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https://github.com/fluencelabs/wasm-bindgen
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Add a basic README which at least attempts
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Alex Crichton
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README.md
332
README.md
@ -6,6 +6,338 @@ Rust (and maybe eventually other languages!)
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[](https://travis-ci.org/alexcrichton/wasm-bindgen)
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[](https://ci.appveyor.com/project/alexcrichton/wasm-bindgen)
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This project is intended to be a framework for interoperating between JS and
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Rust. Currently it's very Rust-focused but it's hoped that one day the
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`wasm-bindgen-cli` tool will not be so Rust-specific and would be amenable to
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bindgen for C/C++ modules.
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Notable features of this project includes:
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* Exposing Rust structs to JS as classes
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* Exposing Rust functions to JS
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* Managing arguments between JS/Rust (strings, numbers, classes, etc)
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Planned features include:
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* Receiving arbitrary JS objects in Rust
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* An optional flag to generate Typescript bindings
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* Field setters/getters in JS through Rust functions
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* ... and more coming soon!
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This project is still very "early days" but feedback is of course always
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welcome!
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## Basic usage
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Let's implement the equivalent of "Hello, world!" for this crate.
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> **Note:** Currently this projects uses *nightly Rust* which you can acquire
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> through [rustup] and configure with `rustup default nightly`
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[rustup]: https://rustup.rs
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First up, let's add the wasm target and generate a Rust project:
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```
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$ rustup target add wasm32-unknown-unknown
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$ cargo new js-hello-world
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```
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Now let's add a dependency on this project inside `Cargo.toml` as well as
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configuring our build output:
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```toml
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[lib]
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crate-type = ["cdylib"]
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[dependencies]
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wasm-bindgen = { git = 'https://github.com/alexcrichton/wasm-bindgen' }
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```
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Next up our actual code! We'll write this in `src/lib.rs`:
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```rust
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#![feature(proc_macro)]
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extern crate wasm_bindgen;
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use wasm_bindgen::prelude::*;
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wasm_bindgen! {
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pub fn greet(name: &str) -> String {
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format!("Hello, {}!", name)
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}
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}
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```
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Here we're wrapping the code we'd like to export to JS in the `wasm_bindgen!`
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macro. We'll see more features later, but it suffices to say that most Rust
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syntax fits inside here, it's not too special beyond what it generates!
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Next up let's build our project:
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```
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$ cargo build --release --target wasm32-unknown-unknown
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```
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Note that we're using `--release` here because unfortunately the current LLVM
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backend for wasm has a few bugs in non-optimized mode. Those bugs will hopefully
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get smoothed out over time!
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After this you'll have a wasm file at
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`target/wasm32-unknown-unknown/release/js_hello_world.wasm`. If you'd like you
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can use [wasm-gc] to make this file a little smaller
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[wasm-gc]: https://github.com/alexcrichton/wasm-gc
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Now that we've generated the wasm module it's time to run the bindgen tool
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itself! Let's install it:
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```
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$ cargo install --git https://github.com/alexcrichton/wasm-bindgen
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```
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This'll install a `wasm-bindgen` binary next to your `cargo` binary. This tool
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will postprocess the wasm file rustc generated, generating a new wasm file and a
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set of JS bindings as well. Let's invoke it!
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```
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$ wasm-bindgen target/wasm32-unknown-unknown/release/js_hello_world.wasm \
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--output-js hello.js \
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--output-wasm hello.wasm
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```
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This'll create a `hello.js` which binds the functions described in
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`js_hello_world.wasm`, and the `hello.wasm` will be a little smaller than the
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input `js_hello_world.wasm`, but it's otherwise equivalent. Note that `hello.js`
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isn't very pretty so to read it you'll probably want to run it through a JS
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formatter.
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Note that `hello.js` uses ES6 modules for easier integration into transpilers
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like webpack/rollup/babel/etc. We're going to test out the syntax in the browser
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with `index.html` below, but your browser may not natively support ES6 modules
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just yet. To see more information about this, you can browse
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[online](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import).
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Ok let's see what this look like on the web!
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```html
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<html>
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<head>
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<meta content="text/html;charset=utf-8" http-equiv="Content-Type"/>
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</head>
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<body>
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<script type='module'>
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import { instantiate } from "./hello.js";
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// Send an async request to fetch the wasm file and get all its contents
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fetch("hello.wasm")
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.then(resp => resp.arrayBuffer())
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// Invoke the wasm-bindgen-generated function `instantiate` which will
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// give us a compiled wasm module when it's resolved
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.then(wasm => instantiate(wasm))
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// Using the module, call our Rust-exported function `greet` and then
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// use `alert` to display it
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.then(mod => {
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alert(mod.greet("world"));
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});
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</script>
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</body>
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</html>
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```
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If you open that in a browser you should see a `Hello, world!` dialog pop up!
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## What just happened?
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Phew! That was a lot of words and a lot ended up happening along the way. There
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were two main pieces of magic happening: the `wasm_bindgen!` macro and the
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`wasm-bindgen` CLI tool.
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**The `wasm_bindgen!` macro**
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This macro, exported from the `wasm-bindgen` crate, is the entrypoint to
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exposing Rust functions to JS. This is a procedural macro (hence requiring the
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nightly Rust toolchain) which will transform the definitions inside and prepare
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appropriate wrappers to receive JS-compatible types and convert them to
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Rust-compatible types.
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There's a more thorough explanation below of the various bits and pieces of the
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macro, but it suffices for now to say that you can have free functions, structs,
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and impl blocks for those structs in the macro right now. Many Rust features
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aren't supported in these blocks like generics, lifetime parameters, etc.
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Additionally not all types can be taken or returned from the functions. In
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general though simple-ish types should work just fine!
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**The `wasm-bindgen` CLI tool**
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The next half of what happened here was all in the `wasm-bindgen` tool. This
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tool opened up the wasm module that rustc generated and found an encoded
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description of what was passed to the `wasm_bindgen!` macro. You can think of
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this as the `wasm_bindgen!` macro created a special section of the output module
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which `wasm-bindgen` strips and processes.
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This information gave `wasm-bindgen` all it needed to know to generate the JS
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file that we then imported. The JS file wraps instantiating the underlying wasm
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module (aka calling `WebAssembly.instantiate`) and then provides wrappers for
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classes/functions within.
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Eventually `wasm-bindgen` will also take a list of imports where you can call
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from Rust to JS without worrying about argument conversions and such. An example
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to come here soon!
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## What else can we do?
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Turns out much more! Here's a taste of various features you can use in this
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project:
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```rust
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// src/lib.rs
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#![feature(proc_macro)]
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extern crate wasm_bindgen;
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use wasm_bindgen::prelude::*;
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wasm_bindgen! {
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// Strings can both be passed in and received
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pub fn concat(a: &str, b: &str) -> String {
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let mut a = a.to_string();
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a.push_str(b);
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return a
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}
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// A struct will show up as a class on the JS side of things
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pub struct Foo {
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contents: u32,
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}
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impl Foo {
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pub fn new() -> Foo {
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Foo { contents: 0 }
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}
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// Methods can be defined with `&mut self` or `&self`, and arguments you
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// can pass to a normal free function also all work in methods.
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pub fn add(&mut self, amt: u32) -> u32 {
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self.contents += amt;
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return self.contents
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}
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// You can also take a limited set of references to other types as well.
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pub fn add_other(&mut self, bar: &Bar) {
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self.contents += bar.contents;
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}
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// Ownership can work too!
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pub fn consume_other(&mut self, bar: Bar) {
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self.contents += bar.contents;
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}
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}
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pub struct Bar {
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contents: u32,
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}
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impl Bar {
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pub fn from_str(s: &str) -> Foo {
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Bar { contents: s.parse().unwrap_or(0) }
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}
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pub fn reset(&mut self, s: &str) {
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if let Ok(n) = s.parse() {
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self.contents = n;
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}
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}
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}
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}
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```
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and this can be worked with similarly to above with:
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```html
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<html>
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<head>
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<meta content="text/html;charset=utf-8" http-equiv="Content-Type"/>
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</head>
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<body>
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<script type='module'>
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import { instantiate } from "./hello.js";
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function assertEq(a, b) {
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if (a !== b)
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throw new Error(`${a} != ${b}`);
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console.log(`found ${a} === ${b}`);
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}
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fetch("hello.wasm")
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.then(resp => resp.arrayBuffer())
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.then(instantiate)
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.then(mod => {
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assertEq(mod.concat('a', 'b'), 'ab');
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// Note the `new Foo()` syntax cannot be used, static function
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// constructors must be used instead. Additionally objects allocated
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// corrseponding to Rust structs will need to be deallocated on the
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// Rust side of things with an explicit call to `free`.
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let foo = mod.Foo.new();
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assertEq(foo.add(10), 10);
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foo.free();
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// Pass objects to one another
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let foo1 = mod.Foo.new();
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let bar = mod.Bar.from_str("22");
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foo1.add_other(bar);
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// We also don't have to `free` the `bar` variable as this function is
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// transferring ownership to `foo1`
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bar.reset('34');
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foo1.consume_other(bar);
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assertEq(foo1.add(2), 22 + 34 + 2);
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foo1.free();
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alert('all passed!')
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});
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</script>
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</body>
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</html>
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```
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## Feature reference
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Here this section will attempt to be a reference for the various features
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implemented in this project.
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In the `wasm_bindgen!` macro you can have three items: functions, structs, and
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impls. Impls can only contain functions. No lifetime parameters or type
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parameters are allowed on any of these types.
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All structs referenced through arguments to functions should be defined in the
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macro itself. Arguments allowed are:
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* Integers (not u64/i64)
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* Floats
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* Borrowed strings (`&str`)
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* Owned strings (`String`)
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* Owned structs (`Foo`) defined in the same bindgen macro
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* Borrowed structs (`&Foo` or `&mut Bar`) defined in the same bindgen macro
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All of the above can also be returned except borrowed references. Strings are
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implemented with shim functions to copy data in/out of the Rust heap. That is, a
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string passed to Rust from JS is copied to the Rust heap (using a generated shim
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to malloc some space) and then will be freed appropriately.
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Owned values are implemented through boxes. When you return a `Foo` it's
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actually turned into `Box<RefCell<Foo>>` under the hood and returned to JS as a
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pointer. The pointer is to have a defined ABI, and the `RefCell` is to ensure
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safety with reentrancy and aliasing in JS. In general you shouldn't see
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`RefCell` panics with normal usage.
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All of these constructs currently create relatively straightforward code on the
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JS side of things, mostly haveing a 1:1 match in Rust with JS.
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# License
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This project is licensed under either of
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