问题
In my main code(in order to show UIActivityIndicatorView) I'm calling a function, foo() on a background thread. What happens to the functions called by foo() in turn? Will those functions also be called and executed in the same background thread?
回答1:
Yes, it will also be called in that same background thread.
It's actually pretty easy: everything you call will be run in the same thread as the caller, unless you explicitly use methods to create a new thread and run a method there or cause a method to be run in another thread (e.g. performSelectorOnMainThread:withObject:waitUntilDone:). But from those methods on it's the same again: they will be run in the very same thread as their callers.
来源：https://stackoverflow.com/questions/5151521/calling-a-function-on-a-background-thread-ios-what-happens-to-the-nested-functio