Make canvas in WindowBuilder safe (#3320)

src/platform/web.rs

@@ -34,7 +34,6 @@ use crate::event_loop::EventLoopWindowTarget;
 use crate::platform_impl::PlatformCustomCursorBuilder;
 use crate::window::CustomCursor;
 use crate::window::{Window, WindowBuilder};
-use crate::SendSyncWrapper;
 
 use web_sys::HtmlCanvasElement;
 
@@ -105,7 +104,7 @@ pub trait WindowBuilderExtWebSys {
 
 impl WindowBuilderExtWebSys for WindowBuilder {
  fn with_canvas(mut self, canvas: Option<HtmlCanvasElement>) -> Self {
- self.platform_specific.canvas = SendSyncWrapper(canvas);
+ self.platform_specific.set_canvas(canvas);
  self
  }
 

src/platform_impl/web/async/dispatcher.rs

@@ -1,3 +1,4 @@
+use super::super::main_thread::MainThreadMarker;
 use super::{channel, AsyncReceiver, AsyncSender, Wrapper};
 use std::{
  cell::Ref,
@@ -10,10 +11,11 @@ struct Closure<T>(Box<dyn FnOnce(&T) + Send>);
 
 impl<T> Dispatcher<T> {
  #[track_caller]
- pub fn new(value: T) -> Option<(Self, DispatchRunner<T>)> {
+ pub fn new(main_thread: MainThreadMarker, value: T) -> Option<(Self, DispatchRunner<T>)> {
  let (sender, receiver) = channel::<Closure<T>>();
 
  Wrapper::new(
+ main_thread,
  value,
  |value, Closure(closure)| {
  // SAFETY: The given `Closure` here isn't really `'static`, so we shouldn't do anything

src/platform_impl/web/async/waker.rs

@@ -1,3 +1,4 @@
+use super::super::main_thread::MainThreadMarker;
 use super::Wrapper;
 use atomic_waker::AtomicWaker;
 use std::future;
@@ -19,7 +20,7 @@ struct Sender(Arc<Inner>);
 
 impl<T> WakerSpawner<T> {
  #[track_caller]
- pub fn new(value: T, handler: fn(&T, usize)) -> Option<Self> {
+ pub fn new(main_thread: MainThreadMarker, value: T, handler: fn(&T, usize)) -> Option<Self> {
  let inner = Arc::new(Inner {
  counter: AtomicUsize::new(0),
  waker: AtomicWaker::new(),
@@ -31,6 +32,7 @@ impl<T> WakerSpawner<T> {
  let sender = Sender(Arc::clone(&inner));
 
  let wrapper = Wrapper::new(
+ main_thread,
  handler,
  |handler, count| {
  let handler = handler.borrow();
@@ -86,7 +88,7 @@ impl<T> WakerSpawner<T> {
 
  pub fn fetch(&self) -> usize {
  debug_assert!(
- self.0.is_main_thread(),
+ MainThreadMarker::new().is_some(),
  "this should only be called from the main thread"
  );
 

src/platform_impl/web/async/wrapper.rs

@@ -1,9 +1,8 @@
+use super::super::main_thread::MainThreadMarker;
 use std::cell::{Ref, RefCell};
 use std::future::Future;
 use std::marker::PhantomData;
 use std::sync::Arc;
-use wasm_bindgen::prelude::wasm_bindgen;
-use wasm_bindgen::{JsCast, JsValue};
 
 // Unsafe wrapper type that allows us to use `T` when it's not `Send` from other threads.
 // `value` **must** only be accessed on the main thread.
@@ -34,36 +33,15 @@ unsafe impl<const SYNC: bool, V> Send for Value<SYNC, V> {}
 unsafe impl<V> Sync for Value<true, V> {}
 
 impl<const SYNC: bool, V, S: Clone + Send, E> Wrapper<SYNC, V, S, E> {
- thread_local! {
- static MAIN_THREAD: bool = {
- #[wasm_bindgen]
- extern "C" {
- #[derive(Clone)]
- type Global;
-
- #[wasm_bindgen(method, getter, js_name = Window)]
- fn window(this: &Global) -> JsValue;
- }
-
- let global: Global = js_sys::global().unchecked_into();
- !global.window().is_undefined()
- };
- }
-
  #[track_caller]
  pub fn new<R: Future<Output = ()>>(
+ _: MainThreadMarker,
  value: V,
  handler: fn(&RefCell<Option<V>>, E),
  receiver: impl 'static + FnOnce(Arc<RefCell<Option<V>>>) -> R,
  sender_data: S,
  sender_handler: fn(&S, E),
  ) -> Option<Self> {
- Self::MAIN_THREAD.with(|safe| {
- if !safe {
- panic!("only callable from inside the `Window`")
- }
- });
-
  let value = Arc::new(RefCell::new(Some(value)));
 
  wasm_bindgen_futures::spawn_local({
@@ -86,29 +64,16 @@ impl<const SYNC: bool, V, S: Clone + Send, E> Wrapper<SYNC, V, S, E> {
  }
 
  pub fn send(&self, event: E) {
- Self::MAIN_THREAD.with(|is_main_thread| {
- if *is_main_thread {
- (self.handler)(&self.value.value, event)
- } else {
- (self.sender_handler)(&self.sender_data, event)
- }
- })
- }
-
- pub fn is_main_thread(&self) -> bool {
- Self::MAIN_THREAD.with(|is_main_thread| *is_main_thread)
+ if MainThreadMarker::new().is_some() {
+ (self.handler)(&self.value.value, event)
+ } else {
+ (self.sender_handler)(&self.sender_data, event)
+ }
  }
 
  pub fn value(&self) -> Option<Ref<'_, V>> {
- Self::MAIN_THREAD.with(|is_main_thread| {
- if *is_main_thread {
- Some(Ref::map(self.value.value.borrow(), |value| {
- value.as_ref().unwrap()
- }))
- } else {
- None
- }
- })
+ MainThreadMarker::new()
+ .map(|_| Ref::map(self.value.value.borrow(), |value| value.as_ref().unwrap()))
  }
 
  pub fn with_sender_data<T>(&self, f: impl FnOnce(&S) -> T) -> T {