Ensure UI updates to WatchKit Interface Controller only happen while the controller is active.

MyWKInterfaceController.swift

/*
  "WatchKit ignores attempts to set values of interface objects
  while your interface is inactive.... Modifications can be made
  only during initialization of your interface controller and
  between calls to willActivate and [didDeactivate]."
*/

class MyWKInterfaceController: WKInterfaceController {
    
    private var isActive:Bool = true
    private var uiUpdateQ:Array<() -> ()> = []
    
    // Perform any UI updates that may be taking 
    // place when the controller is inactive
    // self.updateUI { /* update UI elements */ }
    func updateUI(updates:() -> ()) {
        if self.isActive {
           if !NSThread.isMainThread() {
              dispatch_async(dispatch_get_main_queue()) {
                 self.updateUI(updates)
              }
           }
           else {
              updates()
           }
        }
        else {
            self.uiUpdateQ.extend([updates])
        }
    }

    override func willActivate() {
        super.willActivate()
        self.isActive = true
        for updates in self.uiUpdateQ {
            updates()
        }
        self.uiUpdateQ = []
    }

    override func didDeactivate() {
        super.didDeactivate()
        self.isActive = false
    }
}