Understanding and Implementing NSNotificationCenter on Apple's Platforms
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Sometimes when you are writing an app, you need to be notified of events occurring somewhere else in the system - either in your own app, or in the operating system - and react to them accordingly. For example, you may be an app like Snapchat and you want to know when a screenshot has been taken. This is a system notification that you can “listen” to in order to react. If you have an app like a photo gallery, you may need to know when the user adds a new photo so you can update all relevant UI and make other necessary updates.