“Allowing third-party apps to communicate with other apps is just one of the problems extensions are meant to solve — third-party keyboards, connecting apps to cloud services other than iCloud, and the new Notification Center widgets are all their own kind of extensions,” Cunningham reports. “Not all parts of iOS can be changed (or ‘extended’) by third parties. If you wanted to replace one of the default apps with your own or add some kind of toggle to the Control Center, you can’t do that. Apple defines a handful of pre-set ‘extension points’ to show developers where they can add stuff.”
“A simple way to summarize all of this: Apple doesn’t want one app to be able to get into another app’s sandbox,” Cunningham reports. “Extensions are like little sandboxes-within-sandboxes that facilitate communication between different apps while never sharing all of their containing app’s data directly with the host app.”
Much more in the full article here.