Rust XCB

Rust-XCB is a safe Rust interface to XCB. Rust-XCB uses under the hood the core XCB functions to connect and communicate with the X server.

[dependencies]
xcb = "1.0.0-beta"

Documentation: https://rust-x-bindings.github.io/rust-xcb/branches/v1.0-dev/xcb/

Rust-XCB is constituted of the following components:

the core library
the X protocol and its extensions

See CONTRIBUTING.md for contributions.

The core library

The main component is the Connection class which is used to connect and communicate to the X server. The Connection class wraps calls to the C XCB functions in a safe to use interface.

In the new API (v1.0+), Rust-XCB takes care of all the heavy lifting of event and error resolution, including the handling of the different kinds of event (regular events, "GeGeneric" events, the specifics about Xkb ...) and present them under a unified and safe to use enum instead of requesting the user to perform unsafe cast as in the C library.

The core library also provides many traits that are used in the protocol implementation. e.g. the Wired trait has implementation for each type that must be serialized for the requests.

The protocol implementation

The core X protocol and all the extensions present in XCB are generated by the build scripts, entirely written in Rust (no more python dependency). The build script do not generate bindings to the C protocol extensions, it generates an actual Rust implementation of the protocol:

Simple structures that have the same memory layout than C are translated in Rust directly (e.g. points etc.)
More complex structures wrap a slice of raw data and provide accessor methods (Debug will still print all the members)
The masks use the bitflags crate macro
The enums are enums!
The unions are also enums, but they carry data
The Xids (handles to windows, pixmaps etc.) are type-safe implementations of the Xid trait, not just integers like with 0.x versions.
The requests are structures that serialize themselves when passed to the Connection.
- Each request has two type of cookie, for checked and unchecked requests. This allows type safe reply fetching and error checking
The protocol and each extension provide an Event and an Error enum, which are unified by the core library.

The new API

Here are some highlights of the new API:

Modules

Previously, the core X protocol was directly accessible under the xcb crate namespace. This is no longer the case, the protocol is under the x module, and each extension gets its own module too like before.

Only the core library is directly accessible under xcb. This helps for a clean separation of concerns and save a lot of confusion in regards to the new event and error types.

Window creation

// v0.x
let window = conn.generate_id(); // this is a u32

let values = [
    (xcb::CW_BACK_PIXEL, screen.white_pixel()),
    (
        xcb::CW_EVENT_MASK,
        xcb::EVENT_MASK_EXPOSURE | xcb::EVENT_MASK_KEY_PRESS,
    ),
];

xcb::create_window(
    &conn,
    xcb::COPY_FROM_PARENT as u8,
    window,
    screen.root(),
    0,
    0,
    150,
    150,
    10,
    xcb::WINDOW_CLASS_INPUT_OUTPUT as u16,
    screen.root_visual(),
    &values,
);

// v1.0

let window: x::Window = conn.generate_id();

conn.send_request(&x::CreateWindow {
    depth: x::COPY_FROM_PARENT as u8,
    wid: window,
    parent: screen.root(),
    x: 0,
    y: 0,
    width: 150,
    height: 150,
    border_width: 10,
    class: x::WindowClass::InputOutput,
    visual: screen.root_visual(),
    value_list: &[
        x::Cw::BackPixel(screen.white_pixel()),
        x::Cw::EventMask(x::EventMask::EXPOSURE | x::EventMask::KEY_PRESS),
    ],
});

Checked void request

// v0.x
// same cookie than for xcb::map_window
let cookie = xcb::map_window_checked(&conn, window);
// this error is a simple wrapper over xcb_generic_error_t
cookie.request_check()?;

// v1.0
// specific cookie type for the checked request
// code would not compile with `conn.send_request(..)`
let cookie = conn.send_request_checked(&x::MapWindow {window});
// reports a resolved error enum (e.g. x::Error::Drawable(..))
conn.check_request(cookie)?;

Event and error handling

// 0.x
loop {
    let event = conn.wait_for_event();
    // Errors are only wrappers around generic C errors and must be casted
    // from the event if `response_type` is 0. That seem very safe Rust...
    // I am the core author and I don't exactly know how to do proper error
    // handling with the 0.x API...
    match event {
        None => {
            break;
        }
        Some(event) => {
            let r = event.response_type();
            if r == 0 {
                // This is an error.
                panic!("received error from the X server");
            } else if r == xcb::KEY_PRESS as u8 {
                let key_press: &xcb::KeyPressEvent = unsafe {
                    // so much for the safe interface
                    xcb::cast_event(&event)
                };

                // do stuff

            } else if r == xkb_first_event {
                // resolving extension events like we would in C
                // very poor support is provided (not better than C)
            }
        }
    }
}

// 1.0
fn main() -> xcb::Result<()> {
    // ...
    loop {
        match conn.wait_for_event()? {
            xcb::Event::X(x::Event::KeyPress(key_press)) => {
                // do stuff
            }
            xcb::Event::Xkb(xkb::Event::MapNotify(ev)) => {
                // do other stuff (pass data to xkbcommon for example)
            }
            _ => {}
        }
    }
}

Debugging

All types in Rust XCB implement Debug in a way that allows recursive debug print. E.g. iterators will not print a useless pointer value, but will recurse down to each element.

There is in addition the optional "debug_atom_names" cargo feature under which each atom will print its name for easier debugging in some situations. For example, Xinput provide some information about input devices with atom identifiers. This allows you to quickly look-up which atoms you need to intern and seek for. E.g. the feature would turn:

Atom {
    res_id: 303,
}

into

Atom("Abs Pressure" ; 303)

The feature sets global variable to have access to the connection in the Debug::fmt call, so it should be activated only when needed.

xcb 1.0.0-beta.3