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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" class="active"><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"><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="#polyfill-for-js-objects-in-wasm" id="polyfill-for-js-objects-in-wasm">Polyfill for "JS objects in wasm"</a></h1>
|
|
<p>One of the main goals of <code>wasm-bindgen</code> is to allow working with and passing
|
|
around JS objects in wasm, but that's not allowed today! While indeed true,
|
|
that's where the polyfill comes in.</p>
|
|
<p>The question here is how we shoehorn JS objects into a <code>u32</code> for wasm to use.
|
|
The current strategy for this approach is to maintain a module-local variable
|
|
in the generated <code>foo.js</code> file: a <code>heap</code>.</p>
|
|
<h3><a class="header" href="#temporary-js-objects-on-the-stack" id="temporary-js-objects-on-the-stack">Temporary JS objects on the "stack"</a></h3>
|
|
<p>The first slots in the <code>heap</code> in <code>foo.js</code> are considered a stack. This stack,
|
|
like typical program execution stacks, grows down. JS objects are pushed on the
|
|
bottom of the stack, and their index in the stack is the identifier that's passed
|
|
to wasm. A stack pointer is maintained to figure out where the next item is
|
|
pushed.</p>
|
|
<p>JS objects are then only removed from the bottom of the stack as well. Removal
|
|
is simply storing null then incrementing a counter. Because of the "stack-y"
|
|
nature of this scheme it only works for when wasm doesn't hold onto a JS object
|
|
(aka it only gets a "reference" in Rust parlance).</p>
|
|
<p>Let's take a look at an example.</p>
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|
# #![allow(unused_variables)]
|
|
#fn main() {
|
|
// foo.rs
|
|
#[wasm_bindgen]
|
|
pub fn foo(a: &JsValue) {
|
|
// ...
|
|
}
|
|
#}</code></pre></pre>
|
|
<p>Here we're using the special <code>JsValue</code> type from the <code>wasm-bindgen</code> library
|
|
itself. Our exported function, <code>foo</code>, takes a <em>reference</em> to an object. This
|
|
notably means that it can't persist the object past the lifetime of this
|
|
function call.</p>
|
|
<p>Now what we actually want to generate is a JS module that looks like (in
|
|
TypeScript parlance)</p>
|
|
<pre><code class="language-ts">// foo.d.ts
|
|
export function foo(a: any);
|
|
</code></pre>
|
|
<p>and what we actually generate looks something like:</p>
|
|
<pre><code class="language-js">// foo.js
|
|
import * as wasm from './foo_bg';
|
|
|
|
const heap = new Array(32);
|
|
heap.push(undefined, null, true, false);
|
|
let stack_pointer = 32;
|
|
|
|
function addBorrowedObject(obj) {
|
|
stack_pointer -= 1;
|
|
heap[stack_pointer] = obj;
|
|
return stack_pointer;
|
|
}
|
|
|
|
export function foo(arg0) {
|
|
const idx0 = addBorrowedObject(arg0);
|
|
try {
|
|
wasm.foo(idx0);
|
|
} finally {
|
|
heap[stack_pointer++] = undefined;
|
|
}
|
|
}
|
|
</code></pre>
|
|
<p>Here we can see a few notable points of action:</p>
|
|
<ul>
|
|
<li>The wasm file was renamed to <code>foo_bg.wasm</code>, and we can see how the JS module
|
|
generated here is importing from the wasm file.</li>
|
|
<li>Next we can see our <code>heap</code> module variable which is to store all JS values
|
|
reference-able from wasm.</li>
|
|
<li>Our exported function <code>foo</code>, takes an arbitrary argument, <code>arg0</code>, which is
|
|
converted to an index with the <code>addBorrowedObject</code> object function. The index
|
|
is then passed to wasm so wasm can operate with it.</li>
|
|
<li>Finally, we have a <code>finally</code> which frees the stack slot as it's no longer
|
|
used, popping the value that was pushed at the start of the function.</li>
|
|
</ul>
|
|
<p>It's also helpful to dig into the Rust side of things to see what's going on
|
|
there! Let's take a look at the code that <code>#[wasm_bindgen]</code> generates in Rust:</p>
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|
# #![allow(unused_variables)]
|
|
#fn main() {
|
|
// what the user wrote
|
|
pub fn foo(a: &JsValue) {
|
|
// ...
|
|
}
|
|
|
|
#[export_name = "foo"]
|
|
pub extern "C" fn __wasm_bindgen_generated_foo(arg0: u32) {
|
|
let arg0 = unsafe {
|
|
ManuallyDrop::new(JsValue::__from_idx(arg0))
|
|
};
|
|
let arg0 = &*arg0;
|
|
foo(arg0);
|
|
}
|
|
#}</code></pre></pre>
|
|
<p>And as with the JS, the notable points here are:</p>
|
|
<ul>
|
|
<li>The original function, <code>foo</code>, is unmodified in the output</li>
|
|
<li>A generated function here (with a unique name) is the one that's actually
|
|
exported from the wasm module</li>
|
|
<li>Our generated function takes an integer argument (our index) and then wraps it
|
|
in a <code>JsValue</code>. There's some trickery here that's not worth going into just
|
|
yet, but we'll see in a bit what's happening under the hood.</li>
|
|
</ul>
|
|
<h3><a class="header" href="#long-lived-js-objects" id="long-lived-js-objects">Long-lived JS objects</a></h3>
|
|
<p>The above strategy is useful when JS objects are only temporarily used in Rust,
|
|
for example only during one function call. Sometimes, though, objects may have a
|
|
dynamic lifetime or otherwise need to be stored on Rust's heap. To cope with
|
|
this there's a second half of management of JS objects, naturally corresponding
|
|
to the other side of the JS <code>heap</code> array.</p>
|
|
<p>JS Objects passed to wasm that are not references are assumed to have a dynamic
|
|
lifetime inside of the wasm module. As a result the strict push/pop of the stack
|
|
won't work and we need more permanent storage for the JS objects. To cope with
|
|
this we build our own "slab allocator" of sorts.</p>
|
|
<p>A picture (or code) is worth a thousand words so let's show what happens with an
|
|
example.</p>
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|
# #![allow(unused_variables)]
|
|
#fn main() {
|
|
// foo.rs
|
|
#[wasm_bindgen]
|
|
pub fn foo(a: JsValue) {
|
|
// ...
|
|
}
|
|
#}</code></pre></pre>
|
|
<p>Note that the <code>&</code> is missing in front of the <code>JsValue</code> we had before, and in
|
|
Rust parlance this means it's taking ownership of the JS value. The exported ES
|
|
module interface is the same as before, but the ownership mechanics are slightly
|
|
different. Let's see the generated JS's slab in action:</p>
|
|
<pre><code class="language-js">import * as wasm from './foo_bg'; // imports from wasm file
|
|
|
|
const heap = new Array(32);
|
|
heap.push(undefined, null, true, false);
|
|
let heap_next = 36;
|
|
|
|
function addHeapObject(obj) {
|
|
if (heap_next === heap.length)
|
|
heap.push(heap.length + 1);
|
|
const idx = heap_next;
|
|
heap_next = heap[idx];
|
|
heap[idx] = obj;
|
|
return idx;
|
|
}
|
|
|
|
export function foo(arg0) {
|
|
const idx0 = addHeapObject(arg0);
|
|
wasm.foo(idx0);
|
|
}
|
|
|
|
export function __wbindgen_object_drop_ref(idx) {
|
|
heap[idx ] = heap_next;
|
|
heap_next = idx;
|
|
}
|
|
</code></pre>
|
|
<p>Unlike before we're now calling <code>addHeapObject</code> on the argument to <code>foo</code> rather
|
|
than <code>addBorrowedObject</code>. This function will use <code>heap</code> and <code>heap_next</code> as a
|
|
slab allocator to acquire a slot to store the object, placing a structure there
|
|
once it's found. Note that this is going on the right-half of the array, unlike
|
|
the stack which resides on the left half. This discipline mirrors the stack/heap
|
|
in normal programs, roughly.</p>
|
|
<p>Another curious aspect of this generated module is the
|
|
<code>__wbindgen_object_drop_ref</code> function. This is one that's actually imported to
|
|
wasm rather than used in this module! This function is used to signal the end of
|
|
the lifetime of a <code>JsValue</code> in Rust, or in other words when it goes out of
|
|
scope. Otherwise though this function is largely just a general "slab free"
|
|
implementation.</p>
|
|
<p>And finally, let's take a look at the Rust generated again too:</p>
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|
# #![allow(unused_variables)]
|
|
#fn main() {
|
|
// what the user wrote
|
|
pub fn foo(a: JsValue) {
|
|
// ...
|
|
}
|
|
|
|
#[export_name = "foo"]
|
|
pub extern "C" fn __wasm_bindgen_generated_foo(arg0: u32) {
|
|
let arg0 = unsafe {
|
|
JsValue::__from_idx(arg0)
|
|
};
|
|
foo(arg0);
|
|
}
|
|
#}</code></pre></pre>
|
|
<p>Ah that looks much more familiar! Not much interesting is happening here, so
|
|
let's move on to...</p>
|
|
<h3><a class="header" href="#anatomy-of-jsvalue" id="anatomy-of-jsvalue">Anatomy of <code>JsValue</code></a></h3>
|
|
<p>Currently the <code>JsValue</code> struct is actually quite simple in Rust, it's:</p>
|
|
<pre><pre class="playpen"><code class="language-rust">
|
|
# #![allow(unused_variables)]
|
|
#fn main() {
|
|
pub struct JsValue {
|
|
idx: u32,
|
|
}
|
|
|
|
// "private" constructors
|
|
|
|
impl Drop for JsValue {
|
|
fn drop(&mut self) {
|
|
unsafe {
|
|
__wbindgen_object_drop_ref(self.idx);
|
|
}
|
|
}
|
|
}
|
|
#}</code></pre></pre>
|
|
<p>Or in other words it's a newtype wrapper around a <code>u32</code>, the index that we're
|
|
passed from wasm. The destructor here is where the <code>__wbindgen_object_drop_ref</code>
|
|
function is called to relinquish our reference count of the JS object, freeing
|
|
up our slot in the <code>slab</code> that we saw above.</p>
|
|
<p>If you'll recall as well, when we took <code>&JsValue</code> above we generated a wrapper
|
|
of <code>ManuallyDrop</code> around the local binding, and that's because we wanted to
|
|
avoid invoking this destructor when the object comes from the stack.</p>
|
|
<h3><a class="header" href="#working-with-heap-in-reality" id="working-with-heap-in-reality">Working with <code>heap</code> in reality</a></h3>
|
|
<p>The above explanations are pretty close to what happens today, but in reality
|
|
there's a few differences especially around handling constant values like
|
|
<code>undefined</code>, <code>null</code>, etc. Be sure to check out the actual generated JS and the
|
|
generation code for the full details!</p>
|
|
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