<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/01/15/expanding-nodes-in-a-flex-tree-control-using-the-openitems-property/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white" viewSourceURL="srcview/index.html">

    <mx:Script>
        <![CDATA[
            private function openAllNodes():void {
                tree.openItems = dp..node;
            }

            private function closeAllNodes():void {
                tree.openItems = [];
            }
        ]]>
    </mx:Script>

    <mx:XML id="dp">
        <root>
            <node label="Parent 1" isExpanded="true">
                <node label="Child 1" />
                <node label="Child 2" isOpen="true">
                    <node label="Grandchild 1" />
                    <node label="Grandchild 2" />
                </node>
                <node label="Child 3" />
                <node label="Child 4" />
            </node>
        </root>
    </mx:XML>

    <mx:ApplicationControlBar dock="true">
        <mx:Button label="Open all nodes" click="openAllNodes();" />
        <mx:Button label="Close all nodes" click="closeAllNodes();" />
    </mx:ApplicationControlBar>

    <mx:Tree id="tree"
            dataProvider="{dp}"
            showRoot="false"
            labelField="@label"
            width="200" />

</mx:Application>