Have you ever wanted to alert other classes of your app of some event from another one? Sure, you could have your Objective-C class poll the other one and keep asking if something has changed, but that seems to be wasteful, since in many cases, there usually won’t be. You could even not bother with polling, and just read it back in all the time, but if there has been no change, that again is wasteful of processing time, power, ...
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