Flex:: How do I prevent in drag and drop, dragging onto self

I have a Flex question. I was wondering, if I have two list boxes, and I want to drag and drop between both of them, how do I prevent the user from 开发者_运维百科dragging onto the same list (thus duplicating the item? I cannot have a situation where that is the case. Thanks guys

Haven't tested it but I guess something like this should work:

Listen to dragStart event on both lists and set a source variable depending on event.target. Now listen to the dragDrop event on both lists and call event.preventDefault() if the source is same as the target.

Here are some simple functions I made while building a working application I used to wrap my head around flex drag and drop. I was looking for a way to have multiple lists with drag and drop functionality that would not interfere with each other. Also, I didn't want to deal with copying list data around.

private function onlyAllowMoveDragOverHandler(event:DragEvent):void {
  event.preventDefault();
  event.currentTarget.showDropFeedback(event);
  DragManager.showFeedback(DragManager.MOVE);
}

private function allowDropOnlyIfInitiatorEqualsComponent(event:DragEvent, component:IUIComponent):void {
  event.preventDefault();
  if (event.dragInitiator == component) {
    DragManager.acceptDragDrop(event.target as IUIComponent);
  }
  else {
    DragManager.showFeedback(DragManager.NONE);
  }
}

And I in use in my mxml:

<mx:List 
    x="10" 
    y="170" 
    id="availableLangsList" 
    dataProvider="{availableLangs}" 
    width="100" 
    height="200"
    dragEnabled="true"
    dragMoveEnabled="true"
    dropEnabled="true"
    dragOver="onlyAllowMoveDragOverHandler(event);"
    dragEnter="allowDropOnlyIfInitiatorEqualsComponent(event, selectedLangsList);"
    dragComplete="selectedLangs.refresh();"
    />

  <mx:Label x="129" y="153" text="list 4"/>
  <mx:List 
    x="129" 
    y="170" 
    id="selectedLangsList" 
    dataProvider="{selectedLangs}" 
    width="100" 
    height="200"
    dragEnabled="true"
    dragMoveEnabled="true"
    dropEnabled="true"
    dragOver="onlyAllowMoveDragOverHandler(event);"
    dragEnter="allowDropOnlyIfInitiatorEqualsComponent(event, availableLangsList);"
    dragComplete="availableLangs.refresh();"
    />

I found the solution, which im not sure would work for anyone else. I basically had in my two lists: `

    <mx:List id="srcList" dataProvider="{_source}"
    allowMultipleSelection="true"
    enabled="{enabled}"
    labelField="{labelField}"
    iconFunction="iconFunction"
    dragEnabled="true" 
    dropEnabled="true" 
    dragDrop="doDragDrop(event);"
    width="100%"
    height="100%"       
    />
</mx:VBox>
<mx:VBox paddingTop="50">
    <mx:Button label="-&gt;" enabled="{enabled}" click="add()"/>
    <mx:Button label="&lt;-" enabled="{enabled}" click="rem()"/>
</mx:VBox>
<mx:VBox width="100%" height="100%">
    <mx:Label text="{right_col_heading}" />
    <mx:List id="dstList" dataProvider="{_destination}"
        allowMultipleSelection="true"
        enabled="{enabled}"
        dragEnabled="true" 
        dropEnabled="true" 
        dragDrop="doDragDrop(event);"
        width="100%"
        height="100%"
        labelField="{labelField}"
        iconFunction="iconFunction"
        verticalAlign="center"
    />`

I basically added a dragMoveEnabled = "true" to both lists and now basically not re-add to the same list an item of itself, but just move the order (which doesnt matter to me as its a soap send and the back-logic would put it in the correct order anyway).

In my case, I used a HashCollection (which extends ArrayCollection) [just google it, you'll find the component]. The dataprovider is bind to this has collection. You would add items to the collection with: dataprovider.put (key, object) instead of dataprovider.addItem(object).

The "hash" will ensure uniqueness in the collection. SO, even if the user drag-drop something that already exists in the hash, the original value would get replaced with the new object (but it wouldn't matter because it's the same value).

The "key", however, must be unique.... otherwise, the hash idea won't work.

Thanks Brice, Those functions were helpful.

For them to work in Spark Lists just update the first function as follows with createDropIndicator instead of showDropFeedback and stop passing the event.

private function onlyAllowMoveDragOverHandlerS(event:DragEvent):void {
    event.preventDefault();
    event.currentTarget.createDropIndicator();
    DragManager.showFeedback(DragManager.MOVE);
}