crates | ||
examples | ||
src | ||
tests | ||
.appveyor.yml | ||
.gitignore | ||
.travis.yml | ||
Cargo.toml | ||
DESIGN.md | ||
LICENSE-APACHE | ||
LICENSE-MIT | ||
package.json | ||
README.md | ||
yarn.lock |
wasm-bindgen
A project for facilitating high-level interactions between wasm modules and JS.
This project is sort of half polyfill for features like the host bindings
proposal and half features for empowering high-level interactions between
JS and wasm-compiled code (currently mostly from Rust). More specifically this
project allows JS/wasm to communicate with strings, JS objects, classes, etc, as
opposed to purely integers and floats. Using wasm-bindgen
for example you can
define a JS class in Rust or take a string from JS or return one. The
functionality is growing as well!
Currently this tool is Rust-focused but the underlying foundation is language-independent, and it's hoping that over time as this tool stabilizes that it can be used for languages like C/C++!
Notable features of this project includes:
- Exposing Rust structs to JS as classes
- Exposing Rust functions to JS
- Managing arguments between JS/Rust (strings, numbers, classes, objects, etc)
- Importing JS functions with richer types (strings, objects)
- Importing JS classes and calling methods
- Receiving arbitrary JS objects in Rust, passing them through to JS
- Catching JS exceptions in imports
Planned features include:
- Field setters/getters in JS through Rust functions
- ... and more coming soon!
This project is still very "early days" but feedback is of course always welcome! If you're curious about the design plus even more information about what this crate can do, check out the design doc.
Basic usage
Let's implement the equivalent of "Hello, world!" for this crate.
Note: Currently this projects uses nightly Rust which you can acquire through rustup and configure with
rustup default nightly
First up, let's install the tools we need
$ rustup target add wasm32-unknown-unknown
$ cargo install --git https://github.com/alexcrichton/wasm-bindgen
The first command here installs the wasm target so you can compile to it, and
the latter will install the wasm-bindgen
CLI tool we'll be using later.
Next up let's make our project
$ cargo new js-hello-world
Now let's add a dependency on this project inside Cargo.toml
as well as
configuring our build output:
[lib]
crate-type = ["cdylib"]
[dependencies]
wasm-bindgen = { git = 'https://github.com/alexcrichton/wasm-bindgen' }
Next up our actual code! We'll write this in src/lib.rs
:
#![feature(proc_macro)]
extern crate wasm_bindgen;
use wasm_bindgen::prelude::*;
#[wasm_bindgen]
extern {
fn alert(s: &str);
}
#[wasm_bindgen]
#[no_mangle]
pub extern fn greet(name: &str) {
alert(&format!("Hello, {}!", name));
}
And that's it! If we were to write the greet
function naively without the
#[wasm_bindgen]
attribute then JS wouldn't be able to communicate with the
types like str
, so slapping a #[wasm_bindgen]
on the function and the import
of alert
ensures that the right shims are generated.
Next up let's build our project:
$ cargo build --release --target wasm32-unknown-unknown
Note that we're using --release
here because unfortunately the current LLVM
backend for wasm has a few bugs in non-optimized mode. Those bugs will hopefully
get smoothed out over time!
After this you'll have a wasm file at
target/wasm32-unknown-unknown/release/js_hello_world.wasm
. If you'd like you
can use wasm-gc to make this file a little smaller
Now that we've generated the wasm module it's time to run the bindgen tool itself! This tool will postprocess the wasm file rustc generated, generating a new wasm file and a set of JS bindings as well. Let's invoke it!
$ wasm-bindgen target/wasm32-unknown-unknown/release/js_hello_world.wasm \
--out-dir .
This is the main point where the magic happens. The js_hello_world.wasm
file
emitted by rustc contains descriptors of how to communicate via richer types
than wasm currently supports. The wasm-bindgen
tool will interpret this
information, emitting a replacement module for the wasm file.
The previous js_hello_world.wasm
file is interpreted as if it were an ES6
module. The js_hello_world.js
file emitted by wasm-bindgen
should have the
intended interface of the wasm file, notably with rich types like strings,
classes, etc.
The wasm-bindgen
tool also emits a few other files needed to implement this
module. For example js_hello_world_wasm.wasm
is the original wasm file but
postprocessed a bit. It's intended that the js_hello_world_wasm.wasm
file,
like before, acts like an ES6 module. The js_hello_world.wasm
file, for
example, uses import
to import functionality from the other *_shims
file
generated (an internal implementation detail here).
Note that you can also pass a --nodejs
argument to wasm-bindgen
for emitting
Node-compatible JS as well as a --typescript
argument to emit a *.d.ts
file
describing the exported contents.
At this point you'll probably plug these files into a larger build system.
Files emitted by wasm-bindgen
act like normal ES6 modules (one just happens to
be wasm). As of the time of this writing there's unfortunately not a lot of
tools that natively do this, but Webpack's 4.0 beta release has native wasm
support!. Let's take a look at that and see how it works.
First create an index.js
file:
const js = import("./js_hello_world");
js.then(js => {
js.greet("World!");
});
Note that we're using import(..)
here because Webpack doesn't
support synchronously importing modules from the main chunk just
yet.
Next our JS dependencies by creating a package.json
:
{
"scripts": {
"serve": "webpack-dev-server"
},
"devDependencies": {
"webpack": "^4.0.1",
"webpack-cli": "^2.0.10",
"webpack-dev-server": "^3.1.0"
}
}
and our webpack configuration
// webpack.config.js
const path = require('path');
module.exports = {
entry: "./index.js",
output: {
path: path.resolve(__dirname, "dist"),
filename: "index.js",
},
mode: "development"
};
Our corresponding index.html
:
<html>
<head>
<meta content="text/html;charset=utf-8" http-equiv="Content-Type"/>
</head>
<body>
<script src='./index.js'></script>
</body>
</html>
And finally:
$ npm run serve
If you open https://localhost:8080 in a browser you should see a Hello, world!
dialog pop up!
If that was all a bit much, no worries! You can follow along online to see all the files necessary as well as a script to set it all up.
What just happened?
Phew! That was a lot of words and a lot ended up happening along the way. There
were two main pieces of magic happening: the #[wasm_bindgen]
attribute and the
wasm-bindgen
CLI tool.
The #[wasm_bindgen]
attribute
This attribute, exported from the wasm-bindgen
crate, is the entrypoint to
exposing Rust functions to JS. This is a procedural macro (hence requiring the
nightly Rust toolchain) which will generate the appropriate shims in Rust to
translate from your type signature to one that JS can interface with. Finally
the attribute also serializes some information to the output artifact which
wasm-bindgen
-the-tool will discard after it parses.
There's a more thorough explanation below of the various bits and pieces of the
attribute, but it suffices for now to say that you can attach it to free
functions, structs, impl blocks for those structs and extern { ... }
blocks.
Some Rust features like generics, lifetime parameters, etc, aren't supported on
functions tagged with #[wasm_bindgen]
right now.
The wasm-bindgen
CLI tool
The next half of what happened here was all in the wasm-bindgen
tool. This
tool opened up the wasm module that rustc generated and found an encoded
description of what was passed to the #[wasm_bindgen]
attribute. You can
think of this as the #[wasm_bindgen]
attribute created a special section of
the output module which wasm-bindgen
strips and processes.
This information gave wasm-bindgen
all it needed to know to generate the JS
file that we then imported. The JS file wraps instantiating the underlying wasm
module (aka calling WebAssembly.instantiate
) and then provides wrappers for
classes/functions within.
What else can we do?
Much more! Here's a taste of various features you can use in this project:
// src/lib.rs
#![feature(proc_macro)]
extern crate wasm_bindgen;
use wasm_bindgen::prelude::*;
// Strings can both be passed in and received
#[wasm_bindgen]
#[no_mangle]
pub extern fn concat(a: &str, b: &str) -> String {
let mut a = a.to_string();
a.push_str(b);
return a
}
// A struct will show up as a class on the JS side of things
#[wasm_bindgen]
pub struct Foo {
contents: u32,
}
#[wasm_bindgen]
impl Foo {
pub fn new() -> Foo {
Foo { contents: 0 }
}
// Methods can be defined with `&mut self` or `&self`, and arguments you
// can pass to a normal free function also all work in methods.
pub fn add(&mut self, amt: u32) -> u32 {
self.contents += amt;
return self.contents
}
// You can also take a limited set of references to other types as well.
pub fn add_other(&mut self, bar: &Bar) {
self.contents += bar.contents;
}
// Ownership can work too!
pub fn consume_other(&mut self, bar: Bar) {
self.contents += bar.contents;
}
}
#[wasm_bindgen]
pub struct Bar {
contents: u32,
opaque: JsValue, // defined in `wasm_bindgen`, imported via prelude
}
#[wasm_bindgen(module = "./index")] // what ES6 module to import from
extern {
fn bar_on_reset(to: &str, opaque: &JsValue);
// We can import classes and annotate functionality on those classes as well
type Awesome;
#[wasm_bindgen(constructor)]
fn new() -> Awesome;
#[wasm_bindgen(method)]
fn get_internal(this: &Awesome) -> u32;
}
#[wasm_bindgen]
impl Bar {
pub fn from_str(s: &str, opaque: JsValue) -> Bar {
let contents = s.parse().unwrap_or_else(|_| {
Awesome::new().get_internal()
});
Bar { contents, opaque }
}
pub fn reset(&mut self, s: &str) {
if let Ok(n) = s.parse() {
bar_on_reset(s, &self.opaque);
self.contents = n;
}
}
}
The generated JS bindings for this invocation of the macro look like this. You can view them in action like so:
and our corresponding index.js
:
import { Foo, Bar, concat } from "./js_hello_world";
import { booted } from "./js_hello_world_wasm";
export function bar_on_reset(s, token) {
console.log(token);
console.log(`this instance of bar was reset to ${s}`);
}
function assertEq(a, b) {
if (a !== b)
throw new Error(`${a} != ${b}`);
console.log(`found ${a} === ${b}`);
}
function main() {
assertEq(concat('a', 'b'), 'ab');
// Note the `new Foo()` syntax cannot be used, static function
// constructors must be used instead. Additionally objects allocated
// corresponding to Rust structs will need to be deallocated on the
// Rust side of things with an explicit call to `free`.
let foo = Foo.new();
assertEq(foo.add(10), 10);
foo.free();
// Pass objects to one another
let foo1 = Foo.new();
let bar = Bar.from_str("22", { opaque: 'object' });
foo1.add_other(bar);
// We also don't have to `free` the `bar` variable as this function is
// transferring ownership to `foo1`
bar.reset('34');
foo1.consume_other(bar);
assertEq(foo1.add(2), 22 + 34 + 2);
foo1.free();
alert('all passed!')
}
export class Awesome {
constructor() {
this.internal = 32;
}
get_internal() {
return this.internal;
}
}
booted.then(main);
Feature reference
Here this section will attempt to be a reference for the various features implemented in this project. This is likely not exhaustive but the tests should also be a great place to look for examples.
The #[wasm_bindgen]
attribute can be attached to functions, structs,
impls, and foreign modules. Impls can only contain functions, and the attribute
cannot be attached to functions in an impl block or functions in a foreign
module. No lifetime parameters or type parameters are allowed on any of these
types. Foreign modules must have the "C"
abi (or none listed). Free functions
with #[wasm_bindgen]
must also have the "C"
abi or none listed and also be
annotated with the #[no_mangle]
attribute.
All structs referenced through arguments to functions should be defined in the
macro itself. Arguments allowed implement the WasmBoundary
trait, and examples
are:
- Integers (not u64/i64)
- Floats
- Borrowed strings (
&str
) - Owned strings (
String
) - Exported structs (
Foo
, annotated with#[wasm_bindgen]
) - Exported C-like enums (
Foo
, annotated with#[wasm_bindgen]
) - Imported types in a foreign module annotated with
#[wasm_bindgen]
- Borrowed exported structs (
&Foo
or&mut Bar
) - The
JsValue
type and&JsValue
(not mutable references) - Vectors and slices of supported integer types and of the
JsValue
type.
All of the above can also be returned except borrowed references. Passing
Vec<JsValue>
as an argument to a function is not currently supported. Strings are
implemented with shim functions to copy data in/out of the Rust heap. That is, a
string passed to Rust from JS is copied to the Rust heap (using a generated shim
to malloc some space) and then will be freed appropriately.
Owned values are implemented through boxes. When you return a Foo
it's
actually turned into Box<RefCell<Foo>>
under the hood and returned to JS as a
pointer. The pointer is to have a defined ABI, and the RefCell
is to ensure
safety with reentrancy and aliasing in JS. In general you shouldn't see
RefCell
panics with normal usage.
JS-values-in-Rust are implemented through indexes that index a table generated as part of the JS bindings. This table is managed via the ownership specified in Rust and through the bindings that we're returning. More information about this can be found in the design doc.
All of these constructs currently create relatively straightforward code on the JS side of things, mostly having a 1:1 match in Rust with JS.
License
This project is licensed under either of
- Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
- MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)
at your option.
Contribution
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this project by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.
Tests
In order to run the tests you will need (node.js)[https://nodejs.org/] version
8.9.4 or above. Running the tests is done by running cargo test
.