[Sugar-devel] How to copy something from the Journal to a pendrive? FotoToon export?

[Sascha Silbe]

Excerpts from tom.staubitz at fhtw-berlin.de's message of Sun Oct 17 09:02:42 +0200 2010:

> Imagine two or more children. Half of them are in country1, the others are in country2.
> They would like to collaborate on an activity, e.g. FotoToon.
> One of the groups starts and adds a picture and some bubbles. Now they want to send their work to the other group so that they can add a picture and a couple of bubbles. This goes back and forth a couple of times. 

Re-reading your post again, it seems what you actually wanted to do was
to save an activity _session_ (the content you worked on), not the
activity itself (the program).

This is still a bit of a sour spot. In order for the system to know what
activity it needs to know either the activity it has been previously
opened with or the MIME type (so it can select activities that are able
to handle this type - that's what the "Start with" submenu shows).

The Sugar data store can store metadata as arbitrary key=value pairs.
Activities use this to set the activity id and the MIME type of the
entry, so the Journal can resume an entry fine as long as it's in the
data store and the metadata is preserved.

External storage media (USB sticks, SD cards) are intended as a means
to interchange data with non-Sugar systems, so we store entries as plain
files instead of putting a data store on them (that no non-Sugar system
would be able to read). And here we enter the messy world of traditional
desktop systems: there is no place (*) to store metadata, especially not
non-standard properties like the activity that wrote the entry. Not even
the MIME type can be preserved properly; we need to rely on mappings
between file extensions and MIME type. There's no single, central
mapping (except for a few standard types) but rather each application
ships a small part of the database with the MIME types and file
extensions it cares about. If you don't have an application installed
that recognises the file extension, you won't know the MIME type.

The same technology is used in Sugar for reading files on external
media. So if an activity wants to handle files of a certain MIME type,
it needs to tell Sugar how to recognise the files (by shipping a
suitable mimetypes.xml [1]). This is what some of the activities use
to make the data store <-> USB stick <-> data store roundtrip work:
They choose a custom MIME type and file extension. When copying files
to the USB stick, the Journal will set the file extension based on the
MIME type. When copying the entry back from the USB stick, the Journal
will map the file extension back to the MIME type (if the activity is
installed, i.e. if the mimetypes.xml is present). If the activity tells
Sugar it can handle entries of this type (by setting mime_types in
activity.info [2]), it will be used for opening (resuming) the entries.

There is a way to transfer a data store entry including all of its
metadata: Journal Entry Bundles [3]. Unfortunately the only way (that I
know of) to _create_ such a bundle is my Backup activity [4] and it
stores the entire data store instead of a single entry. It also uses a
different MIME type and file extension (to make sure that these bundles
are handled by the Restore activity [5] which can cope with JEBs
containing multiple entries, unlike the Journal) but that would be easy
enough to work around (by renaming the file).

Thanks for following me through to the end. ;)

Sascha

(*) There are extended attributes [6], but they are only supported on
    some file systems (in particular not on VFAT, which is what all the
    USB sticks and SD cards come pre-formatted as) and need to be
    enabled explicitly (in /etc/fstab; no idea if it's possible to tell
    the Gnome auto mount magic to do it). Except for OS/2, Mac OS and
    a few special applications I don't know of anything that actually
    uses them for metadata (i.e. MIME type & co).
[1] https://wiki.sugarlabs.org/go/Development_Team/Almanac/Activity_Bundles#Bundle_structure
[2] https://wiki.sugarlabs.org/go/Development_Team/Almanac/Activity_Bundles#.info_file_format
[3] http://wiki.laptop.org/go/Journal_entry_bundles
[4] http://activities.sugarlabs.org/en-US/sugar/addon/4326
[5] http://activities.sugarlabs.org/en-US/sugar/addon/4327
[6] http://en.wikipedia.org/wiki/Extended_attributes
--
http://sascha.silbe.org/
http://www.infra-silbe.de/
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