iOS blocks are called on what thread?

Question

I'm learning about blocks from a Stanford video. I'm now at the part which explains core data. The teachers mentions something about:

- (void)openWithCompletionHandler:(void (^)(BOOL success))completionHandler;

He said that completionhandler block will be called in the thread which called the method. So basically the method runs async but the blocks get called on the thread, lets assume main.

So my question is do all blocks run on the thread from where the method call was made. To illustrate why I ask this question, I have a Async class which does request to a server.

The format of all these methods is like this:

- (void) getSomething:(id <delegateWhatever> const)delegate{
   goto background thread using GCD..
   Got result from server...
   Go back to main thread and call the delegate method...
}

When I use blocks I do not need to worry about going back to main thread if they will be called where the call was made?

Hope this is clear,

Thanks in advance

Answer 1

If something runs asynchronously, you should read a documentation to know on which thread, e.g. the completion block will be executed. If it is your code, you are in charge here, you can use global GCD queues, you can create your own queue and execute it there or whatever...

In general, blocks behaves like a function or a method call, it is executed on thread, which calls it. It is even possible that the same block will be executed from 2 different threads at the same time.

And just to be clear: Even if you are using blocks, you need to care about going back to main thread, of course if it is necessary

Answer 2

Nothing forces blocks to be called on a particular thread, so it depends on the specific method whether or not you need to worry about its callback being on the main thread. (In practice I don't remember ever seeing a library where a method called on the main thread would not call its completion handler also on the main thread. But you still need to read the documentation of the specific library and method you are using, as always.)