Flex4中的皮肤（4）：使用SkinPart约束Skin 小宝马的爸爸

在SkinnableComponent中，可以声明SkinPart元标签对Skin进行约束，同时在组件中提供对Skin元素的引用：

[SkinPart(required="false")]
public var labelElement:SimpleText;

[SkinPart(required="false")] public var labelElement:SimpleText;

如果声明了SkinPart并且required="true"，则Skin中必须包含该类型灯元素并且具有相同的id：

<s:SimpleText id="labelElement" .../>

SkinnableComponent中还提供了partAdded和partRemoved方法：

//--------------------------------------------------------------------------
    //
    //  Methods - Parts
    //
    //--------------------------------------------------------------------------

    /**
     *  Called when a skin part is added.
     *  You do not call this method directly.
     *  For static parts, Flex calls it automatically when it calls the <code>attachSkin()</code> method.
     *  For dynamic parts, Flex calls it automatically when it calls
     *  the <code>createDynamicPartInstance()</code> method.
     *
     *  <p>Override this function to attach behavior to the part.
     *  If you want to override behavior on a skin part that is inherited from a base class,
     *  make sure that you do not call the <code>super.partAdded()</code> method.
     *  Otherwise, you should always call the <code>super.partAdded()</code> method.</p>
     *
     *  @param partname The name of the part.
     *
     *  @param instance The part.
     *
     *  @langversion 3.0
     *  @playerversion Flash 10
     *  @playerversion AIR 1.5
     *  @productversion Flex 4
     */
    protected function partAdded(partName:String, instance:Object):void
    {
    }
    /**
     *  Called when an instance of a skin part is being removed.
     *  You do not call this method directly.
     *  For static parts, Flex calls it automatically when it calls the <code>detachSkin()</code> method.
     *  For dynamic parts, Flex calls it automatically when it calls
     *  the <code>removeDynamicPartInstance()</code> method.
     *
     *  <p>Override this function to remove behavior from the part.</p>
     *
     *  @param partname The name of the part.
     *
     *  @param instance The part.
     *
     *  @langversion 3.0
     *  @playerversion Flash 10
     *  @playerversion AIR 1.5
     *  @productversion Flex 4
     */
    protected function partRemoved(partName:String, instance:Object):void
    {
    }

//-------------------------------------------------------------------------- // // Methods - Parts // //-------------------------------------------------------------------------- /** * Called when a skin part is added. * You do not call this method directly. * For static parts, Flex calls it automatically when it calls the <code>attachSkin()</code> method. * For dynamic parts, Flex calls it automatically when it calls * the <code>createDynamicPartInstance()</code> method. * * <p>Override this function to attach behavior to the part. * If you want to override behavior on a skin part that is inherited from a base class, * make sure that you do not call the <code>super.partAdded()</code> method. * Otherwise, you should always call the <code>super.partAdded()</code> method.</p> * * @param partname The name of the part. * * @param instance The part. * * @langversion 3.0 * @playerversion Flash 10 * @playerversion AIR 1.5 * @productversion Flex 4 */ protected function partAdded(partName:String, instance:Object):void { } /** * Called when an instance of a skin part is being removed. * You do not call this method directly. * For static parts, Flex calls it automatically when it calls the <code>detachSkin()</code> method. * For dynamic parts, Flex calls it automatically when it calls * the <code>removeDynamicPartInstance()</code> method. * * <p>Override this function to remove behavior from the part.</p> * * @param partname The name of the part. * * @param instance The part. * * @langversion 3.0 * @playerversion Flash 10 * @playerversion AIR 1.5 * @productversion Flex 4 */ protected function partRemoved(partName:String, instance:Object):void { }

在增加或删除SkinPart时会调用这些方法。

通过重写这些方法可以对SkinPart进行额外的操作，比如增加SkinPart时为其添加事件监听：

override protected function partAdded(partName:String, instance:Object) : void
{
       super.partAdded(partName, instance);
       if (instance == labelElement)
           labelElement.addEventListener(...);
}