Struct Closure

pub struct Closure<T: ?Sized> { /* private fields */ }

A handle to both a closure in Rust as well as JS closure which will invoke the Rust closure.

A Closure is the primary way that a 'static lifetime closure is transferred from Rust to JS. Closure currently requires that the closures it’s created with have the 'static lifetime in Rust for soundness reasons.

This type is a “handle” in the sense that whenever it is dropped it will invalidate the JS closure that it refers to. Any usage of the closure in JS after the Closure has been dropped will raise an exception. It’s then up to you to arrange for Closure to be properly deallocate at an appropriate location in your program.

The type parameter on Closure is the type of closure that this represents. Currently this can only be the Fn and FnMut traits with up to 7 arguments (and an optional return value).

Examples

Here are a number of examples of using Closure.

Using the setInterval API

Sample usage of Closure to invoke the setInterval API.

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
extern "C" {
    fn setInterval(closure: &Closure<dyn FnMut()>, time: u32) -> i32;
    fn clearInterval(id: i32);

    #[wasm_bindgen(js_namespace = console)]
    fn log(s: &str);
}

#[wasm_bindgen]
pub struct IntervalHandle {
    interval_id: i32,
    _closure: Closure<dyn FnMut()>,
}

impl Drop for IntervalHandle {
    fn drop(&mut self) {
        clearInterval(self.interval_id);
    }
}

#[wasm_bindgen]
pub fn run() -> IntervalHandle {
    // First up we use `Closure::new` to wrap up a Rust closure and create
    // a JS closure.
    let cb = Closure::new(|| {
        log("interval elapsed!");
    });

    // Next we pass this via reference to the `setInterval` function, and
    // `setInterval` gets a handle to the corresponding JS closure.
    let interval_id = setInterval(&cb, 1_000);

    // If we were to drop `cb` here it would cause an exception to be raised
    // whenever the interval elapses. Instead we *return* our handle back to JS
    // so JS can decide when to cancel the interval and deallocate the closure.
    IntervalHandle {
        interval_id,
        _closure: cb,
    }
}

Casting a Closure to a js_sys::Function

This is the same setInterval example as above, except it is using web_sys (which uses js_sys::Function for callbacks) instead of manually writing bindings to setInterval and other Web APIs.

use wasm_bindgen::JsCast;

#[wasm_bindgen]
pub struct IntervalHandle {
    interval_id: i32,
    _closure: Closure<dyn FnMut()>,
}

impl Drop for IntervalHandle {
    fn drop(&mut self) {
        let window = web_sys::window().unwrap();
        window.clear_interval_with_handle(self.interval_id);
    }
}

#[wasm_bindgen]
pub fn run() -> Result<IntervalHandle, JsValue> {
    let cb = Closure::new(|| {
        web_sys::console::log_1(&"interval elapsed!".into());
    });

    let window = web_sys::window().unwrap();
    let interval_id = window.set_interval_with_callback_and_timeout_and_arguments_0(
        // Note this method call, which uses `as_ref()` to get a `JsValue`
        // from our `Closure` which is then converted to a `&Function`
        // using the `JsCast::unchecked_ref` function.
        cb.as_ref().unchecked_ref(),
        1_000,
    )?;

    // Same as above.
    Ok(IntervalHandle {
        interval_id,
        _closure: cb,
    })
}

Using FnOnce and Closure::once with requestAnimationFrame

Because requestAnimationFrame only calls its callback once, we can use FnOnce and Closure::once with it.

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
extern "C" {
    fn requestAnimationFrame(closure: &Closure<dyn FnMut()>) -> u32;
    fn cancelAnimationFrame(id: u32);

    #[wasm_bindgen(js_namespace = console)]
    fn log(s: &str);
}

#[wasm_bindgen]
pub struct AnimationFrameHandle {
    animation_id: u32,
    _closure: Closure<dyn FnMut()>,
}

impl Drop for AnimationFrameHandle {
    fn drop(&mut self) {
        cancelAnimationFrame(self.animation_id);
    }
}

// A type that will log a message when it is dropped.
struct LogOnDrop(&'static str);
impl Drop for LogOnDrop {
    fn drop(&mut self) {
        log(self.0);
    }
}

#[wasm_bindgen]
pub fn run() -> AnimationFrameHandle {
    // We are using `Closure::once` which takes a `FnOnce`, so the function
    // can drop and/or move things that it closes over.
    let fired = LogOnDrop("animation frame fired or canceled");
    let cb = Closure::once(move || drop(fired));

    // Schedule the animation frame!
    let animation_id = requestAnimationFrame(&cb);

    // Again, return a handle to JS, so that the closure is not dropped
    // immediately and JS can decide whether to cancel the animation frame.
    AnimationFrameHandle {
        animation_id,
        _closure: cb,
    }
}

Converting FnOnces directly into JavaScript Functions with Closure::once_into_js

If we don’t want to allow a FnOnce to be eagerly dropped (maybe because we just want it to drop after it is called and don’t care about cancellation) then we can use the Closure::once_into_js function.

This is the same requestAnimationFrame example as above, but without supporting early cancellation.

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
extern "C" {
    // We modify the binding to take an untyped `JsValue` since that is what
    // is returned by `Closure::once_into_js`.
    //
    // If we were using the `web_sys` binding for `requestAnimationFrame`,
    // then the call sites would cast the `JsValue` into a `&js_sys::Function`
    // using `f.unchecked_ref::<js_sys::Function>()`. See the `web_sys`
    // example above for details.
    fn requestAnimationFrame(callback: JsValue);

    #[wasm_bindgen(js_namespace = console)]
    fn log(s: &str);
}

// A type that will log a message when it is dropped.
struct LogOnDrop(&'static str);
impl Drop for LogOnDrop {
    fn drop(&mut self) {
        log(self.0);
    }
}

#[wasm_bindgen]
pub fn run() {
    // We are using `Closure::once_into_js` which takes a `FnOnce` and
    // converts it into a JavaScript function, which is returned as a
    // `JsValue`.
    let fired = LogOnDrop("animation frame fired");
    let cb = Closure::once_into_js(move || drop(fired));

    // Schedule the animation frame!
    requestAnimationFrame(cb);

    // No need to worry about whether or not we drop a `Closure`
    // here or return some sort of handle to JS!
}

Implementations

impl<T> Closure<T>

where T: ?Sized + WasmClosure,

pub fn new<F>(t: F) -> Closure<T>

where F: IntoWasmClosure<T> + 'static,

Creates a new instance of Closure from the provided Rust function.

Note that the closure provided here, F, has a few requirements associated with it:

• It must implement Fn or FnMut (for FnOnce functions see Closure::once and Closure::once_into_js).

• It must be 'static, aka no stack references (use the move keyword).

• It can have at most 7 arguments.

• Its arguments and return values are all types that can be shared with JS (i.e. have #[wasm_bindgen] annotations or are simple numbers, etc.)

pub fn wrap(data: Box<T>) -> Closure<T>

A more direct version of Closure::new which creates a Closure from a Box<dyn Fn>/Box<dyn FnMut>, which is how it’s kept internally.

pub fn into_js_value(self) -> JsValue

Release memory management of this closure from Rust to the JS GC.

When a Closure is dropped it will release the Rust memory and invalidate the associated JS closure, but this isn’t always desired. Some callbacks are alive for the entire duration of the program or for a lifetime dynamically managed by the JS GC. This function can be used to drop this Closure while keeping the associated JS function still valid.

If the platform supports weak references, the Rust memory will be reclaimed when the JS closure is GC’d. If weak references is not supported, this can be dangerous if this function is called many times in an application because the memory leak will overwhelm the page quickly and crash the wasm.

pub fn forget(self)

Same as into_js_value, but doesn’t return a value.

impl Closure<dyn FnOnce()>

pub fn once<F, A, R>(fn_once: F) -> Closure<F::FnMut>

where F: 'static + WasmClosureFnOnce<A, R>,

Create a Closure from a function that can only be called once.

Since we have no way of enforcing that JS cannot attempt to call this FnOne(A...) -> R more than once, this produces a Closure<dyn FnMut(A...) -> R> that will dynamically throw a JavaScript error if called more than once.

Example

use wasm_bindgen::prelude::*;

// Create an non-`Copy`, owned `String`.
let mut s = String::from("Hello");

// Close over `s`. Since `f` returns `s`, it is `FnOnce` and can only be
// called once. If it was called a second time, it wouldn't have any `s`
// to work with anymore!
let f = move || {
    s += ", World!";
    s
};

// Create a `Closure` from `f`. Note that the `Closure`'s type parameter
// is `FnMut`, even though `f` is `FnOnce`.
let closure: Closure<dyn FnMut() -> String> = Closure::once(f);

pub fn once_into_js<F, A, R>(fn_once: F) -> JsValue

where F: 'static + WasmClosureFnOnce<A, R>,

Convert a FnOnce(A...) -> R into a JavaScript Function object.

If the JavaScript function is invoked more than once, it will throw an exception.

Unlike Closure::once, this does not return a Closure that can be dropped before the function is invoked to deallocate the closure. The only way the FnOnce is deallocated is by calling the JavaScript function. If the JavaScript function is never called then the FnOnce and everything it closes over will leak.

use wasm_bindgen::{prelude::*, JsCast};

let f = Closure::once_into_js(move || {
    // ...
});

assert!(f.is_instance_of::<js_sys::Function>());

Trait Implementations

impl<T: ?Sized> AsRef<JsValue> for Closure<T>

fn as_ref(&self) -> &JsValue

Converts this type into a shared reference of the (usually inferred) input type.

impl<T> Debug for Closure<T>

where T: ?Sized,

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<T> Drop for Closure<T>

where T: ?Sized,

fn drop(&mut self)

Executes the destructor for this type.

impl<T> IntoWasmAbi for &Closure<T>

where T: WasmClosure + ?Sized,

type Abi = u32

The Wasm ABI type that this converts into when crossing the ABI boundary.

fn into_abi(self) -> u32

Convert self into Self::Abi so that it can be sent across the wasm ABI boundary.

impl<T> OptionIntoWasmAbi for &Closure<T>

where T: WasmClosure + ?Sized,

fn none() -> Self::Abi

Returns an ABI instance indicating “none”, which JS will interpret as the None branch of this option.