associating widgets with mimetypes

One of the neat features in Plasma is the ability to associated widgets with mimetypes. To associate a widget with a mimetype, you add a X-Plasma-DropMimeTypes key to the .desktop file for the widget.

For instance, the picture frame widget has this in its .desktop file:

X-Plasma-DropMimeTypes=image/jpeg,image/png

When I drop an image file onto the desktop, I get asked if I'd like to make a picture frame or an icon out of it. In 4.2, I can also drop raw image data onto the desktop and it does the same (except it doesn't give the option to create an icon for it as it isn't actually a file).

The way this works is that Plasma::Containment queries the widgets to see which, if any, supports the mimetype(s) in the drag object. It's smart enough to follow the mimetype priority if there is more than one type offered in the drag, so html content will be dropped as html content and not plain text. On my 4.2 system, that results in the mini-browser widget being created.

Inside the widget itself, the first argument passed in on the args list will be the URL to a file containing the data. In this way, the widget doesn't need to concern itself with caring about the difference between a "real" file and a blob of data dragged onto the Containment.

Inside the widget, the code can check for this in the constructor with:

KUrl url;
if (args.count() > 0) {
    url = args.value(0).toString();
} else {
    // do some sort of default set up instead
}

That's all there is to it: one line in the .desktop file and a couple lines in the widget itself.

For the user, it means they can drop "stuff" onto their desktop and the Right Thing(tm) happens depending on the content in the "stuff" without them having to do anything particularly special at all.

I'll be extending this to wallpapers as well so that wallpapers can also say, "oh yeah, i support that kind of data", which will result in the addition of a "Set as wallpaper" to the menu that pops up. Which means you will eventually be able to drop an image file onto a containment and choose between creating an icon shortcut for the image, creating an image frame widget for it or setting it as the background. And by "image file" I mean any file type that is supported by wallpapers or the image frame.

I think this could have some really cool applications if people use their imaginations a bit. To get you started: what could happen when a contact is dropped onto the desktop (e.g. a vcard or maybe even a contact listing from a social networking site)? what could happen when an email is dropped from kmail/thunderbird/whatever? what could happen when a movie is dropped? what could happen when a vnc connection is dropped? what could happen when a torrent is dropped?

What might be extra neat here is when combined with other widgets, like folderview and nepomuk. If the user is looking at a folderview showing all video files, once we have a working video player plasmoid the user could just drag one of these videos to the desktop and watch the video play without starting up another application. In fact, they could drag a couple of them onto the desktop and have multiple videos playing. And it would all Just Work(tm).

Now consider using something like this within an application ... say for instance Amarok2. You drag a movie into its plasma-driven context view, and it could quite easily pop up the movie widget with approximately 0 lines of code inside of Amarok2. I don't know if the Amarok2 peeps have such a thing planned, but it demonstrates the concept, one that could probably be extended to other applications as well.

I plan on playing with this auto-widget-generation-based-on-mimetype on small form factor devices as well as a way to perhaps limit the crazyness of opening entire apps for simple processes. Of course, on those devices it probably won't be triggered by physical drag 'n drops, but the idea and code paths will be the same. I'm not sure yet just how useful it will be in that context, but there's only one way to find out, right? =)

Note that in 4.1, drops are limited to applets or files. It's only in 4.2 that we have the non-file drop support (e.g. directly from a web page to the desktop, or from the image thumbnail in ksnapshot to the desktop). Props to Ivan for pointing out that the drop support should be extended to non-file drops as well, prompting me to flesh out this feature to its logical conclusion.