Subclassing NSControl, IBAction is not called in Swift

I've subclassed an NSSlider that behaves differently whether the option key is pressed. In order to do that, I overrode the mouseDown func. It seems to do the job. The thing is, I've noticed the connected @IBAction in my ViewController is only triggered when the option key is unpressed (i.e. when the mouseDown method is passed to super). What am I missing in order to allow the @IBAction to perform?

Many thanks Besides the issue, improvement advices on the code are welcome... :-)

Josh

class ViewController: NSViewController {

    @IBOutlet weak var theSlider: MySlider!
    @IBAction func moveSlider(sender: NSSlider) {
        print(sender.floatValue) //works only with optionKey unpressed
    }
}

class MySlider: NSSlider { //Implemented in another source file

    @IBInspectable var multiplier: Float = 0.5
    private var modifierKeys = NSEventModifierFlags.AlternateKeyMask
    private var optionKeyPressed = false
    private var previousSliderPosition: Float = 0.0

//MARK: Init with NSCoder
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        Swift.print("init Coder called")

        self.continuous = true
        NSEvent.addLocalMonitorForEventsMatchingMask(.FlagsChangedMask) { (theEvent) -> NSEvent? in
            self.flagsChanged(theEvent)
            return theEvent
        }
    }

//MARK: Mouse tracking
    override func mouseDown(theEvent: NSEvent) {

        if optionKeyPressed {

            var keepOn = true
            previousSliderPosition = self.floatValue * Float(self.bounds.width) / Float(self.maxValue)

            while keepOn {
                if let nextEvent = self.window?.nextEventMatchingMask(Int(NSEventMask.LeftMouseUpMask.rawValue) | Int(NSEventMask.LeftMouseDraggedMask.rawValue))
                {
                    switch nextEvent.type
                    {
                    case .LeftMouseDragged:
                         let mouseInnerLocationX = Float(self.convertPoint(nextEvent.locationInWindow, fromView: self.superview).x)
                        let mouseDelta = mouseInnerLocationX - previousSliderPosition
                        let newSliderPosition = previousSliderPosition + (mouseDelta) * multiplier

                        self.floatValue = newSliderPosition * Float(self.maxValue) / Float(self.bounds.width)
                        break

                    case .LeftMouseUp:
                        keepOn = false
                        break

                    default:
                        break
                    }
                }
            }
        } else {
            super.mouseDown(theEvent)
        }
    }

//MARK: Option key handling
    override func flagsChanged(theEvent: NSEvent) {
        if (theEvent.modifierFlags.rawValue & NSEventModifierFlags.DeviceIndependentModifierFlagsMask.rawValue) == NSEventModifierFlags.AlternateKeyMask.rawValue {
            optionKeyPressed = true
        } else {
            optionKeyPressed = false
        }
    }
}

If you're not calling super.mouseDown, you need to send the action yourself:

sendAction(action, to: target)

sendAction(_:to:), action and target are existing members of NSControl.