- FVWM: How Styles are Applied
FVWM: How Styles are Applied
This is a copy of the following article by Thomas Adam: https://linuxgazette.net/127/adam.html
Configuring FVWM can seem like a chore at times. Indeed, there are certain aspects of it that are easy - and some that are less so. I’ve been helping people configure FVWM for some time now, and while I have delved into some of the more esoteric regions of FVWM, it seems that many people find the use of Style lines the hardest aspect to grasp. Hopefully this article will help clarify things.
What are Style lines?
Style lines (in FVWM parlance) are those lines in an FVWM configuration file which apply some specific style to a window. It could be, for instance, that one would want all windows called foobar to be sticky by default. Hence in one’s .fvwm2rc file:
… would ensure that fact. One can also add multiple properties to a given window. For instance, it might be desirable that the same window , foobar, have no visible title and a border width of eight pixels. This can be expressed as:
More style lines can then be added, line by line, with a specific window for each style. Here’s an example:
FVWM also allows for the use of wildcards when matching a window’s name as in the above example. The ‘*’ matches for anything after what precedes it, whereas the ‘?’ matches a single character.
What’s even more important is that the matching of Style lines is case sensitive. This means that for the following, both are separate entities:
How FVWM Interprets Styles
So far, everything’s going great. Window names are being added as style options, and everything’s working just fine - until you have a series of lines which look like the following:
At first glance there’s nothing wrong with them. Sure, FVWM is doing exactly what you asked for… except that ‘’ is a greedy match, which is what one would expect in using it. In the example above, Mozilla has, in theory, only been told to display no title as a Style directive - but it may also produce entirely unexpected results due to that greediness. As an example, it may match an earlier declaration (e.g., ‘Style Fvwm’) if that string exists in the window title.
In all of the problems encountered with style lines, this has to be the most common. The reason for this isn’t that Mozilla or Firefox are misbehaving, but usually that there’s a lack of understanding of how Style lines are applied.
With applications such as Mozilla and Firefox, titles are dynamic - they often change as a tab or page loads in them. Assuming that we’re using the style lines from above, and that we’re looking at, say, a webpage that has the title: “Fvwm: my nice screenshot”:
…this matches (in part) some of Firefox’s title . If one were to then restart FVWM with this page still showing in Firefox (or issue a Recapture command), then the window would become sticky - annoying, and certainly not what we want. Most people will also try something like this to remedy the situation:
…which also has the same problems, and perhaps even more so, since that’s matching ‘Firefox’ anywhere within the title of a window.
To get around this, something unique needs to be used. With dynamically changing titles such as those in a web browser, specifying the full name of the window just won’t work. However, FVWM also allows us to match by a window’s class, as well.
Take Firefox. That will either have a class of Firefox-bin or Gecko - which will provide a unique class match.
The reason one wants to match on a window’s class in this instance is that it’s less ambiguous than the title of the window, which might be something like this:
Fvwm Forums :: Post a reply - Mozilla Firefox
There are a few ways to obtain a specific window’s class. Perhaps the preferred option is using the module FvwmIdent, although window manager-agnostic commands such as xwininfo and xprop can also be used. Using the window class instead of the title, the previous style command would be replaced with:</p>
You can be fairly well assured that the Class of a window tends to be unique to that application (the exceptions are things like RXVT which sometimes have been known to set their class to that of XTerm.) The problem here though is that the same application will generally always have the same Class.
Indeed, you might be wondering how FVWM knows which attribute style lines match. Truth is, it doesn’t really know, however FVWM defaults to cycling through a known series of window attributes. Hence, FVWM will match your window’s style line thus:
Title --> Class --> Resource
So, FVWM checks the title of a window first. If a match is unsuccessful, it will then look at the Class, and if that fails, it will then look at the Resource of that window for a match. By and large, where wildcards are used in style lines – it’s normally the window’s title that gets matched in the first instance.
Remedying the Situation
There are other considerations that need to be taken into account. Style lines are ANDed. That is, for successive lines that are specified one after the other for the same application, both lines are considered. So for the following:
The window ‘foo’ would be displayed without a title and would become sticky. Because of this, the ordering of style lines is VERY important to prevent race conditions, or other oddities that can creep in.
But that’s not the entire story, either. Specificity is important. Yes, for the same window title, the styles are ANDed together. The order that the style lines appear within your .fvwm2rc also matters. For those of you who are familiar with the concepts of Object Oriented programming, you can consider style lines as following the rules for inheritance. The rule of thumb for style lines is:
“Always generalise, before you specialise.”
That means, aggregate styles for all windows (Style * […]) before you specify the style lines for specific applications. FVWM’s parsing is quite literal in that sense. When FVWM parses its configuration file, it reads it line-by-line. This is why it’s important to think of Style lines as an inheritance model.
Hence, if you wanted all windows to be sticky, and a window whose name is ‘foofoo’ to not be sticky and have no borders, the correct order to write that in would be:
Note that because we had previously declared a global style in which all windows are sticky, it is necessary to negate that Sticky condition for the specific application. Otherwise it would be “inherited”.
Writing that the other way around, however, gets one into trouble:
The greedy match of “*” for all windows, irrespective of the specific condition for ‘foofoo’ above, means that the greedy match takes precedent.
It was mentioned earlier that style lines are ANDed. This is indeed true, and you can see that in operation. But we have another rule that applies: given two contradictory Style statements, the latter one always wins. So, for example (and I see this a lot in people’s configs), assume you had written this:
… because they’re both focus policies being applied to all windows, FocusFollowsMouse would win because it was the last one specified.
So, to recap:
- Style lines are matched on a case-sensitive basis.
- When specifying a window to be matched, the order of operation is: Name –> Class –> Resource.
- Specificity is important. Generalise the styles for all programs at the top, and define specific window styles lower down.
: Although the term title is used throughout this document, it should be noted that it’s really the window’s name that is used (see xprop(1)). For convenience’s sake, the window title is often set from the name of the window.