Does synchronous mean that by definition it is run on and called from separate threads, or does it have to be that way because of the way things are?

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Created Jul ’22

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Does an operation that is synchronous have to be run on a separate thread? I think so, otherwise it would deadlock right from the beginning, if it’s run on and called from the main thread. What about if it’s run on and called from a thread other than the main thread? Will it deadlock?

This question arose when I was reading the following content at

Documentation/Foundation/Task Management/Operation

Asynchronous Versus Synchronous Operations

When you add an operation to an operation queue, the queue ignores the value of the isAsynchronous property and always calls the start() method from a separate thread. Therefore, if you always run operations by adding them to an operation queue, there is no reason to make them asynchronous.

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ShinehahGnolaum OP

Jul ’22

Apple documentation says that Operations run synchronously. Why then does the code continue to run after an operation is added to a queue?

Here is my code:

let op = BlockOperation(block: { print("Done") })
                        
let qu = OperationQueue()
                        
qu.addOperation(op)

print("after!")

Here is the debug results:

after!

Done

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