<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/10/02/positioning-controls-in-flex-3-using-constraint-columns/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white" viewSourceURL="srcview/index.html">

    <mx:ApplicationControlBar dock="true">
        <mx:Form>
            <mx:FormItem label="col1 width:">
                <mx:HSlider id="slider1"
                        minimum="50"
                        maximum="250"
                        value="200"
                        liveDragging="true"
                        snapInterval="1"
                        tickInterval="20"
                        dataTipPrecision="0" />
            </mx:FormItem>
            <mx:FormItem label="col2 width:">
                <mx:HSlider id="slider2"
                        minimum="50"
                        maximum="250"
                        value="200"
                        liveDragging="true"
                        snapInterval="1"
                        tickInterval="20"
                        dataTipPrecision="0" />
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

    <mx:Canvas width="100%">
        <mx:constraintColumns>
            <mx:ConstraintColumn id="col1" width="{slider1.value}" />
            <mx:ConstraintColumn id="col2" width="{slider2.value}" />
        </mx:constraintColumns>

        <mx:Button label="OK" left="col1:0" right="col1:0" />
        <mx:Button label="Cancel" left="col2:0" right="col2:0"/>
    </mx:Canvas>

</mx:Application>