以下是错误:
TypeError: Error #1009: Cannot access a property or method of a null object reference. at mx.styles::StyleProtoChain$/initProtoChainForUIComponentStyleName()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\styles\StyleProtoChain.as:72] at mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::initProtoChain()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\UIComponent.as:7469] at mx.core::UIComponent/regenerateStyleCache()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\UIComponent.as:7690] at mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::addingChild()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\UIComponent.as:5239] at mx.core::UIComponent/addChild()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\UIComponent.as:4955] at mx.controls.listClasses::ListBase/createChildren()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\controls\listClasses\ListBase.as:3103] at mx.core::UIComponent/initialize()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\UIComponent.as:5370] at mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::childAdded()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\UIComponent.as:5267] at mx.core::Container/http://www.adobe.com/2006/flex/mx/internal::childAdded()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\Container.as:3305] at mx.core::Container/addChildAt()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\Container.as:2217] at mx.core::Container/addChild()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\Container.as:2140] at model::MessageBoard()[C:\Documents and Settings\dbabbitt\My Documents\Flex Builder 3\ListExample\src\model\MessageBoard.as:56] at model::MessageBoard$cinit() at global$init()[C:\Documents and Settings\dbabbitt\My Documents\Flex Builder 3\ListExample\src\model\MessageBoard.as:7] at main()[C:\Documents and Settings\dbabbitt\My Documents\Flex Builder 3\ListExample\src\main.mxml:56] at _main_mx_managers_SystemManager/create() at mx.managers::SystemManager/initializeTopLevelWindow()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\managers\SystemManager.as:3188] at 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] at mx.managers::SystemManager/docFrameListener()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\managers\SystemManager.as:2916]
这是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
也许你可以告诉我如何保持空对象引用不受复杂类的影响?
感谢名单
戴夫
答案 0 :(得分:2)
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。我建议覆盖createChildren()方法并在那里添加子元素。
override protected function createChildren():void
{
super.createChildren();
_messageTitle = new Label();
_messageTitle.text = "Message Board";
this.addChildAt(_messageTitle, 0);
}