Use the <setter> tag to declare or override attribute setter methods. For example:

  <setter name="y" args="y" >
    super.setAttribute('y', Math.max(parent.arrowbuttonheight-1 , Math.min( y , parent.height - (parent.thumbheight + parent.arrowbuttonheight))));
  </setter>

Here's another example.

  <text x="${Math.round((parent.width-this.width)/2)}" font="vera_super_bold" fgcolor="0x4d4d4d" datapath="@username" resize="true">
    <setter name="text">
      if (typeof(this.datapath.data) != 'undefined') {
      super.setAttribute('text', this.datapath.data + "'s information");
      }
    </setter>
  </text>

For a complete description of <setter>s, including how to update from previous versions of OpenLaszlo, see Section 2.2, “Attribute setters defined with <setter/>” of the Developer's Guide.