https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/Multithreading/ThreadSafety/ThreadSafety.html#//apple_ref/doc/uid/10000057i-CH8-SW3
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/Multithreading/ThreadSafety/ThreadSafety.html#//apple_ref/doc/uid/10000057i-CH8-SW3