Alert with Shake to Undo

I want to implement a shake to undo on my iPhone App, in Swift.

For now it works, but it didn't display an Alert asking to validate the undo gesture (with buttons "Cancel" and "Undo"). I can add this alert in the right place myself, but I'm not certain I should. Some articles make me think that the alert should appear automatically, in the undo/redo process, so there's something I missed, perhaps...

Here are the relevant bits of code

In the viewController

override func becomeFirstResponder() -> Bool {
    return true
}

and

override func motionEnded(_ motion: UIEvent.EventSubtype, with event: UIEvent?) {
   if motion == .motionShake {
       snowflake.undoLastAction()
   }
}

and in my snowflake class, the action is inserting or modifying point in an array, so I store the value before the change in oldPathPoints, then

   // Action du undo
   undoManager?.registerUndo(withTarget: self, handler: {   (targetSelf) in
        snowflake.pathPoints = oldPathPoints
    })

and finally the undo method

func undoLastAction() {
    undoManager?.undo()
    createPath()
    }

So,

I succeeded to do it (with the help of a 5-years Book from Matt Neuburg!) So, there's nothing to make in the viewController, I deleted becomeFirstResponder and motionEnded.

All is in the SnowFlake class: I added

    let undoer = UndoManager()

    override var undoManager : UndoManager? {
       return self.undoer
    }
    override var canBecomeFirstResponder: Bool { return true }

So I communicate the undoManager I use and the class (a view here) can become firstResponder

after the manipulation of the view that can be undone (in my case in touchesEnded) , I call

    self.becomeFirstResponder()

I deleted the undoLastAction function (useless now), and changed the undo action to a fonction

    changePathUndoable(newPathPoints: finalPathPoints)

the function is like that

    func changePathUndoable(newPathPoints: [CGPoint]) {
        let oldPathPoints = Snowflake.sharedInstance.pathPoints
        Snowflake.sharedInstance.pathPoints = newPathPoints
        // Action du undo
        undoer.setActionName("cutting")
        undoer.registerUndo(withTarget: self, handler: {   (targetSelf) in
            targetSelf.changePathUndoable(newPathPoints: oldPathPoints)
        })

        Snowflake.sharedInstance.createPath()
        self.setNeedsDisplay()
     }

I had to create the function changePathUndoable instead of calling undoer.registerUndo directly just for having the redo action to work (I didn't test redo before).

I think that all the steps are necessary to do what I wanted: make the view receive the shake event, displays this dialog and be able to undo/redo the modifications.