Adding scrollbars using ActionScript3

Having some problems adding the default spark.components.VScrollBar scrollbar. All I'm doing is creating a new VScrollBar, setting it's coordinates, a width and height and doing an addChild but it's not showing up. I'm adding other components too that do show up in the same code so that shouldn't not be the problem.

What might I be missing?

开发者_开发百科var a:VScrollBar = new VScrollBar();
a.x = 100;
a.y = 100;
a.height = 500;
a.width = 100;
addChild(a);

i think you should define the viewport property