How to make a Button continually call a function w

I'm making a game using sprite kit and I want my Character to move across the screen when you hold down the left/right move button. The problem is that he only moves when the button is tapped, not held. I have looked everywhere for a solution but nothing seems to work!

Here's my code;

class Button: SKNode
{
   var defaultButton: SKSpriteNode // defualt state
   var activeButton: SKSpriteNode  // active state

   var timer = Timer()

   var action: () -> Void

   //default constructor
   init(defaultButtonImage: String, activeButtonImage: String, buttonAction: @escaping () -> Void )
   {
      //get the images for both button states
      defaultButton = SKSpriteNode(imageNamed: defaultButtonImage)
      activeButton = SKSpriteNode(imageNamed: activeButtonImage)

      //hide it while not in use
      activeButton.isHidden = true 
      action = buttonAction

      super.init()

      isUserInteractionEnabled = true

      addChild(defaultButton)
      addChild(activeButton)    
   }

   //When user touches button
   override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?)
   {
      action()

      //using timer to repeatedly call action, doesnt seem to work...
      self.timer = Timer.scheduledTimer(timeInterval: 0.1, target: self, selector: #selector(getter: Button.action), userInfo: nil, repeats: true)

      //swtich the image of our button
      activeButton.isHidden = false
      defaultButton.isHidden = true

   }

   code..........

In my game scene...

// *** RIGHT MOVEMENT ***
      let rightMovementbutton = Button(defaultButtonImage: "arrow", activeButtonImage: "arrowActive", buttonAction:
      {

         let moveAction = SKAction.moveBy(x: 15, y: 0, duration: 0.1)
         self.player.run(moveAction)


      })

You know when the button is touched because touchesBegan is called. You then have to set a flag to indicate that the button is pressed.

override func touchesBegan(touches: Set<UITouch>, withEvent event: UIEvent?) {
    let touch = touches.first!
    if leftButton.containsPoint(touch.locationInNode(self)) {
        leftButtonIsPressed = true
    }
    if rightButton.containsPoint(touch.locationInNode(self)) {
        rightButtonIsPressed = true
    }
}

In update(), call your function that flag is true:

update() {
   if leftButtonIsPressed == true {
        moveLeft()
    }

   if rightButtonIsPressed == true {
        moveRight()
    }

}

You set the flag to false when touchesEnded is called for that button:

override func touchesEnded(touches: Set<UITouch>, withEvent event: UIEvent?) {
    let touch = touches.first!
    if leftButton.containsPoint(touch.locationInNode(self)) {
        leftButtonIsPressed = false
    }
    if rightButton.containsPoint(touch.locationInNode(self)) {
        rightButtonIsPressed = flase
    }
}

Edit:

As pointed out by KoD, a cleaner way to do this (for movement buttons) is with SKAction which removes the need for the flag:

Define SKActions for moveTo x:0 and moveTo x:frame.width in didMoveTo(View:)
In touchesBegan, run the correct SKAction on the correct object specifying a key for the SKAction.
In touchesEnded, remove the relevant SKAction.

You'll have to do some maths to calculate how many points your object will have to move and then set a duration for the SKAction based upon this distance and a movement speed (in points per second).

Alternatively, (thanks to KnightOfDragons for this) create a SKAction.MoveBy x: which moves a small distance (based upon your desired movement speed) and with a duration of 1/60s. Repeat this action forever (SKAction.repeatForever) when the button is touched and remove the repeating SKAction when the button is released.