custom controls : How to force redraw ?

Hi,

I'm playing with custom controls, this sample shows how to move squares in an image. The only way to force redraw I found is to hide then to show the control. Is there a nicer solution ?

var myWindow = new Window ("dialog", "Draw squares");

myWindow.add ("button",undefined,"cancel")

var CompoPreview  = myWindow.add ("image", undefined)
CompoPreview.size = [200,200];
CompoPreview.onDraw = generatePreviewPhoto

var myEvent = new UIEvent("resize");

var MySlider = myWindow.add ("slider", undefined, 50, 0, 100);
var coef=MySlider.value

MySlider.onChanging = function ()
{
    coef = this.value;
    CompoPreview.hide()
    CompoPreview.show()
}

function generatePreviewPhoto(){
   
var x,y,l,h,idx,gl;


nbPhotos = 5;
for (idx=0;idx < nbPhotos;idx++)
    {
        y = x = idx*coef    ;
        l= 30;
        h= 30;
        gl  = 1-idx/(nbPhotos-1);
       
        this.graphics.newPath();
        this.graphics.rectPath(x,y,l,h);
        this.fillBrush = this.graphics.newBrush( this.graphics.BrushType.SOLID_COLOR,[gl,gl,gl,1] );
         this.graphics.fillPath(this.fillBrush);
    }

}

myWindow.show ();