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<ol class="chapter"><li class="affix"><a href="../../introduction.html">Introduction</a></li><li class="spacer"></li><li><a href="../../examples/index.html"><strong aria-hidden="true">1.</strong> Examples</a></li><li><ol class="section"><li><a href="../../examples/hello-world.html"><strong aria-hidden="true">1.1.</strong> Hello, World!</a></li><li><a href="../../examples/console-log.html"><strong aria-hidden="true">1.2.</strong> Using console.log</a></li><li><a href="../../examples/add.html"><strong aria-hidden="true">1.3.</strong> Small wasm files</a></li><li><a href="../../examples/without-a-bundler.html"><strong aria-hidden="true">1.4.</strong> Without a Bundler</a></li><li><a href="../../examples/wasm2js.html"><strong aria-hidden="true">1.5.</strong> Converting WebAssembly to JS</a></li><li><a href="../../examples/import-js.html"><strong aria-hidden="true">1.6.</strong> Importing functions from JS</a></li><li><a href="../../examples/char.html"><strong aria-hidden="true">1.7.</strong> Working with char</a></li><li><a href="../../examples/wasm-in-wasm.html"><strong aria-hidden="true">1.8.</strong> js-sys: WebAssembly in WebAssembly</a></li><li><a href="../../examples/dom.html"><strong aria-hidden="true">1.9.</strong> web-sys: DOM hello world</a></li><li><a href="../../examples/closures.html"><strong aria-hidden="true">1.10.</strong> web-sys: Closures</a></li><li><a href="../../examples/performance.html"><strong aria-hidden="true">1.11.</strong> web-sys: performance.now</a></li><li><a href="../../examples/fetch.html"><strong aria-hidden="true">1.12.</strong> web-sys: using fetch</a></li><li><a href="../../examples/2d-canvas.html"><strong aria-hidden="true">1.13.</strong> web-sys: canvas hello world</a></li><li><a href="../../examples/julia.html"><strong aria-hidden="true">1.14.</strong> web-sys: canvas Julia set</a></li><li><a href="../../examples/web-audio.html"><strong aria-hidden="true">1.15.</strong> web-sys: WebAudio</a></li><li><a href="../../examples/webgl.html"><strong aria-hidden="true">1.16.</strong> web-sys: WebGL</a></li><li><a href="../../examples/websockets.html"><strong aria-hidden="true">1.17.</strong> web-sys: WebSockets</a></li><li><a href="../../examples/webrtc_datachannel.html"><strong aria-hidden="true">1.18.</strong> web-sys: WebRTC DataChannel</a></li><li><a href="../../examples/request-animation-frame.html"><strong aria-hidden="true">1.19.</strong> web-sys: requestAnimationFrame</a></li><li><a href="../../examples/paint.html"><strong aria-hidden="true">1.20.</strong> web-sys: A Simple Paint Program</a></li><li><a href="../../examples/raytrace.html"><strong aria-hidden="true">1.21.</strong> Parallel Raytracing</a></li><li><a href="../../examples/todomvc.html"><strong aria-hidden="true">1.22.</strong> web-sys: A TODO MVC App</a></li></ol></li><li><a href="../../reference/index.html"><strong aria-hidden="true">2.</strong> Reference</a></li><li><ol class="section"><li><a href="../../reference/deployment.html"><strong aria-hidden="true">2.1.</strong> Deployment</a></li><li><a href="../../reference/js-snippets.html"><strong aria-hidden="true">2.2.</strong> JS snippets</a></li><li><a href="../../reference/passing-rust-closures-to-js.html"><strong aria-hidden="true">2.3.</strong> Passing Rust Closures to JS</a></li><li><a href="../../reference/receiving-js-closures-in-rust.html"><strong aria-hidden="true">2.4.</strong> Receiving JS Closures in Rust</a></li><li><a href="../../reference/js-promises-and-rust-futures.html"><strong aria-hidden="true">2.5.</strong> Promises and Futures</a></li><li><a href="../../reference/iterating-over-js-values.html"><strong aria-hidden="true">2.6.</strong> Iterating over JS Values</a></li><li><a href="../../reference/arbitrary-data-with-serde.html"><strong aria-hidden="true">2.7.</strong> Arbitrary Data with Serde</a></li><li><a href="../../reference/accessing-properties-of-untyped-js-values.html"><strong aria-hidden="true">2.8.</strong> Accessing Properties of Untyped JS Values</a></li><li><a href="../../reference/working-with-duck-typed-interfaces.html"><strong aria-hidden="true">2.9.</strong> Working with Duck-Typed Interfaces</a></li><li><a href="../../reference/cli.html"><strong aria-hidden="true">2.10.</strong> Command Line Interface</a></li><li><a href="../../reference/optimize-size.html"><strong aria-hidden="true">2.11.</strong> Optimizing for Size</a></li><li><a href="../../reference/rust-targets.html"><strong aria-hidden="true">2.12.</strong> Supported Rust Targets</a></li><li><a href="../../reference/browser-support.html"><strong aria-hidden="true">2.13.</strong> Supported Browsers</a></li><li><a href="../../reference/types.html"><strong aria-hidden="true">2.14.</strong> Supported Types</a></li><li><ol class="section"><li><a href="../../reference/types/imported-js-types.html"><strong aria-hidden="true">2.14.1.</strong> Imported JavaScript Types</a></li><li><a href="../../reference/types/exported-rust-types.html"><strong aria-hidden="true">2.14.2.</strong> Exported Rust Types</a></li><li><a href="../../reference/types/jsvalue.html"><strong aria-hidden="true">2.14.3.</strong> JsValue</a></li><li><a href="../../reference/types/boxed-jsvalue-slice.html"><strong aria-hidden="true">2.14.4.</strong> Box<[JsValue]></a></li><li><a href="../../reference/types/pointers.html"><strong aria-hidden="true">2.14.5.</strong> *const T and *mut T</a></li><li><a href="../../reference/types/numbers.html"><strong aria-hidden="true">2.14.6.</strong> Numbers</a></li><li><a href="../../reference/types/bool.html"><strong aria-hidden="true">2.14.7.</strong> bool</a></li><li><a href="../../reference/types/char.html"><strong aria-hidden="true">2.14.8.</strong> char</a></li><li><a href="../../reference/types/str.html"><strong aria-hidden="true">2.14.9.</strong> str</a></li><li><a href="../../reference/types/string.html"><strong aria-hidden="true">2.14.10.</strong> String</a></li><li><a href="../../reference/types/number-slices.html"><strong aria-hidden="true">2.14.11.</strong> Number Slices</a></li><li><a href="../../reference/types/boxed-number-slices.html"><strong aria-hidden="true">2.14.12.</strong> Boxed Number Slices</a></li><li><a href="../../reference/types/result.html"><strong aria-hidden="true">2.14.13.</strong> Result<T, JsValue></a></li></ol></li><li><a href="../../reference/attributes/index.html"><strong aria-hidden="true">2.15.</strong> #[wasm_bindgen] Attributes</a></li><li><ol class="section"><li><a href="../../reference/attributes/on-js-imports/index.html"><strong aria-hidden="true">2.15.1.</strong> On JavaScript Imports</a></li><li><ol class="section"><li><a href="../../reference/attributes/on-js-imports/catch.html"><strong aria-hidden="true">2.15.1.1.</strong> catch</a></li><li><a href="../../reference/attributes/on-js-imports/constructor.html"><strong aria-hidden="true">2.15.1.2.</strong> constructor</a></li><li><a href="../../reference/attributes/on-js-imports/extends.html"><strong aria-hidden="true">2.15.1.3.</strong> extends</a></li><li><a href="../../reference/attributes/on-js-imports/getter-and-setter.html"><strong aria-hidden="true">2.15.1.4.</strong> getter and setter</a></li><li><a href="../../reference/attributes/on-js-imports/final.html"><strong aria-hidden="true">2.15.1.5.</strong> final</a></li><li><a href="../../reference/attributes/on-js-imports/indexing-getter-setter-deleter.html"><strong aria-hidden="true">2.15.1.6.</strong> indexing_getter, indexing_setter, and indexing_deleter</a></li><li><a href="../../reference/attributes/on-js-imports/js_class.html"><strong aria-hidden="true">2.15.1.7.</strong> js_class = "Blah"</a></li><li><a href="../../reference/attributes/on-js-imports/js_name.html"><strong aria-hidden="true">2.15.1.8.</strong> js_name</a></li><li><a href="../../reference/attributes/on-js-imports/js_namespace.html"><strong aria-hidden="true">2.15.1.9.</strong> js_namespace</a></li><li><a href="../../reference/attributes/on-js-imports/method.html"><strong aria-hidden="true">2.15.1.10.</strong> method</a></li><li><a href="../../reference/attributes/on-js-imports/module.html"><strong aria-hidden="true">2.15.1.11.</strong> module = "blah"</a></li><li><a href="../../reference/attributes/on-js-imports/raw_module.html"><strong aria-hidden="true">2.15.1.12.</strong> raw_module = "blah"</a></li><li><a href="../../reference/attributes/on-js-imports/static_method_of.html"><strong aria-hidden="true">2.15.1.13.</strong> static_method_of = Blah</a></li><li><a href="../../reference/attributes/on-js-imports/structural.html"><strong aria-hidden="true">2.15.1.14.</strong> structural</a></li><li><a href="../../reference/attributes/on-js-imports/variadic.html"><strong aria-hidden="true">2.15.1.15.</strong> variadic</a></li><li><a href="../../reference/attributes/on-js-imports/vendor_prefix.html"><strong aria-hidden="true">2.15.1.16.</strong> vendor_prefix</a></li></ol></li><li><a href="../../reference/attributes/on-rust-exports/index.html"><strong aria-hidden="true">2.15.2.</strong> On Rust Exports</a></li><li><ol class="section"><li><a href="../../reference/attributes/on-rust-exports/constructor.html"><strong aria-hidden="true">2.15.2.1.</strong> constructor</a></li><li><a href="../../reference/attributes/on-rust-exports/js_name.html"><strong aria-hidden="true">2.15.2.2.</strong> js_name = Blah</a></li><li><a href="../../reference/attributes/on-rust-exports/readonly.html"><strong aria-hidden="true">2.15.2.3.</strong> readonly</a></li><li><a href="../../reference/attributes/on-rust-exports/skip.html"><strong aria-hidden="true">2.15.2.4.</strong> skip</a></li><li><a href="../../reference/attributes/on-rust-exports/start.html"><strong aria-hidden="true">2.15.2.5.</strong> start</a></li><li><a href="../../reference/attributes/on-rust-exports/typescript_custom_section.html"><strong aria-hidden="true">2.15.2.6.</strong> typescript_custom_section</a></li><li><a href="../../reference/attributes/on-rust-exports/getter-and-setter.html"><strong aria-hidden="true">2.15.2.7.</strong> getter and setter</a></li><li><a href="../../reference/attributes/on-rust-exports/inspectable.html"><strong aria-hidden="true">2.15.2.8.</strong> inspectable</a></li><li><a href="../../reference/attributes/on-rust-exports/skip_typescript.html"><strong aria-hidden="true">2.15.2.9.</strong> skip_typescript</a></li><li><a href="../../reference/attributes/on-rust-exports/typescript_type.html"><strong aria-hidden="true">2.15.2.10.</strong> typescript_type</a></li></ol></li></ol></li></ol></li><li><a href="../../web-sys/index.html"><strong aria-hidden="true">3.</strong> web-sys</a></li><li><ol class="section"><li><a href="../../web-sys/using-web-sys.html"><strong aria-hidden="true">3.1.</strong> Using web-sys</a></li><li><a href="../../web-sys/cargo-features.html"><strong aria-hidden="true">3.2.</strong> Cargo Features</a></li><li><a href="../../web-sys/function-overloads.html"><strong aria-hidden="true">3.3.</strong> Function Overloads</a></li><li><a href="../../web-sys/type-translations.html"><strong aria-hidden="true">3.4.</strong> Type Translations</a></li><li><a href="../../web-sys/inheritance.html"><strong aria-hidden="true">3.5.</strong> Inheritance</a></li><li><a href="../../web-sys/unstable-apis.html"><strong aria-hidden="true">3.6.</strong> Unstable APIs</a></li></ol></li><li><a href="../../wasm-bindgen-test/index.html"><strong aria-hidden="true">4.</strong> Testing with wasm-bindgen-test</a></li><li><ol class="section"><li><a href="../../wasm-bindgen-test/usage.html"><strong aria-hidden="true">4.1.</strong> Usage</a></li><li><a href="../../wasm-bindgen-test/asynchronous-tests.html"><strong aria-hidden="true">4.2.</strong> Writing Asynchronous Tests</a></li><li><a href="../../wasm-bindgen-test/browsers.html"><strong aria-hidden="true">4.3.</strong> Testing in Headless Browsers</a></li><li><a href="../../wasm-bindgen-test/continuous-integration.html"><strong aria-hidden="true">4.4.</strong> Continuous Integration</a></li></ol></li><li><a href="../../contributing/index.html"><strong aria-hidden="true">5.</strong> Contributing to wasm-bindgen</a></li><li><ol class="section"><li><a href="../../contributing/testing.html"><strong aria-hidden="true">5.1.</strong> Testing</a></li><li><a href="../../contributing/design/index.html"><strong aria-hidden="true">5.2.</strong> Internal Design</a></li><li><ol class="section"><li><a href="../../contributing/design/js-objects-in-rust.html"><strong aria-hidden="true">5.2.1.</strong> JS Objects in Rust</a></li><li><a href="../../contributing/design/exporting-rust.html"><strong aria-hidden="true">5.2.2.</strong> Exporting a function to JS</a></li><li><a href="../../contributing/design/exporting-rust-struct.html" class="active"><strong aria-hidden="true">5.2.3.</strong> Exporting a struct to JS</a></li><li><a href="../../contributing/design/importing-js.html"><strong aria-hidden="true">5.2.4.</strong> Importing a function from JS</a></li><li><a href="../../contributing/design/importing-js-struct.html"><strong aria-hidden="true">5.2.5.</strong> Importing a class from JS</a></li><li><a href="../../contributing/design/rust-type-conversions.html"><strong aria-hidden="true">5.2.6.</strong> Rust Type conversions</a></li><li><a href="../../contributing/design/describe.html"><strong aria-hidden="true">5.2.7.</strong> Types in wasm-bindgen</a></li></ol></li><li><a href="../../contributing/js-sys/index.html"><strong aria-hidden="true">5.3.</strong> js-sys</a></li><li><ol class="section"><li><a href="../../contributing/js-sys/testing.html"><strong aria-hidden="true">5.3.1.</strong> Testing</a></li><li><a href="../../contributing/js-sys/adding-more-apis.html"><strong aria-hidden="true">5.3.2.</strong> Adding More APIs</a></li></ol></li><li><a href="../../contributing/web-sys/index.html"><strong aria-hidden="true">5.4.</strong> web-sys</a></li><li><ol class="section"><li><a href="../../contributing/web-sys/overview.html"><strong aria-hidden="true">5.4.1.</strong> Overview</a></li><li><a href="../../contributing/web-sys/testing.html"><strong aria-hidden="true">5.4.2.</strong> Testing</a></li><li><a href="../../contributing/web-sys/logging.html"><strong aria-hidden="true">5.4.3.</strong> Logging</a></li><li><a href="../../contributing/web-sys/supporting-more-web-apis.html"><strong aria-hidden="true">5.4.4.</strong> Supporting More Web APIs</a></li></ol></li><li><a href="../../contributing/publishing.html"><strong aria-hidden="true">5.5.</strong> Publishing</a></li><li><a href="../../contributing/team.html"><strong aria-hidden="true">5.6.</strong> Team</a></li></ol></li></ol>
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<div id="content" class="content">
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<main>
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<h1><a class="header" href="#exporting-a-struct-to-js" id="exporting-a-struct-to-js">Exporting a struct to JS</a></h1>
|
|
<p>So far we've covered JS objects, importing functions, and exporting functions.
|
|
This has given us quite a rich base to build on so far, and that's great! We
|
|
sometimes, though, want to go even further and define a JS <code>class</code> in Rust. Or
|
|
in other words, we want to expose an object with methods from Rust to JS rather
|
|
than just importing/exporting free functions.</p>
|
|
<p>The <code>#[wasm_bindgen]</code> attribute can annotate both a <code>struct</code> and <code>impl</code> blocks
|
|
to allow:</p>
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|
# #![allow(unused_variables)]
|
|
#fn main() {
|
|
#[wasm_bindgen]
|
|
pub struct Foo {
|
|
internal: i32,
|
|
}
|
|
|
|
#[wasm_bindgen]
|
|
impl Foo {
|
|
pub fn new(val: i32) -> Foo {
|
|
Foo { internal: val }
|
|
}
|
|
|
|
pub fn get(&self) -> i32 {
|
|
self.internal
|
|
}
|
|
|
|
pub fn set(&mut self, val: i32) {
|
|
self.internal = val;
|
|
}
|
|
}
|
|
#}</code></pre></pre>
|
|
<p>This is a typical Rust <code>struct</code> definition for a type with a constructor and a
|
|
few methods. Annotating the struct with <code>#[wasm_bindgen]</code> means that we'll
|
|
generate necessary trait impls to convert this type to/from the JS boundary. The
|
|
annotated <code>impl</code> block here means that the functions inside will also be made
|
|
available to JS through generated shims. If we take a look at the generated JS
|
|
code for this we'll see:</p>
|
|
<pre><code class="language-js">import * as wasm from './js_hello_world_bg';
|
|
|
|
export class Foo {
|
|
static __construct(ptr) {
|
|
return new Foo(ptr);
|
|
}
|
|
|
|
constructor(ptr) {
|
|
this.ptr = ptr;
|
|
}
|
|
|
|
free() {
|
|
const ptr = this.ptr;
|
|
this.ptr = 0;
|
|
wasm.__wbg_foo_free(ptr);
|
|
}
|
|
|
|
static new(arg0) {
|
|
const ret = wasm.foo_new(arg0);
|
|
return Foo.__construct(ret)
|
|
}
|
|
|
|
get() {
|
|
const ret = wasm.foo_get(this.ptr);
|
|
return ret;
|
|
}
|
|
|
|
set(arg0) {
|
|
const ret = wasm.foo_set(this.ptr, arg0);
|
|
return ret;
|
|
}
|
|
}
|
|
</code></pre>
|
|
<p>That's actually not much! We can see here though how we've translated from Rust
|
|
to JS:</p>
|
|
<ul>
|
|
<li>Associated functions in Rust (those without <code>self</code>) turn into <code>static</code>
|
|
functions in JS.</li>
|
|
<li>Methods in Rust turn into methods in wasm.</li>
|
|
<li>Manual memory management is exposed in JS as well. The <code>free</code> function is
|
|
required to be invoked to deallocate resources on the Rust side of things.</li>
|
|
</ul>
|
|
<p>To be able to use <code>new Foo()</code>, you'd need to annotate <code>new</code> as <code>#[wasm_bindgen(constructor)]</code>.</p>
|
|
<p>One important aspect to note here, though, is that once <code>free</code> is called the JS
|
|
object is "neutered" in that its internal pointer is nulled out. This means that
|
|
future usage of this object should trigger a panic in Rust.</p>
|
|
<p>The real trickery with these bindings ends up happening in Rust, however, so
|
|
let's take a look at that.</p>
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|
# #![allow(unused_variables)]
|
|
#fn main() {
|
|
// original input to `#[wasm_bindgen]` omitted ...
|
|
|
|
#[export_name = "foo_new"]
|
|
pub extern "C" fn __wasm_bindgen_generated_Foo_new(arg0: i32) -> u32
|
|
let ret = Foo::new(arg0);
|
|
Box::into_raw(Box::new(WasmRefCell::new(ret))) as u32
|
|
}
|
|
|
|
#[export_name = "foo_get"]
|
|
pub extern "C" fn __wasm_bindgen_generated_Foo_get(me: u32) -> i32 {
|
|
let me = me as *mut WasmRefCell<Foo>;
|
|
wasm_bindgen::__rt::assert_not_null(me);
|
|
let me = unsafe { &*me };
|
|
return me.borrow().get();
|
|
}
|
|
|
|
#[export_name = "foo_set"]
|
|
pub extern "C" fn __wasm_bindgen_generated_Foo_set(me: u32, arg1: i32) {
|
|
let me = me as *mut WasmRefCell<Foo>;
|
|
wasm_bindgen::__rt::assert_not_null(me);
|
|
let me = unsafe { &*me };
|
|
me.borrow_mut().set(arg1);
|
|
}
|
|
|
|
#[no_mangle]
|
|
pub unsafe extern "C" fn __wbindgen_foo_free(me: u32) {
|
|
let me = me as *mut WasmRefCell<Foo>;
|
|
wasm_bindgen::__rt::assert_not_null(me);
|
|
(*me).borrow_mut(); // ensure no active borrows
|
|
drop(Box::from_raw(me));
|
|
}
|
|
#}</code></pre></pre>
|
|
<p>As with before this is cleaned up from the actual output but it's the same idea
|
|
as to what's going on! Here we can see a shim for each function as well as a
|
|
shim for deallocating an instance of <code>Foo</code>. Recall that the only valid wasm
|
|
types today are numbers, so we're required to shoehorn all of <code>Foo</code> into a
|
|
<code>u32</code>, which is currently done via <code>Box</code> (like <code>std::unique_ptr</code> in C++).
|
|
Note, though, that there's an extra layer here, <code>WasmRefCell</code>. This type is the
|
|
same as <a href="https://doc.rust-lang.org/std/cell/struct.RefCell.html"><code>RefCell</code></a> and can be mostly glossed over.</p>
|
|
<p>The purpose for this type, if you're interested though, is to uphold Rust's
|
|
guarantees about aliasing in a world where aliasing is rampant (JS).
|
|
Specifically the <code>&Foo</code> type means that there can be as much aliasing as you'd
|
|
like, but crucially <code>&mut Foo</code> means that it is the sole pointer to the data
|
|
(no other <code>&Foo</code> to the same instance exists). The <a href="https://doc.rust-lang.org/std/cell/struct.RefCell.html"><code>RefCell</code></a> type in libstd
|
|
is a way of dynamically enforcing this at runtime (as opposed to compile time
|
|
where it usually happens). Baking in <code>WasmRefCell</code> is the same idea here,
|
|
adding runtime checks for aliasing which are typically happening at compile
|
|
time. This is currently a Rust-specific feature which isn't actually in the
|
|
<code>wasm-bindgen</code> tool itself, it's just in the Rust-generated code (aka the
|
|
<code>#[wasm_bindgen]</code> attribute).</p>
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