The following is a brief code overview / general introduction for those wanting to hack on sway.
In Wayland the compositor is the display server. That's a design decision that brings several advantages, but the downside is that all compositors need to implement an entire display server as well.
To aid the situation there are several wayland display servers being
implemented as libraries so that compositors can stick to doing compositing and
leave the low level details to one of those libraries. In sway that library is
wlc, and it handles tty switching, logind sessions, input, everything that
deals with the GPU, and just about everything concerning the Wayland protocol
itself (as of writing there's not a single call to any wayland functions inside
of sway). sway communicates with wlc via a callback api found in
sway/handlers (wlc_interface). The code in that file deals with all the
entry points from wlc to sway.
Being a tiling window manager, controlling it via commands is an important part
of its functionality, and sway/commands which deals with that is by far the
biggest file in the codebase.
There are multiple ways to trigger a command: via the keyboard, via the config file, or via the IPC interface.
i3 has an IPC interface (it creates a socket that applications can connect to
and issue commands or queries via its protocol), and sway replicates that
protocol (so e.g. i3-msg can be used with sway by simply changing the socket,
e.g. i3-msg -s $(sway --get-socketpath)). The code for that lies in
sway/ipc.
The config state and loading the config file lies in sway/config. Since the
config file is simply a list of commands, that code mostly just parses the text
and then hands commands off to commands for execution.
The mouse has buttons, state (due to buttons pressed, e.g. "dragging",
"resizing" etc.) and movement. Most code related to that lies in
sway/input_state.
In traditional floating window managers, all windows (or views as they're called in sway) are placed anywhere on the screen. In a tiling window manager like sway the views are arranged by the compositor, and the user mostly just manipulates the arrangement via commands (floating views are also supported).
In sway, each output (a physical monitor) has one or more workspaces which has one or more views (the actual windows). In order to keep track of the arrangement of the views, sway organizes everything in a tree of containers. Each of the previously mentioned things is a type of container. In addition there's a type of container called container which is needed to arrange other containers as siblings (horizontal or vertical layout), and a root container which exists for practical reasons.
sway/containers contains the code for this and understanding containers is
essential in understanding sway.
Also, the code that actually arranges the different views lays in
sway/layout.
When changing workspace, changing output, changing view or just moving the
pointer you change which view has focus. The code for handling this and
e.g. deciding what view receives input events is handled in sway/focus.
As sway is a work in progress, as of writing it is still not versioned. Use the
master branch of sway and wlc for now.