There is a class inherited from the NSSliderCell, with an overridden method drawBar(inside aRect: NSRect, flipped: Bool). I need, apparently, to call this method every time you change just added by me in the child class parameter.
In didSet'e this parameter cause the aforesaid method, but it doesn't work. The slider is updated (redrawn) only when changes in the values of its progress (floatValue or doubleValue), when you change the position and knob'. Certainly, even when you set maxValue - not tested. There are of course wild workaroud is to change self.doubleValue some tiny value here and there. Then the method is run, apparently, from didSet'and this variable.
But this, of course, is not the case.