dispatch_once_t cannot be an instance variable

[steipete]

BOXAPIMultipartToJSONOperation.m uses dispatch_once_t as an instance variable which does not work.

https://github.com/box/box-ios-sdk/blob/master/BoxContentSDK/BoxContentSDK/Operations/BOXAPIMultipartToJSONOperation.m#L368

This StackOverflow question is a surprisingly good an thorough analysis why this doesn't work:
http://stackoverflow.com/questions/13856037/can-i-declare-dispatch-once-t-predicate-as-a-member-variable-instead-of-static/19845164#19845164

Greg Parker, who works on the Objective-C Runtime at Apple.

dispatch_once_t must not be an instance variable. The implementation of dispatch_once() requires that the dispatch_once_t is zero, and has never been non-zero. The previously-not-zero case would need additional memory barriers to work correctly, but dispatch_once() omits those barriers for performance reasons.

Xcode 8.3 Beta 1 now has a new Analyzer Warning step which we found while working on Box integration for PSPDFKit's PDF Viewer for iOS: https://pdfviewer.io

I'll follow up with a PR that fixes this.

[boxtcarpel]

Thanks, @steipete! Nice catch.