????? 在mx的列表组件中，可以通过设置rowCount，但是到了spark中缺“丢失”该属性，看过Commbox的skin类之后，才知道rowCount其实layout中设置的。
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<!--- @private -->
<s:Scroller id="scroller" left="0" top="0" right="0" bottom="0" hasFocusableChildren="false" minViewportInset="1">
<!--- @copy spark.components.SkinnableDataContainer#dataGroup-->
<s:DataGroup id="dataGroup" itemRenderer="spark.skins.spark.DefaultItemRenderer">
<s:layout>
<s:VerticalLayout gap="0" horizontalAlign="contentJustify" requestedMinRowCount="1" requestedMaxRowCount="6"/>
</s:layout>
</s:DataGroup>
</s:Scroller>
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?? 从上面的代码也以看出，最大的rowCont是6，这就意味着如果列表组件的数据源长度大于6将会出现垂直滚动条。
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?? 有了上面的代码，我们可以对ComboBox或DropDownList组件设置布局layout，在layout中设置requestedRowCount就可以了。
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<s:DropDownList >
<s:layout>
<s:VerticalLayout requestedRowCount="10"/>
</s:layout>
<s:dataProvider>
<s:ArrayList>
<fx:Object label="AIR" icon="{imgCls}"/>
<fx:Object label="ColdFusion" icon="{imgCls2}"/>
<fx:Object label="Dreamweaver" icon="{imgCls}"/>
<fx:Object label="Flash" icon="{imgCls2}"/>
<fx:Object label="Flex" icon="{imgCls}"/>
<fx:Object label="Photoshop2" icon="{imgCls2}"/>
<fx:Object label="AIR2" icon="{imgCls}"/>
<fx:Object label="ColdFusion2" icon="{imgCls2}"/>
<fx:Object label="Dreamweaver2" icon="{imgCls}"/>
<fx:Object label="Flash2" icon="{imgCls2}"/>
<fx:Object label="Flex2" icon="{imgCls}"/>
<fx:Object label="Photoshop2" icon="{imgCls2}"/>
</s:ArrayList>
</s:dataProvider>
</s:DropDownList>
<s:ComboBox >
<s:layout>
<s:VerticalLayout requestedRowCount="10"/>
</s:layout>
<s:dataProvider>
<s:ArrayList>
<fx:Object label="AIR" icon="{imgCls}"/>
<fx:Object label="ColdFusion" icon="{imgCls2}"/>
<fx:Object label="Dreamweaver" icon="{imgCls}"/>
<fx:Object label="Flash" icon="{imgCls2}"/>
<fx:Object label="Flex" icon="{imgCls}"/>
<fx:Object label="Photoshop2" icon="{imgCls2}"/>
<fx:Object label="AIR2" icon="{imgCls}"/>
<fx:Object label="ColdFusion2" icon="{imgCls2}"/>
<fx:Object label="Dreamweaver2" icon="{imgCls}"/>
<fx:Object label="Flash2" icon="{imgCls2}"/>
<fx:Object label="Flex2" icon="{imgCls}"/>
<fx:Object label="Photoshop2" icon="{imgCls2}"/>
</s:ArrayList>
</s:dataProvider>
</s:ComboBox>
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