显示/隐藏ComboBox控件的滚动条提示.

ComboBox控件的dropdown 属性中的showScrollTips属性.
示例：

代码：

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/07/01/displaying-scroll-tips-on-a-combobox-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="top"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            private function init():void {
                checkBox.selected = comboBox.dropdown.showScrollTips;
            }

            private function checkBox_change(evt:Event):void {
                comboBox.dropdown.showScrollTips = checkBox.selected;
            }
        ]]>
    </mx:Script>

    <mx:Array id="arr">
        <mx:Object label="One" />
        <mx:Object label="Two" />
        <mx:Object label="Three" />
        <mx:Object label="Four" />
        <mx:Object label="Five" />
        <mx:Object label="Six" />
        <mx:Object label="Seven" />
        <mx:Object label="Eight" />
        <mx:Object label="Nine" />
        <mx:Object label="Ten" />
    </mx:Array>

    <mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="showScrollTips:">
                <mx:CheckBox id="checkBox"
                        change="checkBox_change(event);" />
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

    <mx:ComboBox id="comboBox"
            dataProvider="{arr}"
            creationComplete="init();" />

</mx:Application>

也可以通过AS实现:

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/07/01/displaying-scroll-tips-on-a-combobox-control-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="top"
        backgroundColor="white"
        creationComplete="init();">

    <mx:Script>
        <![CDATA[
            import mx.containers.ApplicationControlBar;
            import mx.containers.Form;
            import mx.containers.FormItem;
            import mx.controls.ComboBox;
            import mx.controls.CheckBox;

            private var arr:Array;
            private var checkBox:CheckBox;
            private var comboBox:ComboBox;

            private function init():void {
                arr = [];
                arr.push({label:"One"});
                arr.push({label:"Two"});
                arr.push({label:"Three"});
                arr.push({label:"Four"});
                arr.push({label:"Five"});
                arr.push({label:"Six"});
                arr.push({label:"Seven"});
                arr.push({label:"Eight"});
                arr.push({label:"Nine"});
                arr.push({label:"Ten"});

                checkBox = new CheckBox();
                checkBox.addEventListener(Event.CHANGE, checkBox_change);

                var formItem:FormItem = new FormItem();
                formItem.label = "showScrollTips:";
                formItem.addChild(checkBox);

                var form:Form = new Form();
                form.styleName = "plain";
                form.addChild(formItem);

                var appControlBar:ApplicationControlBar;
                appControlBar = new ApplicationControlBar();
                appControlBar.dock = true;
                appControlBar.addChild(form);
                Application.application.addChildAt(appControlBar, 0);

                comboBox = new ComboBox();
                comboBox.dataProvider = arr;
                addChild(comboBox);

                callLater(delayedInit);
            }

            private function delayedInit():void {
                checkBox.selected = comboBox.dropdown.showScrollTips;
            }

            private function checkBox_change(evt:Event):void {
                comboBox.dropdown.showScrollTips = checkBox.selected;
            }
        ]]>
    </mx:Script>

</mx:Application>