print - Background or main thread operation

This might sound quite basic and stupid but it has been bothering me for a while. How can print be classified in terms of operation - main or background ?
As a small test, on putting print in a background task - web service call :

Webservice().loadHeadlinesForSource(source: source) { headlines in
            print("background print")
            self.headlineViewModels = headlines.map(HeadlineViewModel.init)

            DispatchQueue.main.async {
                print("main thread print")
                completion()
            }
        }  

Both the print statements get printed. From previous experience, if print was a main thread task, Xcode would have given me a warning saying that I need to put that in main thread. This is an evidence that print is not a main thread operation. Note that I am not saying print is a background task.
However, I have this understanding that since print displays output on Console, it is not a background operation. As a matter of fact all logging operations are not.
How would one justify the classification ?

1 answer

  • answered 2018-04-17 05:50 Matic Oblak

    It seems what you consider to be a main thread operation is a call that needs to be performed on the main thread. From that perspective you are correct and have found an evidence of this call not being a main thread operation.

    But does this have anything to do with anything else? Internally if needed this method may still execute its real operation on the main thread or any other thread for what we care. So in this sense a main thread operation is a restriction that call needs to be performed on main thread but has nothing to do with its execution or multithreading.

    Without looking into what print does in terms of coding we can see that it works across multiple "computers". You can run your app on your device (iPhone) while plugged and Xcode on your computer will print out logs. This makes a suspicion that print is much like call to the remote server in which case the server is responsible for serializing the events so it makes no difference what thread the client is on. There are other possibilities such as dropping logs into file and then sending it which really makes little difference.

    So How can print be classified in terms of operation - main or background? The answer is probably none. The call is not restricted to any thread so it is not main. It will probably lock whatever thread it is on until the operation is complete so it is not background either. Think of it like Data(contentsOf: <#T##URL#>) which will block the thread until data from given URL is retrieved (or exception is thrown).