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Making My Own Desktop Manager: Starting at the Beginning

The hardest part of this whole project is the lack of people talking about X Window Managers. There are countless forums post for people asking how to start learning about Window Managers and either get no response or someone just says "read the source code of {insert open-source window manager here}".

While, as a programmer, I probably agree with learning from others' source code; but I also think there is a big gap of information missing from this equation. There are plenty of high-level explanations of the roles of Window Managers and there is source-code. That's about it.

So just to clarify, here are the roles my Window Manager will be responsible for:
  • Listening for events on the root window and all child windows
  • Size and placement of windows when they are first created
  • Show the difference between the focused/active window and all other windows
  • Iconified/Hidden states of windows
  • Resizing windows
  • Moving floating windows
  • Window decoration (If I choose to go down this route?)
  • Launching programs

I decided the next phase would be to write the simplest possible Window Manager. This window manager would allow programs to accept the programs preferred location & size and would also draw a white border around the focused/active window.

The code for this example can also be found in 'simple-wm' in my GitHub project-sandbox. (Including the MakeFile)

#include <X11/Xlib.h>
#include <stdio.h>

#include <stdlib.h>


static Window focused_win;

int handle_xerror(Display *dpy, XErrorEvent *ee) {
   int i=0, alen = sizeof(ee) / sizeof(XErrorEvent);
  for (i=0; i
<alen; i++) {
    fprintf(stderr, "An error occurred: %d\n", ee[i].error_code);
  }
  return 0;
}

int main()
{
  printf("opening display\n");
  Display *d = XOpenDisplay(NULL);

   printf("grabbing default screen\n");
  int screen = DefaultScreen(d);
  printf("default screen = %d\n", screen);

  printf("grabbing root window\n");
  Window root = DefaultRootWindow(d);

  // tell root window you'll be capturing it's events
  XSetWindowAttributes a;
  a.event_mask =   StructureNotifyMask|SubstructureNotifyMask|EnterWindowMask|LeaveWindowMask;   // root + child (sub) windows created, window-in, window-out
  XSelectInput(d, root, a.event_mask);

  // handle errors
  XSetErrorHandler(handle_xerror);

  // capture those events
  XEvent e;

  // main event loop
  for (;;) {
    XNextEvent(d, &e);

    if (e.type == CreateNotify) {
      printf("Event: create\n");
      XSelectInput(e.xcreatewindow.display, e.xcreatewindow.window, a.event_mask);
      XSetWindowBorderWidth(e.xcreatewindow.display, e.xcreatewindow.window, 1); // always set border to 1px
     }

    else if (e.type == ConfigureNotify) {
      printf("EVENT: configure\n");
    }

    else if (e.type == EnterNotify) {
      printf("EVENT: focus-in\n");
      focused_win = e.xcrossing.window;
      a.border_pixel = WhitePixel(e.xcrossing.display, screen);
      XChangeWindowAttributes(e.xcrossing.display, focused_win, CWBorderPixel, &a);
    }

    else if (e.type == LeaveNotify) {
      printf("EVENT: focus-out\n");
      a.border_pixel = BlackPixel(e.xcrossing.display, screen);
      XChangeWindowAttributes(e.xcrossing.display, e.xcrossing.window, CWBorderPixel, &a);
    }
   }
   return 0;
}


To see this window manager in action you will need to edit your .xinitrc file to launch and wait for an xterm and run the window manager form here. I realise I could launch the window manager from .xinitrc as a background process, but I want to watch the events as they happen from my logging in the host xterm shell.

So I:

  • Edited the .xinitrc file for my wmtest user (click here for my .xinitrc file)
  • Logged in as that user via SLiM
  • Started simple-wm:
    • $ /path/to/simple-wm/program&
  • Then launched another terminal window on the other side of the screen:
    • $ xterm -geometry 50x50+500+500

Notice how I didn't need to do anything in the code for the xterm window to position correctly at x=500,y=500 and size of 50charsx50chars? If you don't change the values on the ConfigureNotify event, the client window's hints are accepted! Now when I hover over the xterm window, I get a white border! Hover out and the border goes black! :)

To exit the session just type exit on the xterm shell.

 

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