Is [NSOperationQueue mainQueue] guaranteed to be serial?

Question

That is, if we queue the same thing several time there will be no concurrency. The one we queued first will be executed first.

I mean there is only one main thread right?

Answer 1

I have found a nice answer here:

NSOperationQueue and concurrent vs non-concurrent

So make all added operations serial you can always set:

[[NSOperationQueue mainQueue] setMaxConcurrentOperationCount:1];

Answer 2

And the answer is... YES and NO

when you create a new NSOperation to add to your queue, you can use

- (void)setQueuePriority:(NSOperationQueuePriority)priority

according to the documentation, the queue will use this priority, and other factors as inter dependency to decide what operation will be executed next.

As long as your operations have the same priority and no inter-operation dependencies, they should be executed in the same order you added them, maybe with other, system related operations, inserted between them.