如何摆脱空的 StyleProtoChain 对象引用？

Question

这是错误：

TypeError：错误 #1009：无法访问空对象引用的属性或方法。 在 mx.styles::StyleProtoChain$/initProtoChainForUIComponentStyleName()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\styles\StyleProtoChain.as:72] 在 mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::initProtoChain()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\UIComponent.as:7469] 在 mx.core::UIComponent/regenerateStyleCache()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\UIComponent.as:7690] 在 mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::addingChild()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\UIComponent.as:5239] 在 mx.core::UIComponent/addChild()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\UIComponent.as:4955] 在 mx.controls.listClasses::ListBase/createChildren()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\controls\listClasses\ListBase.as:3103] 在 mx.core::UIComponent/initialize()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\UIComponent.as:5370] 在 mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::childAdded()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\UIComponent.as:5267] 在 mx.core::Container/http://www.adobe.com/2006/flex/mx/internal::childAdded()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\Container.as:3305] 在 mx.core::Container/addChildAt()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\Container.as:2217] 在 mx.core::Container/addChild()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\Container.as:2140] 在模型::MessageBoard()[C:\Documents and Settings\dbabbitt\My Documents\Flex Builder 3\ListExample\src\model\MessageBoard.as:56] 在模型::MessageBoard$cinit() 在 global$init()[C:\Documents and Settings\dbabbitt\My Documents\Flex Builder 3\ListExample\src\model\MessageBoard.as:7] 在 main()[C:\Documents and Settings\dbabbitt\My Documents\Flex Builder 3\ListExample\src\main.mxml:56] 在 _main_mx_managers_SystemManager/create() 在 mx.managers::SystemManager/initializeTopLevelWindow()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\managers\SystemManager.as:3188] 在 mx.managers::SystemManager/http://www.adobe.com/2006/flex/mx/internal::docFrameHandler()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\managers\SystemManager.as:3064] 在 mx.managers::SystemManager/docFrameListener()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\managers\SystemManager.as:2916]

这里是 main.mxml：

<?xml version="1.0"?> <mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" > <mx:Script> <![CDATA[ import model.MessageBoard; public var _messageBoard:MessageBoard = MessageBoard.instance; ]]> </mx:Script> </mx:Application> 这是 MessageBoard.as：

package model {
    import mx.containers.VBox;
    import mx.controls.Label;

    [Bindable]
    public class MessageBoard extends VBox {

        /** The message list title. */
        private var _messageTitle:Label;

        /** The maximum message count. */
        private var _maxMessageCount:int = 4;

        /** Storage for the singleton instance. */
        private static const _instance:MessageBoard = new MessageBoard( SingletonLock );

        /** Provides singleton access to the instance. */
        public static function get instance():MessageBoard {
            return _instance;
        }

        /**
         * Constructor
         * 
         * @param lock The Singleton lock class to pevent outside instantiation.
         */
        public function MessageBoard( lock:Class ) {
            super();

            // Verify that the lock is the correct class reference.
            if ( lock != SingletonLock ) {
                throw new Error( "Invalid Singleton access.  Use MessageBoard.instance." );
            }

            _messageTitle = new Label();
            _messageTitle.text = "Message Board";
            this.addChild(_messageTitle);
        }

    } // end class
} // end package

class SingletonLock {
} // end class

也许你可以教我如何将空对象引用排除在复杂的类之外？

感谢

戴夫

Answer 1

看起来错误可能来自对 MessageBoard 类的构造函数中的 addChild() 方法的调用。您很可能在 UIComponent 生命周期中过早地调用此方法。我建议重写 createChildren() 方法并在那里添加您的子元素。

override protected function createChildren():void
{
    super.createChildren();
    _messageTitle = new Label();
    _messageTitle.text = "Message Board";
    this.addChildAt(_messageTitle, 0);
}