AS3错误代码1009-无法访问空对象引用的属性或方法

Question

Yes, I'm sure there are millions of questions like this on SO, but this one has stumped me. Observe the code sample below:

<s:Group xmlns:fx="http://ns.adobe.com/mxml/2009" 
     xmlns:s="library://ns.adobe.com/flex/spark" 
     xmlns:mx="library://ns.adobe.com/flex/mx"
     creationComplete="init(event)">

<fx:Script>
    <![CDATA[
        import mx.events.FlexEvent;

        [Bindable]
        private var data:Object;

        private function init(e:FlexEvent):void {
            this.data.item = new Array();
        }
    ]]>
</fx:Script>
</s:Group>

Why can't I create an item array on the data object? I thought the Object class was innately declared as dynamic? Here is the error I get at runtime because of it:

TypeError: Error #1009: Cannot access a property or method of a null object reference.

Could someone please help me figure out how to create my own property on this object. Usually this is easy. lol

Thank you for your time.

Answer 1

The issue is that data has not been instantiated and is null.

<s:Group xmlns:fx="http://ns.adobe.com/mxml/2009" 
     xmlns:s="library://ns.adobe.com/flex/spark" 
     xmlns:mx="library://ns.adobe.com/flex/mx"
     creationComplete="init(event)">

<fx:Script>
    <![CDATA[
        import mx.events.FlexEvent;

        [Bindable]
        private var data:Object;

        private function init(e:FlexEvent):void {
            this.data = {};
            this.data.item = new Array();
        }
    ]]>
</fx:Script>
</s:Group>

Answer 2

Make sure data exists first.

private function init(e:FlexEvent):void {
   this.data = {};
   this.data.item = new Array();
}

Answer 3

Better still, you can do this:

private function init(e:FlexEvent):void {
    if(this.data == null) 
        this.data = {};
    this.data.item = new Array();
}

That way, whenever init is called, if data exists, it will not be overwritten with a blank object.

Answer 4

You are confusing the data type and the data instance. All classes derive from object though not all objects are dynamic.

In other words, if data is a data type that is not dynamic, then you can't set the item property. Classes in AS3 are defined as dynamic with the dynamic keyword. Anonymous objects are dynamic by default:

So, if you set this.data = {} then you will be able to set the item property.