NSProgress lets you easily and accurately report the progress of work in your app. Understand the concepts behind progress reporting and how to design code that accurately and efficiently reports its progress. Gain specific insights on how to receive progress updates from framework APIs, fold that into your own progress reporting, and show that progress in your app. Hear from the experts about best practices and how to master the NSProgress APIs.
PhotoProgress: Using NSProgress
WWDC 2015 > App Frameworks > Best Practices for Progress Reporting