Getting informed by DBus about files to be thumbnailed

[probonopd]

Objective

We have a daemon that handles AppImages, and among other things, can generate thumbnails for them and can integrate them into the system.

We want to be notified whenever AppImages (or more generally, ELF executable files) are displayed so that we can generate a thumbnail and, while we are at it, do desktop integration.

Thubnailer1 dbus

The Thumbnail management DBus specification at https://wiki.gnome.org/DraftSpecs/ThumbnailerSpec describes how to interact with thumbnailers over DBus.

We can see requests for files to be thumbnailed on the system using

dbus-monitor "type='signal',interface='org.freedesktop.thumbnails.Thumbnailer1'"

We can send a request to get a thumbnail using

dbus-send \
    --session \
    --print-reply \
    --dest=org.freedesktop.thumbnails.Thumbnailer1 \
    /org/freedesktop/thumbnails/Thumbnailer1 \
    org.freedesktop.thumbnails.Thumbnailer1.Queue \
    array:string:"file:///usr/share/pixmaps/xoo.png" \
    array:string:"image/png" \
    string:"normal" \
    string:"default" \
    uint32:0

Notes

On Xfce, thumbnails are requested when

• A window in the file manager is opened that contains files with a mime type that is registered to be thumbnailed
• Applications are shown in the applications menu

Thumbnails are not requested when

• Firefox downloads a file with a mime type that is registered to be thumbnailed

Register a specialized thumbnailer

Note that by default, thumbnails get requested for files that have mime types that are registered to be thumbnailed, such as PNG and JPG, but not for ELF executables such as AppImages. How can we make it happen so that thumbnails get requested for ELF files, too?

Apparently we need to register a "specialized thumbnailer" over DBus so that we get notifications. Appaerntly we need to do this using the org.freedesktop.thumbnails.Manager1 interface. The following does not seem to work, why?

dbus-send \
    --session \
    --print-reply \
    --dest=org.freedesktop.thumbnails.Manager1 \
    /org/freedesktop/thumbnails/Manager1 \
    org.freedesktop.thumbnails.Manager1.Register \
    string:"/com/company/Thumbnailer1" \
    array:string:"file" \
    array:string:"application/x-executable"

This only results in

Error org.freedesktop.DBus.Error.InvalidArgs: Type of message, '(sasas)', does not match expected type '(ss)'

"Register a specialized thumbnailer dynamically" at https://wiki.gnome.org/DraftSpecs/ThumbnailerSpec shows some XML. Why XML? Where am I supposed to enter that XML? Why does the documentation not show how to send such a message using dbus-send? What is object_path supposed to be?

I interpret the XML on https://wiki.gnome.org/DraftSpecs/ThumbnailerSpec to translate to the dbus-send command above, but apparently I am wrong. What is the proper way of doing it?

Help please

Apparently registering a "specialized thumbnailer" over DBus is little-used, at least I was not able to find ample examples on GitHub.

Can someone please help me, @pvanhoof @robtaylor maybe?

[pvanhoof]

This happened too long ago for me ...
Specialized thumbnailers was used on the Nokia N9. But it only had one which was never made open source. I don't think any open source project ever attempted to implement one. We provided the DBus APIs because the thumbnailer was not allowed to become open source.