COAS: A Flexible Approach to System Administration Tools

The first thing that will probably strike you as odd about this example is that it has no label strings anywhere. Nevertheless, the dialog is supposed to have a title, edit fields are supposed to have a label to their left, etc.

The answer is that COAS generates NLS strings for you out of the information it has. For instance, when creating the prompt dialog, we inconspicuously passed the string mouse into the function. As a consequence, COAS will create tags such as |":MOUSE_TITLE:"| for the dialog's title and attempt to look it up in the module's message catalog. (The message catalog name was specified in the class constructor.) Likewise, for the protocol edit field, it will generate the tag |":MOUSE_PROTOCOL_LABEL:"|. All you need to do is write the message catalog, mapping these funky strings to intelligible English (or French, German, etc.) and install the file.

Looking at the sample code above, you may also have thought: I understand where they put the data in the dialog, but how do they put it back into the data model?

This is the interesting part about the data editing process. If you have ever programmed Motif, you know how tedious it can be to extract the value to be edited from the data model, put it into the dialog and write the resulting value back to the data model when the user hits the OK button.

The approach taken by COAS is to tie data model nodes into the dialog directly and let the dialog select an appropriate widget type (string, combo box, toggle button, spin button, etc.). When the user provides a new value, the dialog will automatically check the value's syntax against data model constraints and write it back into the data model.

In our example, the dialog would create a simple string edit field for deviceName, a pop-up list for protocol (since it is limited to a set of choices) and a toggle button for emulation.

What's more, this mechanism offers you easy-to-use context help for each input field, bound to the f2 key. Adding this type of help to a data item is as easy as adding the HELP attribute to the data definition in the schema file:

device         RECORD {
    model      STRING HELP "HELP_MODEL"
    protocol    MouseProtocol HELP "HELP_PROTOCOL"
    ...
}

These help messages will be looked up in the message catalog associated with the schema file (remember the MSGCATALOG keyword in the schema file?) and displayed in a pop-up dialog whenever the user presses f2.

Of course, every scheme you devise has a drawback. In this case, it is how to cancel changes made during the execution of the dialog. When the user presses the Cancel button, he wants all changes to go away.

This is where the marker object comes into play. The data node's getMarker method obtains a marker for the node's change log (called a journal in COAS lingo). When the user requests a discard of all changes, the CLAM base class invokes self.mouse.cancel(marker), which reverts all changes made after the marker object was obtained.

I have to admit that the above example, in its simplicity, is a bit deceptive. What I'm showing here is the simplest version of a dialog. In fact, what you see here is just a glorious interface to the configuration file because it does not offer the user any help or guidance. A good dialog would automatically choose the appropriate device file when a selection is made (e.g., a bus mouse) and keep the user from enabling three-button emulation for mice that already have three buttons. As a consequence, your average COAS module will have a lot more than those 20-odd lines in the example above.

However, the greatest advantage COAS offers in this context is that it relieves you of the usual hassle when working with a GUI and lets you concentrate on the data flow instead.

Dependency Model

If this article has piqued your interest and you would like to take a closer look at COAS, you can find out more about it on http://www.coas.org/ and http://developer.coas.org/. If you want to participate in the development of COAS, don't hesitate to contact me.

All listings referred to in this article are available by anonymous download in the file ftp.linuxjournal.com/pub/lj/listings/issue58/3019.tgz.