# Dependencies and the window

# Boring, I know

Some of you reading this are very experienced with opening up windows in Rust and probably have your favorite windowing library, but this guide is designed for everybody, so it's something that we need to cover. Luckily, you don't need to read this if you know what you're doing. One thing that you do need to know is that whatever windowing solution you use needs to support the raw-window-handle (opens new window) crate.

# What crates are we using?

For the beginner stuff, we're going to keep things very simple, we'll add things as we go, but I've listed the relevant Cargo.toml bits below.

[dependencies]
winit = "0.26"
env_logger = "0.9"
log = "0.4"
wgpu = "0.12"

# Using Rust's new resolver

As of version 0.10, wgpu requires cargo's newest feature resolver (opens new window), which is the default in the 2021 edition (any new project started with Rust version 1.56.0 or newer). However, if you are still using the 2018 edition, you must include resolver = "2" in either the [package] section of Cargo.toml if you are working on a single crate, or the [workspace] section of the root Cargo.toml in a workspace.

# env_logger

It is very important to enable logging via env_logger::init();. When wgpu hits any error it panics with a generic message, while logging the real error via the log crate. This means if you don't include env_logger::init(), wgpu will fail silently, leaving you very confused!
(This has been done in the code below)

# Create a new project

run cargo new project_name where project_name is the name of the project.
(In the example below I have used 'tutorial1_window')

# The code

There's not much going on here yet, so I'm just going to post the code in full. Just paste this into your lib.rs or equivalent.

use winit::{
    event::*,
    event_loop::{ControlFlow, EventLoop},
    window::WindowBuilder,
};

pub fn run() {
    env_logger::init();
    let event_loop = EventLoop::new();
    let window = WindowBuilder::new().build(&event_loop).unwrap();

    event_loop.run(move |event, _, control_flow| match event {
        Event::WindowEvent {
            ref event,
            window_id,
        } if window_id == window.id() => match event {
            WindowEvent::CloseRequested
            | WindowEvent::KeyboardInput {
                input:
                    KeyboardInput {
                        state: ElementState::Pressed,
                        virtual_keycode: Some(VirtualKeyCode::Escape),
                        ..
                    },
                ..
            } => *control_flow = ControlFlow::Exit,
            _ => {}
        },
        _ => {}
    });
}

All this does is create a window, and keep it open until the user closes it, or presses escape. Next, we'll need a main.rs to run the code. It's quite simple, it just imports run() and, well, runs it!

use tutorial1_window::run;

fn main() {
    run();
}

(Where 'tutorial1_window' is the name of the project you created with cargo earlier)

If you only want to support desktops, that's all you have to do! In the next tutorial, we'll start using wgpu!

# Added support for the web

If I go through this tutorial about WebGPU and never talk about using it on the web, then I'd hardly call this tutorial complete. Fortunately getting a wgpu application running in a browser is not too difficult once you get things set up.

Let's start with the changes we need to make to are Cargo.toml:

[lib]
crate-type = ["cdylib", "rlib"]

These lines tell cargo that we want to allow our crate to build a native Rust static library (rlib) and a C/C++ compatible library (cdylib). We need rlib if we want to run wgpu in a desktop environment. We need cdylib to create the Web Assembly that the browser will run.

# Web Assembly

Web Assembly i.e. WASM, is a binary format supported by most modern browsers that allows lower-level languages such as Rust to run on a web page. This allows us to write the bulk of our application in Rust and use a few lines of Javascript to get it running in a web browser.

Now, all we need are some more dependencies that are specific to running in WASM:

[dependencies]
cfg-if = "1"
# the other regular dependencies...

[target.'cfg(target_arch = "wasm32")'.dependencies]
console_error_panic_hook = "0.1.6"
console_log = "0.2.0"
wgpu = { version = "0.12", features = ["webgl"]}
wasm-bindgen = "0.2"
wasm-bindgen-futures = "0.4.30"
web-sys = { version = "0.3", features = [
    "Document",
    "Window",
    "Element",
]}

The cfg-if (opens new window) crate adds a macro that makes using platform-specific code more manageable.

The [target.'cfg(target_arch = "wasm32")'.dependencies] line tells cargo to only include these dependencies if we are targeting the wasm32 architecture. The next few dependencies just make interfacing with javascript a lot easier.

  • console_error_panic_hook (opens new window) configures the panic! macro to send errors to the javascript console. Without this when you encounter panics, you'll be left in the dark about what caused them.
  • console_log (opens new window) implements the log (opens new window) API. It sends all logs to the javascript console. It can be configured to only send logs of a particular log level. This is also great for debugging.
  • We need to enable WebGL feature on wgpu if we want to run on most current browsers. Support is in the works for using the WebGPU api directly, but that is only possible on experimental versions of browsers such as Firefox Nightly and Chrome Canary.
    You're welcome to test this code on these browsers (and the wgpu devs would appreciate it as well), but for sake of simplicity, I'm going to stick to using the WebGL feature until the WebGPU api gets to a more stable state.
    If you want more details check out the guide for compiling for the web on wgpu's repo (opens new window)
  • wasm-bindgen (opens new window) is the most important dependency in this list. It's responsible for generating the boilerplate code that will tell the browser how to use our crate. It also allows us to expose methods in Rust that can be used in Javascript, and vice-versa.
    I won't get into the specifics of wasm-bindgen, so if you need a primer (or just a refresher) check out this (opens new window)
  • web-sys (opens new window) is a crate that includes many methods and structures that are available in a normal javascript application: get_element_by_id, append_child. The features listed are only the bare minimum of what we need currently.

# More code

First, we need to import wasm-bindgen in lib.rs:

#[cfg(target_arch="wasm32")]
use wasm_bindgen::prelude::*;

Next, we need to tell wasm-bindgen to run our run() function when the WASM is loaded:

#[cfg_attr(target_arch="wasm32", wasm_bindgen(start))]
pub async fn run() {
    // snipped...
}

Then we need to toggle what logger we are using based on if we are in WASM land or not. Add the following to the top of the run function replacing the env_logger::init() line:

cfg_if::cfg_if! {
    if #[cfg(target_arch = "wasm32")] {
        std::panic::set_hook(Box::new(console_error_panic_hook::hook));
        console_log::init_with_level(log::Level::Warn).expect("Couldn't initialize logger");
    } else {
        env_logger::init();
    }
}

This will set up console_log and console_error_panic_hook in a web build, and will initialize env_logger in a normal build. This is important as env_logger doesn't support Web Assembly at the moment.

Next, after we create our event loop and window, we need to add a canvas to the HTML document that we will host our application:

#[cfg(target_arch = "wasm32")]
{
    // Winit prevents sizing with CSS, so we have to set
    // the size manually when on web.
    use winit::dpi::PhysicalSize;
    window.set_inner_size(PhysicalSize::new(450, 400));
    
    use winit::platform::web::WindowExtWebSys;
    web_sys::window()
        .and_then(|win| win.document())
        .and_then(|doc| {
            let dst = doc.get_element_by_id("wasm-example")?;
            let canvas = web_sys::Element::from(window.canvas());
            dst.append_child(&canvas).ok()?;
            Some(())
        })
        .expect("Couldn't append canvas to document body.");
}

The "wasm-example" id is specific to my project (aka. this tutorial). You can substitute this for whatever id you're using in your HTML. Alternatively, you could add the canvas directly to the <body> as they do in the wgpu repo. This part is ultimately up to you.

That's all the web-specific code we need for now. The next thing we need to do is build the Web Assembly itself.

# Wasm Pack

Now you can build a wgpu application with just wasm-bindgen, but I ran into some issues doing that. For one, you need to install wasm-bindgen on your computer as well as include it as a dependency. The version you install as a dependency needs to exactly match the version you installed, otherwise, your build will fail.

To get around this shortcoming, and to make the lives of everyone reading this easier, I opted to add wasm-pack (opens new window) to the mix. Wasm-pack handles installing the correct version of wasm-bindgen for you, and it supports building for different types of web targets as well: browser, NodeJS, and bundlers such as webpack.

To use wasm-pack, first, you need to install it (opens new window).

Once you've done that, we can use it to build our crate. If you only have one crate in your project, you can just use wasm-pack build. If you're using a workspace, you'll have to specify what crate you want to build. Imagine your crate is a directory called game, you would use:

wasm-pack build game

Once wasm-pack is done building you'll have a pkg directory in the same directory as your crate. This has all the javascript code needed to run the WASM code. You'd then import the WASM module in javascript:

const init = await import('./pkg/game.js');
init().then(() => console.log("WASM Loaded"));

This site uses Vuepress (opens new window), so I load the WASM in a Vue component. How you handle your WASM will depend on what you want to do. If you want to check out how I'm doing things take a look at this (opens new window).

If you intend to use your WASM module in a plain HTML website, you'll need to tell wasm-pack to target the web:

wasm-pack build --target web

You'll then need to run the WASM code in an ES6 Module:

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Pong with WASM</title>
</head>

<body>
  <script type="module">
      import init from "./pkg/pong.js";
      init().then(() => {
          console.log("WASM Loaded");
      });
  </script>
  <style>
      canvas {
          background-color: black;
      }
  </style>
</body>

</html>

Press the button below and you will see the code running!

Last Updated: 7/1/2022, 7:15:05 PM