed on the main thread. If this method is called from the background thread, control will return to that thread instantaneously even when UI seems to be frozen. This is because async call is made on DispatchQueue.main

user_9660860
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