编译器警告我绑定不起作用，但为什么我运行它确实起作用的应用程序！

Question

下面的 Flex 应用程序生成编译器警告：数据绑定将无法检测对“dp”的分配。这似乎是正确的，因为变量“dp”不是可绑定属性（没有 [Bindable] 元数据标记）。我添加了一个按钮，单击该按钮时会将项目附加到“dp”的后面。尽管编译器警告我不会看到“dp”的更改，但每次单击按钮时列表都会显示新项目！

我不明白为什么我可以看到列表中出现新项目。有人可以解释为什么尽管“dp”不可绑定，但它仍然有效吗？

<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="vertical" minWidth="955" minHeight="600">
<mx:Script>
    <![CDATA[
        import mx.collections.ArrayCollection;
        import mx.rpc.events.ResultEvent;

        private var arrayData:Array = [
            {name:"banana", cat:"fruit", cost:0.99},
            {name:"bread", cat:"bakery", cost:1.99},
            {name:"orange", cat:"fruit", cost:0.52},
            {name:"donut", cat:"bakery", cost:0.33},
            {name:"apple", cat:"fruit", cost:1.05}];

        private var dp:ArrayCollection = new ArrayCollection(arrayData);

        private function onButtonClick(event:MouseEvent):void
        {
            var obj:Object = new Object();
            obj.name="test";
            obj.cat="testcat";
            obj.cost=666;
            dp.addItem(obj);
        }
    ]]>
</mx:Script>        
<mx:HorizontalList dataProvider="{dp}" labelField="name" columnWidth="100" width="80%" height="50"/>
<mx:Button label="Click me" click="onButtonClick(event)" /> 

Answer 1

编译器的警告是正确的。

编译器警告您，将不会检测到将 dp 值从您指定的初始 ArrayCollection 更改为另一个 ArrayCollection 的赋值。

但是，如果您保留 dp 的值，而仅更改其内容，则您的 将继续工作。

这可能看起来微不足道，但它是一个重要的区别，并且可能会导致应用程序中出现一些非常令人困惑的错误。

不会检测到变量dp 的赋值。但是，对 ArrayCollection 的 list 的更改将会发生，因为它们会调度 CollectionChangeEvent。

例如：

private var dp:ArrayCollection = new ArrayCollection();

private function test():void
{
    // Here, we don't change the value of dp directly,
    // instead we just modify it's list.
    // The DataGroup will show the strings One,Two
    dp.addItem("One")
    dp.addItem("Two") 

    // Here, we change the actual value of dp, by assigning a 
    // new ArrayCollection to it.
    // This change would not be detected, and the list would continue to show 
    // the contents of the previous value.
    // Additionally, the label will show the string "Length: 2",
    // even though the length is clearly now 3. 
    dp = new ArrayCollection(); 
    dp.addItem("Tahi");
    dp.addItem("Rua");
    dp.addItem("Toru");
}


<s:DataGroup dataProvider="{dp}" />
<s:Label text="Length: {dp.length}" />

Answer 2

尝试使用：

[Bindable("__NoChangeEvent__")]
private var dp:ArrayCollection = new ArrayCollection(arrayData);

在列表中添加元素怎么样，请参见ListBase的代码：

public function set dataProvider(value:Object):void
{
    if (collection)
    {
        collection.removeEventListener(CollectionEvent.COLLECTION_CHANGE, collectionChangeHandler);
    }

    if (value is Array)
    {
        collection = new ArrayCollection(value as Array);
    }
    else if (value is ICollectionView)
    {
        collection = ICollectionView(value);
    }
    else if (value is IList)
    {
        collection = new ListCollectionView(IList(value));
    }
    else if (value is XMLList)
    {
        collection = new XMLListCollection(value as XMLList);
    }
    else if (value is XML)
    {
        var xl:XMLList = new XMLList();
        xl += value;
        collection = new XMLListCollection(xl);
    }
    else
    {
        // convert it to an array containing this one item
        var tmp:Array = [];
        if (value != null)
            tmp.push(value);
        collection = new ArrayCollection(tmp);
    }
    // get an iterator for the displaying rows.  The CollectionView's
    // main iterator is left unchanged so folks can use old DataSelector
    // methods if they want to
    iterator = collection.createCursor();
    collectionIterator = collection.createCursor(); //IViewCursor(collection);
    // trace("ListBase added change listener");
    collection.addEventListener(CollectionEvent.COLLECTION_CHANGE, collectionChangeHandler, false, 0, true);

    clearSelectionData();

    var event:CollectionEvent = new CollectionEvent(CollectionEvent.COLLECTION_CHANGE);
    event.kind = CollectionEventKind.RESET;
    collectionChangeHandler(event);
    dispatchEvent(event);

    itemsNeedMeasurement = true;
    invalidateProperties();
    invalidateSize();
    invalidateDisplayList();
}

所以看一下行：

    collection.addEventListener(CollectionEvent.COLLECTION_CHANGE, collectionChangeHandler, false, 0, true);