In my Notification Center Today Extension/Widget I need to update a portion of the UI every time Notification Center is activated. It never needs to update while Notification Center is in use, nor while it's in the background. In what method should I place that code?
viewDidLoad and viewWillAppear are both called every time it will be displayed, for example if you scroll up and down they will be called again, so that's too often.
widgetPerformUpdateWithCompletionHandler is not called at all before it's displayed for the first time it seems (at least with iOS 8.2 beta), and this method is automatically called whenever iOS feels like it to update the UI even when it's in the background which is not appropriate either.
loadView is only called a single time, never to be called again unless the widget is removed from memory. So if you open Notification Center and view the widget then dismiss Notification Center and reopen it later, it may not call that method again depending on whether or not it's been cleared from memory.
I'd just use viewDidLoad and not worry about the possibility of multiple calls. Unless the method takes a long time to run, there's no reason not to do it that way. [And if it does take a long time to run, your today extension is going to suck, so fix that.]
If for some reason you only want it to happen once, add a BOOL ivar to the class. Set it to YES in initWithCoder: Then in viewDidLoad, check that value. If it's YES, do your update and set the value to NO. If it's already NO, skip your update.