         
         var constraints: [NSLayoutConstraint] = []
         
-        constraints.append(contentsOf: self.bar.setup(leading: self.barLeading,
+        constraints.append(contentsOf: self.bar.setup(leftKnob: self.leftKnob,
+                                                      rightKnob: self.rightKnob,
+                                                      leading: self.barLeading,
                                                       trailing: self.barTrailing,
                                                       height: self.barHeight))
         

        ]
        
        return barConstraints
    }
}


     
     - returns: an arrays of constraints to be activated.
     */
    fileprivate func setup(leftKnob: Knob,
                           rightKnob: Knob,
                           leading: CGFloat,
                           trailing: CGFloat,
                           height: CGFloat) -> [NSLayoutConstraint] {
        
        self.leftKnob = leftKnob
        self.rightKnob = rightKnob
        
        self.translatesAutoresizingMaskIntoConstraints = false
        
