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A Guide to BeOS System Objects


app_registrar Icon

The Registrar Icon
Location in the BeOS: /beos/system/servers/registrar

One of the things that makes using the BeOS feel smooth is the extensive use of MIME typing throughout the system, and much of what makes MIME graceful on the BeOS is the Registrar.


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 communications.)

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!

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