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FvwmEvent: conditional window checking by example

By Thomas Adam


Most window managers have some form of automation that allows the user to 'script' various aspects of its operation. Indeed, the 'kahakai' 1 window manager has long since defined Python as way of scripting its capabilities.

In FVWM, there are a few ways of scripting events. The use of FvwmPerl is one such way. However, in almost all cases, when people say they want to define actions, what they're really after is some way of conditionally checking windows when they're created, or something similar. The combination of this ability coupled with a series of commands grouped together to form what FVWM calls a function is something that can be quite powerful.

What is FvwmEvent?

FvwmEvent is a module - a piece of code that is separate from the core of FVWM. There are a lot of different modules in FVWM, all of which share that important distinction; there's no point in loading extra code which might never be used, or loading it on an ad-hoc basis where the user never requested it.

FvwmEvent is a module which allows listening for various events, and acting upon them when they occur. Originally it was known as FvwmAudio, since its job was primarily in playing sounds when various things happened (such as a window being closed, iconified, etc). FvwmEvent still retains that functionality, but now also has the capability of running specific tasks based on those events.

So what are these events? They're triggers associated with the operations of windows (many of which are wrappers around various low-level Xlib library calls). Whenever an event that FvwmEvent has been told to listen for occurs, it will look for an associated action and execute it. A sample valid list of events that FvwmEvent knows to listen for can be seen in its man page2. The one this article will examine is add_window; note that this is only a working example, and should be thoroughly tested before using in production.

FvwmEvent Example

A generic FvwmEvent configuration looks like the following (note that the line numbers have been added as a convenient reference point, and are not part of the configuration):

   1   DestroyModuleConfig FvwmEvent: *
   2   *FvwmEvent: <some_event_name> <some_action>
   4   Module FvwmEvent

The very first thing that happens is that the module config is destroyed (Line 1). This might seem a little strange at first given that nothing has been declared yet, but the point of it here is that for any previous definitions of it (say via multiple parsings of one's '.fvwm2rc' file during restarts), it gets destroyed and then recreated; otherwise, the module definition would just be added to continuously - something that is most undesirable. What follows next (line 2) is the start of the alias definition that FvwmEvent will eventually read. Obviously <some_name> and <some_action> are dependent on the event and the action required. <some_action> might be a function, or a single command. Line 4 simply tells FVWM to load the FvwmEvent module.

Note the concept of what's happening. Most modules have the concept of aliases - that is, an identifier that the module can be told to use (in earlier version of FVWM, in order for multiple aliases of a specific module to be used, one had to symlink the alias name to the module). In the case of the generic example above, that's using *FvwmEvent which is fine until more instances of FvwmEvent need to be loaded. It's permissible, of course, to just have one instance of FvwmEvent running and declare all the events it will listen for in there. The problem is that it's often desirable to run different actions on the same event - something you can't do with one alias. So the heuristic approach is to define a unique alias to FvwmEvent, which isn't *FvwmEvent. Any name can be used, as will become apparent.

add_window event: maximising windows

This is perhaps the most common event people use when they consider running commands on a window. This event is triggered whenever a window is mapped (created) to the screen3. Perhaps one of the most common operations that people wish to perform when this happens is maximizing a window. This has been covered in part in the main FVWM FAQ4.

Of course, in encountering this question, it is often the case that when people say 'maximised', they also mean so-called 'full-screen' - which implies the removal of any title bars and borders, and other such window decorations. That's fine, and can be dealt with at a later stage, although the premise of maximisation has to be discussed first of all.

It also seems to surprise many people that FVWM has no 'StartMaximised' style option. The reason for this is that in introducing such an option it would break the ICCCM5 - since clients set their own geometry, either by themselves or via user interaction.

The first thing to be done is setting up FvwmEvent:

  DestroyModuleConfig FE-StartMaximised: *
  *FE-StartMaximised: Cmd Function
  *FE-StartMaximised: add_window StartAppMaximised

  Module FvwmEvent FE-StartMaximised

This tells FvwmEvent a few things. One is that the alias we're using for it is *FE-StartMaximised. Secondly, we've informed the module that the command specified for the event is a function. Thirdly, the event we're listening for is add_window. Then the module is started.

The function we'll declare is quite simple to start off with (again, line numbers are for point of reference only, and are not part of the syntax):

   1   DestroyFunc StartAppMaximised
   2   AddToFunc   StartAppMaximised
   3   + I Maximize

Line 1 destroys the previous function definition. It's generally a good idea to do this when declaring functions, since it removes a previous definition for it. Indeed, the AddToFunc command (line 2) is cumulative. Each time it is used, it just adds to the definition of the function. If it doesn't exist, the function is created. Quite often this cumulative nature isn't wanted, so removing the definition beforehand is advised. Line 3 is the important line since it is the line which defines the action for the function.

One can define as many actions within a function as is necessary. There are a few prefixes as well which define when and how those actions are to be invoked:

Function Operators
Context Meaning
I Immediate - executed as soon as the function is called.
C Click - executed when the mouse button is clicked once.
D Double-click - executed when the mouse button is double-clicked.
M Motion - executed when the mouse is moved.
H Hold - executed when the mouse button is held down.

Usually the most common operator is I for non-interactive functions, since those commands will always get executed when the function is called. So within this example, the command Maximize is run whenever a window is created. Try it and see; start up an xterm. It will then be maximised. Start up any application in fact, and all of those windows will be maximised. Clearly this is suboptimal, but a start nevertheless.

Function Context

So far, it's been shown how one can use FvwmEvent plus a function to define actions for events. But there will be times in loading applications (which produce windows mapped to the screen) when some windows won't get maximised. The reason for this has to do with the context in which the function is being run.

In most cases, functions are designed to run within a window context. This means that, when they're run, it's known which window or windows the function is to start operating from. Without the proper context, a function will prompt for one, or not run at all. So it's important to ensure a context is forced wherever it's not apparent.

One can achieve this is in a number of ways, and a lot of it depends upon the situation the function is likely to be called in. Recall the definition for StartAppMaximised - at the moment the line looks like:

+ I Maximize

This already assumes a window context. But one can always make sure by using the ThisWindow command, as in:

+ I ThisWindow Maximize

ThisWindow is extremely useful to refer to windows directly without implying any presumptions. Indeed, there are other conditional commands, such as Current, which is quite a common way to imply context:

+ I Current Maximize

However, its use implies that the window already has focus. Sometimes this is useful to refer to the specific window; however, in the case of the StartAppMaximised function one cannot assume the operand windows will always have the focus - hence, the use of ThisWindow is preferable. Where one is unsure as to the operand window (i.e., it is to be decided when the function runs), one can use the Pick command which will prompt for a window to operate on if it is not already known.

Improving StartAppMaximised

Whilst the function does work, it needs improving. For a start, the function is maximising every window that is created - this is clearly not something likely to be desirable. One can conditionally place restrictions on windows by matching against their name, class or resource by using any of the conditional commands mentioned earlier:

  DestroyFunc StartAppMaximised
  AddToFunc   StartAppMaximised
  + I ThisWindow ("name of window") Maximize

What happens here is that only the window with the name 'name of window' is considered. If it matches the window just created, then it is maximised; otherwise, nothing happens. Of course, the maximize command has a toggling action to it. If the said window 'name of window' were to already be maximised at the time it was created (presumably via some command-line flag) then the maximize command would have the opposite effect, "unmaximising" it. Luckily FVWM has a conditional test, Maximized that can be used to test if the window is maximised. The negation of this is !Maximized:

  DestroyFunc StartAppMaximised
  AddToFunc   StartAppMaximised
  + I ThisWindow ("name of window",!Maximized) Maximize

Looking better, certainly. There's still room for improvement, though. In FVWM 2.5.X, one is able to specify multiple windows to match on, if more than one window need be considered:

  DestroyFunc StartAppMaximised
  AddToFunc   StartAppMaximised
  + I ThisWindow ("name of window|another window", \
      !Maximized) Maximize

The '|' operator acts as a logical OR command, matching either of the titles and applying the maximized condition to the (possibly) matched window. In FVWM 2.4.X, one would have to use multiple lines one after the other:

  DestroyFunc StartAppMaximised
  AddToFunc   StartAppMaximised
  + I ThisWindow ("name of window",!Maximized) Maximize
  + I ThisWindow ("some_window",!Maximized) Maximize

There's still one more condition to consider: different window types. Up until now, the assumption has been that normal windows are considered. Whilst in most cases that's true, FVWM has (at the simplest level) two different window types that it manages; ordinary application windows and transient windows. By its very nature, a transient window is one which is generally only on screen for a short length of time. Also known as 'popup' windows, they're typically used for 'Open' and 'Save' dialogue windows. It's not likely (due to their implementation) that one is going to be able to maximise them anyway, but it's worth excluding them. FVWM allows for this via the Transient conditional check, which can be negated to !Transient:

  DestroyFunc StartAppMaximised
  AddToFunc   StartAppMaximised
  + I ThisWindow ("name of window|another window", \
      !Maximized, !Transient) Maximize

Full-screen mode

The basis and functionality for the StartAppMaximized function is complete. The last remaining item is to make certain windows borderless and to remove their title so that they appear to cover the entire viewport. In the simplest case, the window's name or class is known beforehand, and an appropriate style line can be set6. For example:

Style "name of window" !Title, !Borders, HandleWidth 0, BorderWidth 0, ResizeHintOverride

That line ought to be pretty self-explanatory. The ResizeHintOverride condition makes those applications which are column-sized aware (such as XTerm, GVim, XV, etc) not to be so. Without it, some applications would leave a noticeable gap at the bottom of the screen.


This has been a very brief look into how FvwmEvent can be used to monitor and react to various events. The most important thing to remember about the use of FvwmEvent is specificity: always be as specific as possible when operating on windows. Where a certain amount of automation is required, always enforce a given context, unless it's a requirement that the user is to select an appropriate operand window at the time the event is triggered.


Some general links that might be of interest:


This is a slight simplification, since a window can be mapped and in a 'withdrawn' state.
Inter-client Communications Convention Manual (http://tronche.com/gui/x/icccm/)
In FVWM 2.4.X, one would have to use: Style "name of window" NoTitle, NoBorder, HandleWidth 0, BorderWidth 0

Talkback: Discuss this article with The Answer Gang


I used to write the long-running series "The Linux Weekend Mechanic", which was started by John Fisk (the founder of Linux Gazette) in 1996 and continued until 1998. Articles in that format have been intermittent, but might still continue in the future. I currently write occasional articles for LG, whilst doing a few things behind the scenes. I'm also a member of The Answer Gang.

I was born in Hammersmith (London UK) in 1983. When I was 13, I moved to the sleepy, thatched roofed, village of East Chaldon in the county of Dorset. I am very near the coast (at Lulworth Cove) which is where I used to work. Since then I have moved to Southampton, and currently attend University there, studying for a degree in Software Engineering.

I first got interested in Linux in 1996 having seen a review of it in a magazine (Slackware 2.0). I was fed up with the instability that the then-new operating system Win95 had and so I decided to give it a go. Slackware 2.0 was great. I have been a massive Linux enthusiast ever since. I ended up with running SuSE on both my desktop and laptop computers. Although I now use Debian as my primary operating system.

I am actively involved with the FVWM project, writing documentation, providing user-support, writing ad-hoc and somewhat esoteric patches for it.

Other hobbies include reading. I especially enjoy reading plays (Henrik Ibsen, Chekhov, George Bernard Shaw), and I also enjoy literature (Edgar Allan Poe, Charles Dickens, Jane Austen to name but a few).

I am also a keen musician. I play the piano in my spare time.

Some would consider me an arctophile (teddy bear collector).

I listen to a variety of music.

Copyright © 2006, Thomas Adam. Released under the Open Publication license unless otherwise noted in the body of the article. Linux Gazette is not produced, sponsored, or endorsed by its prior host, SSC, Inc.

Published in Issue 127 of Linux Gazette, June 2006

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