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Hacking i3: How To
==================
Michael Stapelberg <michael+i3@stapelberg.de>
March 2009

This document is intended to be the first thing you read before looking and/or touching
i3’s source code. It should contain all important information to help you understand
why things are like they are. If it does not mention something you find necessary, please
do not hesitate to contact me.

== Window Managers

A window manager is not necessarily needed to run X, but it is usually used in combination
to facilitate some things. The window manager's job is to take care of the placement of
windows, to provide the user some mechanisms to change the position/size of windows and
to communicate with clients to a certain extent (for example handle fullscreen requests
of clients such as MPlayer).

There are no different contexts in which X11 clients run, so a window manager is just another
client, like all other X11 applications. However, it handles some events which normal clients
usually don’t handle.

In the case of i3, the tasks (and order of them) are the following:

. Grab the key bindings (events will be sent upon keypress/keyrelease)
. Iterate through all existing windows (if the window manager is not started as the first
  client of X) and manage them (= reparent them, create window decorations)
. When new windows are created, manage them
. Handle the client’s `_WM_STATE` property, but only the `_WM_STATE_FULLSCREEN`
. Handle the client’s `WM_NAME` property
. Handle the client’s size hints to display them proportionally
. Handle enter notifications (focus follows mouse)
. Handle button (as in mouse buttons) presses for focus/raise on click
. Handle expose events to re-draw own windows such as decorations
. React to the user’s commands: Change focus, Move windows, Switch workspaces,
Change the layout mode of a container (default/stacking), Start a new application,
Restart the window manager

In the following chapters, each of these tasks and their implementation details will be discussed.

=== Tiling window managers

Traditionally, there are two approaches to managing windows: The most common one nowadays is
floating, which means the user can freely move/resize the windows. The other approach is called
tiling, which means that your window manager distributing windows to use as much space as
possible while not overlapping.

The idea behind tiling is that you should not need to waste your time moving/resizing windows
while you usually want to get some work done. After all, most users sooner or later tend to
lay out their windows in a way which corresponds to tiling or stacking mode in i3. Therefore,
why not let i3 do this for you? Certainly, it’s faster than you could ever do it.

The problem with most tiling window managers is that they are too unflexible. In my opinion, a
window manager is just another tool, and similar to vim which can edit all kinds of text files
(like source code, HTML, …) and is not limited to a specific file type, a window manager should
not limit itself to a certain layout (like dwm, awesome, …) but provide mechanisms for you to
easily create the layout you need at the moment.

=== The layout table

To accomplish flexible layouts, we decided to simply use a table. The table grows and shrinks
as you need it. Each cell holds a container which then holds windows (see picture below). You
can use different layouts for each container (default layout and stacking layout).

So, when you open a terminal and immediately open another one, they reside in the same container,
in default layout. The layout table has exactly one column, one row and therefore one cell.
When you move one of the terminals to the right, the table needs to grow. It will be expanded
to two columns and one row. This enables you to have different layouts for each container.
The table then looks like this:

[width="15%",cols="^,^"]
|========
| T1 | T2
|========

When moving terminal 2 to the bottom, the table will be expanded again.

[width="15%",cols="^,^"]
|========
| T1 |
|    | T2
|========

You can really think of the layout table like a traditional HTML table, if you’ve ever
designed one. Especially col- and rowspan work equally. Below you see an example of
colspan=2 for the first container (which has T1 as window).

[width="15%",cols="^asciidoc"]
|========
| T1
|
[cols="^,^",frame="none"]
!========
! T2 ! T3
!========
|========

Furthermore, you can freely resize table cells.

== Files

include/data.h::
Contains data definitions used by nearly all files. You really need to read this first.

include/*.h::
Contains forward definitions for all public functions.

src/commands.c::
Parsing commands

src/config.c::
Parses the configuration file

src/debug.c::
Contains debugging functions to print unhandled X events

src/handlers.c::
Contains all handlers for all kind of X events

src/layout.c::
Renders your layout (screens, workspaces, containers)

src/mainx.c::
Initializes the window manager

src/table.c::
Manages the most important internal data structure, the design table.

src/util.c::
Contains useful functions which are not really dependant on anything.

src/xcb.c::
Contains wrappers to use xcb more easily.

src/xinerama.c::
(Re-)initializes the available screens and converts them to virtual screens (see below).

== Data structures

See include/data.h for documented data structures. The most important ones are explained
right here.

TODO: We need a slick graphic here

=== Virtual screens

A virtual screen (type `i3Screen`) is generated from the connected screens obtained
through Xinerama. The difference to the raw Xinerama monitors as seen when using xrandr(1)
is that it falls back to the lowest common resolution of the logical screens.

For example, if your notebook has 1280x800 and you connect a video projector with
1024x768, set up in clone mode (xrandr \--output VGA \--mode 1024x768 \--same-as LVDS),
i3 will have one virtual screen.

However, if you configure it using xrandr \--output VGA \--mode 1024x768 \--right-of LVDS,
i3 will generate two virtual screens. For each virtual screen, a new workspace will be
assigned. New workspaces are created on the screen you are currently on.

=== Workspace

A workspace is identified by its number. Basically, you could think of workspaces
as different desks in your bureau, if you like the desktop methaphor. They just contain
different sets of windows and are completely separate of each other. Other window
managers also call this ``Virtual desktops''.

=== The layout table

Each workspace has a table, which is just a two-dimensional dynamic array containing
Containers (see below). This table grows and shrinks as you need it (by moving windows
to the right you can create a new column in the table, by moving them to the bottom
you create a new row).

=== Container

A container is the content of a table’s cell. It holds an arbitrary amount of windows
and has a specific layout (default layout or stack layout). Containers can consume
multiple table cells by modifying their colspan/rowspan attribute.

=== Client

A client is x11-speak for a window.

== List/queue macros

i3 makes heavy use of the list macros defined in BSD operating systems. To ensure
that the operating system on which i3 is compiled has all the awaited features,
i3 comes with `include/queue.h`. On BSD systems, you can use man `queue(3)`. On Linux,
you have to use google.

The lists used are `SLIST` (single linked lists) and `CIRCLEQ` (circular queues).
Usually, only forward traversal is necessary, so an `SLIST` works fine. However,
for the windows inside a container, a `CIRCLEQ` is necessary to go from the currently
selected window to the window above/below.

== Naming conventions

There is a row of standard variables used in many events. The following names should be
chosen for those:

 * ``conn'' is the xcb_connection_t
 * ``event'' is the event of the particular type
 * ``container'' names a container
 * ``client'' names a client, for example when using a `CIRCLEQ_FOREACH`

== Startup (src/mainx.c, main())

 * Establish the xcb connection
 * Check for XKB extension on the separate X connection
 * Check for Xinerama screens
 * Grab the keycodes for which bindings exist
 * Manage all existing windows
 * Enter the event loop

== Keybindings

=== Grabbing the bindings

Grabbing the bindings is quite straight-forward. You pass X your combination of modifiers and
the keycode you want to grab and whether you want to grab them actively or passively. Most
bindings (everything except for bindings using Mode_switch) are grabbed passively, that is,
just the window manager gets the event and cannot replay it.

We need to grab bindings that use Mode_switch actively because of a bug in X. When the window
manager receives the keypress/keyrelease event for an actively grabbed keycode, it has to decide
what to do with this event: It can either replay it so that other applications get it or it
can prevent other applications from receiving it.

So, why do we need to grab keycodes actively? Because X does not set the state-property of
keypress/keyrelease events properly. The Mode_switch bit is not set and we need to get it
using XkbGetState. This means we cannot pass X our combination of modifiers containing Mode_switch
when grabbing the key and therefore need to grab the keycode itself without any modiffiers.
This means, if you bind Mode_switch + keycode 38 ("a"), i3 will grab keycode 38 ("a") and
check on each press of "a" if the Mode_switch bit is set using XKB. If yes, it will handle
the event, if not, it will replay the event.

=== Handling a keypress

As mentioned in "Grabbing the bindings", upon a keypress event, i3 first gets the correct state.

Then, it looks through all bindings and gets the one which matches the received event.

The bound command is parsed directly in command mode.

== Manage windows (src/mainx.c, manage_window() and reparent_window())

`manage_window()` does some checks to decide whether the window should be managed at all:

 * Windows have to be mapped, that is, visible on screen
 * The override_redirect must not be set. Windows with override_redirect shall not be
   managed by a window manager

Afterwards, i3 gets the intial geometry and reparents the window if it wasn’t already
managed.

Reparenting means that for each window which is reparented, a new window, slightly larger
than the original one, is created. The original window is then reparented to the bigger one
(called "frame").

After reparenting, the window type (`_NET_WM_WINDOW_TYPE`) is checked to see whether this
window is a dock (`_NET_WM_WINDOW_TYPE_DOCK`), like dzen2 for example. Docks are handled
differently, they don’t have decorations and are not assigned to a specific container.
Instead, they are positioned at the bottom of the screen. To get the height which needsd
to be reserved for the window, the `_NET_WM_STRUT_PARTIAL` property is used.

== What happens when an application is started?

i3 does not care for applications. All it notices is when new windows are mapped (see
`src/handlers.c`, `handle_map_notify_event()`). The window is then reparented (see section
"Manage windows").

After reparenting the window, `render_layout()` is called which renders the internal
layout table. The window was placed in the currently focused container and
therefore the new window and the old windows (if any) need te be moved/resized
so that the currently active layout (default mode/stacking mode) is rendered
correctly. To move/resize windows, a window is ``configured'' in X11-speak.

Some applications, such as MPlayer obivously assume the window manager is stupid
and therefore configure their windows by themselves. This generates an event called
configurenotify. i3 handles these events and pushes the window back to its position/size.

== _NET_WM_STATE

Only the _NET_WM_STATE_FULLSCREEN atom is handled. It calls ``toggle_fullscreen()'' for the
specific client which just configures the client to use the whole screen on which it
currently is. Also, it is set as fullscreen_client for the i3Screen.

== WM_NAME

When the WM_NAME property of a window changes, its decoration (containing the title)
is re-rendered.

== Size hints

TODO

== Rendering (src/layout.c, render_layout() and render_container())

There are two entry points to rendering: render_layout() and render_container(). The
former one renders all virtual screens, the currently active workspace of each virtual
screen and all containers (inside the table cells) of these workspaces using
render_container(). Therefore, if you need to render only a single container, for
example because a window was removed, added or changed its title, you should directly
call render_container().

Rendering consists of two steps: In the first one, in render_layout(), each container
gets its position (screen offset + offset in the table) and size (container's width
times colspan/rowspan). Then, render_container() is called:

render_container() then takes different approaches, depending on the mode the container
is in.

=== Common parts

On the frame (the window which was created around the client’s window for the decorations),
a black rectangle is drawn as a background for windows like MPlayer, which don’t completely
fit into the frame.

=== Default mode

Each clients gets the container’s width and an equal amount of height.

=== Stack mode

In stack mode, a window containing the decorations of all windows inside the container
is placed at the top. The currently focused window is then given the whole remaining
space.

=== Window decorations

The window decorations consist of a rectangle in the appropriate color (depends on whether
this window is the currently focused one or the last focused one in a not focused container
or not focused at all) forming the background. Afterwards, two lighter lines are drawn
and the last step is drawing the window’s title (see WM_NAME) onto it.

=== Fullscreen windows

For fullscreen windows, the `rect` (x, y, width, height) is not changed to allow the client
to easily go back to its previous position. Instead, fullscreen windows are skipped
when rendering.

=== Resizing containers

By clicking and dragging the border of a container, you can resize it freely.

TODO

== User commands / commandmode (src/commands.c)

Like in vim, you can control i3 using commands. They are intended to be a powerful
alternative to lots of shortcuts, because they can be combined. There are a few special
commands, which are the following:

exec <command>::
Starts the given command by passing it to `/bin/sh`.

restart::
Restarts i3 by executing `argv[0]` (the path with which you started i3) without forking.

w::
"With". This is used to select a bunch of windows. Currently, only selecting the whole
container in which the window is in, is supported by specifying "w".

f, s, d::
Toggle fullscreen, stacking, default mode for the current window/container.

The other commands are to be combined with a direction. The directions are h, j, k and l,
like in vim (h = left, j = down, k = up, l = right). When you just specify the direction
keys, i3 will move the focus in that direction. You can provide "m" or "s" before the
direction to move a window respectively or snap.

== Gotchas

* Forgetting to call `xcb_flush(conn);` after sending a request. This usually leads to
  code which looks like it works fine but which does not work under certain conditions.