[whatwg] Proposal for public data in drag events

Ian Hickson ian at hixie.ch
Thu Aug 30 10:27:58 PDT 2012

On Thu, 21 Jun 2012, Trevor Burnham wrote:
> I've been using HTML drag-and-drop 
> (http://www.whatwg.org/specs/web-apps/current-work/multipage/dnd.html) 
> in a project, but I've run into one limitation that seems severe to me: 
> There is no direct way to determine what the source node is from a 
> dragenter, dragover, or dragleave event. This makes it difficult to 
> support use cases where elements react to those events differently 
> depending on what is being dragged over them.

This is intentional, because that source node could be from a Web page in 
another origin, another browser, or indeed, an app that isn't even a 
browser. So there's no real sane way to do it.

> I understand that the reason for this is cross-document drags: In 
> addition to security implications, obtaining a reference to a DOM node 
> in another document simply wouldn't make sense. Therefore, the 
> dataTransfer object only allows serialized data. Unfortunately, 
> dataTransfer is only appropriate for carrying data to the drop target. 
> There is no mechanism for providing data to intermediate drag event 
> receivers, except for the "types" attributes on the dataTransfer object. 
> "types" can be used to carry data that you want to make public 
> (http://stackoverflow.com/a/11089592/66226), but this is clearly a hack 
> and it carries some limitations. Most notably, the spec requires that 
> data type strings be converted to ASCII lowercase.
> Therefore, I'd like to propose the addition of a "publicData" object on 
> all drag events. It would have the same interface and behavior as the 
> dataTransfer object, with the sole exception that it would be read-only 
> in all events where dataTransfer is protected. That is, publicData would 
> be read/write in dragStart, and read-only in all other drag-and-drop 
> events.

That's an interesting idea. I suppose we could expose it using a custom 
type in cross-app OS dnd situations, too.

Could you elaborate on your use case? Are there cross-window use cases for 
this? (For in-window cases, you could instead just use a global.)

Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'

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