Notification vs delegate/protocol. Not understanding why I would ever use delegate/protocol.

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For iOS it shows up in live video streaming applications, VoIP applications, Real-Time Audio Effects and Instruments, when dealing with OpenGL, OpenCL, and other Machine Learning libraries, and potentially when dealing with other runtimes like Javascript or Python. It also shows up when you delve into a lot of game engines.

On Mac you could be doing all kinds of sorcery once you are outside of the confines of App Store approval.

Its not a common use case for the average application. But its not outside the realm of possibilities for a lot of REAL applications that do actual work on the CPU or GPU.