Full code after the jump.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/09/05/creating-fixed-width-buttons-in-a-flex-buttonbar-component/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            private function slider_change(evt:Event):void {
                if (checkBox.selected) {
                    buttonBar.setStyle("buttonWidth", slider.value);
                } else {
                    buttonBar.setStyle("buttonWidth", NaN);
                }
            }
        ]]>
    </mx:Script>

    <mx:ApplicationControlBar dock="true">
        <mx:CheckBox id="checkBox"
                label="fixed width:"
                labelPlacement="left"
                change="slider_change(event)" />

        <mx:Spacer width="50" />

        <mx:Label text="buttonWidth:" />
        <mx:HSlider id="slider"
                enabled="{checkBox.selected}"
                minimum="50"
                maximum="150"
                value="75"
                liveDragging="true"
                snapInterval="5"
                dataTipPrecision="0"
                tickInterval="10"
                change="slider_change(event)" />
    </mx:ApplicationControlBar>

    <mx:ButtonBar id="buttonBar">
        <mx:dataProvider>
            <mx:Array>
                <mx:Object label="One" />
                <mx:Object label="Two" />
                <mx:Object label="Thirteen" />
            </mx:Array>
        </mx:dataProvider>
    </mx:ButtonBar>

</mx:Application>

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