Poking around in the BeOS, you've probably come across the funny icon of
the teacher at the chalkboard, and wondered what the Registrar is. The
average end-user will never use the Registrar -- or at least never
know that they are using the Registrar. The Registrar works behind
the scenes, and everyone who boots the BeOS uses the Registrar whether they
know it or not.
One of the Registrar's most important functions is to handle
communications between running applications. When one application wants to
talk to another application (for example, when you have two BeBounce
windows open, and the ball needs to go from one window to the other or, a
little more commonly, when you drag a text clipping from StyledEdit to the
Desktop), it contacts the Registrar to get the address of the other
application, so it will know where to send the messages. (Once the
application gets the address from the Registrar, it will cut the Registrar
out of the loop, and talk directly to the other application, speeding
In a similar vein, the Registrar handles the passage of data in a
clipboard between applications. (Side note: Did you know that applications
on the BeOS can have multiple clipboards? They can, and the Registrar will
handle much the details, making it easy for developers to add this useful
feature to their applications.)
But the Registrar's most important function is to make MIME work on the
BeOS. The Registrar is the glue that holds MIME together.
First of all, the Registrar runs a background "sniffer" routine, that
scans your BeOS volumes for applications and files, adding the MIME
information it finds to the MIME database that the BeOS keeps on your
system, making sure files have MIME types associated with them, etc. The sniffer generally only runs when your system is idle.
Second, the Registrar answers inquiries about appropriate MIME types.
For example, when you double-click a file in the Tracker, what application
should open it? The Tracker asks the Registrar, the Registrar looks in the
MIME database, and gives the Tracker the answer.
This functionality is used in many places. If you use the new Open With
functionality in Release 3, that requires MIME information, and so the
Registrar is involved there. If you drag a file's icon onto an
application's icon in the Tracker, if the application knows how to open
that kind of file, the icon will darken, or highlight. While the Tracker
does the drawing, it's the Registrar that knows what apps can open what
kinds of files.
And any application can use the Registrar to get this kind
of information, to implement whatever functionality the developer has
dreamed up. Probably many of your favorite applications use the Registrar
quite a bit!
So, though you'll never see his icon in the Deskbar, the Registrar is
always working for you!