API proposal: drag view

Pavel Safrata pavel.safrata at oracle.com
Tue Jan 15 01:20:49 PST 2013

Hi Richard, Steve,

On 14.1.2013 20:51, steve.x.northover at oracle.com wrote:
> +1
> I'd like to see some example code where the tree and table set a 
> default image as part of the implementation of these controls. 
> Applications would get a reasonable image for free and be able to 
> override it as necessary.  Having and getter is necessary for this.

Is it? What will be done with it? User will get the image and 
programatically check if it looks good?

(please see my answers to Richard's comments below).

> BTW, the last time I looked, on Windows, when you set image content, 
> then you will get a drag image for free.  This doesn't happen on the 
> Mac.  We should stop doing this on Windows.
> Steve
> On 14/01/2013 2:09 PM, Richard Bair wrote:
>> HI Pavel!
>> Overall great. I don't see why you can't have a getter. The only way 
>> to set the drag view is via an API we control so it seems pretty easy 
>> to implement the getter as well. 

No. DnD is an OS feature. You can drag data from native applications to 
FX. Is it possible on all systems to find out what did the native 
application use as drag image? I'm really not sure. Anybody has the 

>> You could either have a DragView class which encapsulates the 
>> node/image and the offsetX and offsetY, or you could just break it 
>> out into three properties on the DragBoard and save yourself the 
>> extra class.

If we are OK with the properties having value only during starting DnD 
and not during the rest of the gesture, then yes, this could be done. 
But I still don't see the value.

>> I think you should just have an image based dragImage property and 
>> dragImageOffsetX and dragImageOffsetY properties. We shouldn't by 
>> default put anything in as the drag image, except for a few UI 
>> controls. We'll want a ListView, TreeView, TableView, TreeTableView 
>> to automatically pre-populate the DragBoard before the onDragDetected 
>> code is called, so that those control give you a nice drag image for 
>> free but the developer is free to replace it or clear it as they see 
>> fit.

I don't think this is possible with the proposed API. The DragBoard 
instance doesn't exist until you start DnD in the DRAG_DETECTED handler, 
so you cannot really pre-populate it. Would it be sufficient if the 
controls contained getDefaultDragImage() method that users would 
explicitly call?

>> The developer could use the synchronous version of snapshot. But I 
>> think either snapshot should be fixed to be transparent by default 
>> instead of a white fill by default or add another snapshot method 
>> which produces a transparent fill by default.

True. Kevin, how easy/hard it would be to add such snapshot method?


>> Basically:
>> source.setOnDragDetected(new EventHandler<MouseEvent>() {
>>      public void handle(MouseEvent event) {
>>          DragBoard db = source.startDragAndDrop(TransferMode.ANY);
>>          db.setContent(…);
>>          db.setDragImage(source.snapshot());
>>          db.setDragImageOffsetX(event.getX());
>>          db.setDragImageOffsetY(event.getY());
>>          event.consume();
>>      }
>> }
>> Maybe also have a convenience method on DragBoard:
>>      db.updateDragImage(node, x, y);
>> which then just calls the 3 setters as appropriate.
>> Richard
>> On Jan 13, 2013, at 11:59 PM, Pavel 
>> Safrata<pavel.safrata at oracle.com>  wrote:
>>> Hello,
>>> this is a proposal of an API allowing to specify the image floating 
>>> with mouse cursor during a drag&drop operation.
>>> Jira: http://javafx-jira.kenai.com/browse/RT-14730
>>> I propose to add two methods to DragBoard:
>>> setDragView(Image image, double offsetX, double offsetY)
>>> setDragView(Node node, double offsetX, double offsetY)
>>> The first one simply uses the given image for the drag view with the 
>>> offsetX and offsetY specifying cursor position over the image. The 
>>> second one renders the given node to an image and uses the result 
>>> (the coordinates being in the node's local space).
>>> The typical usage will look like this:
>>>         sourceNode.setOnDragDetected(new EventHandler<MouseEvent>() {
>>>             public void handle(MouseEvent event) {
>>>                 Dragboard db = 
>>> source.startDragAndDrop(TransferMode.ANY);
>>>                 ClipboardContent content = ...
>>>                 db.setContent(content);
>>>                 db.setDragView(sourceNode, event.getX(), 
>>> event.getY()); // that's it
>>>                 event.consume();
>>>             }
>>>         });
>>> This API is meant for telling the operating system what visual cues 
>>> to provide, I don't think it is useful (and I'm not sure it is even 
>>> possible) to provide getters.
>>> There is a possibility to provide default drag view - if none of 
>>> those methods is called, the default drag view would be an image of 
>>> the drag gesture source. This should work nice most of the times. 
>>> However, it may cause inconveniences to some existing apps - for 
>>> instance a text editor node which puts the selected text on the 
>>> DragBoard - after updating FX the application starts to show the 
>>> entire editor in the drag view. For this reason I think the default 
>>> behavior should remain unchanged.
>>> Thanks,
>>> Pavel

More information about the openjfx-dev mailing list