Web Async Rework (#3082)

src/platform_impl/web/async/dispatcher.rs

@@ -0,0 +1,113 @@
+use super::{channel, AsyncReceiver, AsyncSender, Wrapper};
+use std::{
+    cell::Ref,
+    sync::{Arc, Condvar, Mutex},
+};
+
+pub struct Dispatcher<T: 'static>(Wrapper<true, T, AsyncSender<Closure<T>>, Closure<T>>);
+
+struct Closure<T>(Box<dyn FnOnce(&T) + Send>);
+
+impl<T> Dispatcher<T> {
+    #[track_caller]
+    pub fn new(value: T) -> Option<(Self, DispatchRunner<T>)> {
+        let (sender, receiver) = channel::<Closure<T>>();
+
+        Wrapper::new(
+            value,
+            |value, Closure(closure)| {
+                // SAFETY: The given `Closure` here isn't really `'static`, so we shouldn't do anything
+                // funny with it here. See `Self::queue()`.
+                closure(value.borrow().as_ref().unwrap())
+            },
+            {
+                let receiver = receiver.clone();
+                move |value| async move {
+                    while let Ok(Closure(closure)) = receiver.next().await {
+                        // SAFETY: The given `Closure` here isn't really `'static`, so we shouldn't do anything
+                        // funny with it here. See `Self::queue()`.
+                        closure(value.borrow().as_ref().unwrap())
+                    }
+                }
+            },
+            sender,
+            |sender, closure| {
+                // SAFETY: The given `Closure` here isn't really `'static`, so we shouldn't do anything
+                // funny with it here. See `Self::queue()`.
+                sender.send(closure).unwrap()
+            },
+        )
+        .map(|wrapper| (Self(wrapper.clone()), DispatchRunner { wrapper, receiver }))
+    }
+
+    pub fn value(&self) -> Option<Ref<'_, T>> {
+        self.0.value()
+    }
+
+    pub fn dispatch(&self, f: impl 'static + FnOnce(&T) + Send) {
+        if let Some(value) = self.0.value() {
+            f(&value)
+        } else {
+            self.0.send(Closure(Box::new(f)))
+        }
+    }
+
+    pub fn queue<R: Send>(&self, f: impl FnOnce(&T) -> R + Send) -> R {
+        if let Some(value) = self.0.value() {
+            f(&value)
+        } else {
+            let pair = Arc::new((Mutex::new(None), Condvar::new()));
+            let closure = Box::new({
+                let pair = pair.clone();
+                move |value: &T| {
+                    *pair.0.lock().unwrap() = Some(f(value));
+                    pair.1.notify_one();
+                }
+            }) as Box<dyn FnOnce(&T) + Send>;
+            // SAFETY: The `transmute` is necessary because `Closure` requires `'static`. This is
+            // safe because this function won't return until `f` has finished executing. See
+            // `Self::new()`.
+            let closure = Closure(unsafe { std::mem::transmute(closure) });
+
+            self.0.send(closure);
+
+            let mut started = pair.0.lock().unwrap();
+
+            while started.is_none() {
+                started = pair.1.wait(started).unwrap();
+            }
+
+            started.take().unwrap()
+        }
+    }
+}
+
+impl<T> Drop for Dispatcher<T> {
+    fn drop(&mut self) {
+        self.0.with_sender_data(|sender| sender.close())
+    }
+}
+
+pub struct DispatchRunner<T: 'static> {
+    wrapper: Wrapper<true, T, AsyncSender<Closure<T>>, Closure<T>>,
+    receiver: AsyncReceiver<Closure<T>>,
+}
+
+impl<T> DispatchRunner<T> {
+    pub fn run(&self) {
+        while let Some(Closure(closure)) = self
+            .receiver
+            .try_recv()
+            .expect("should only be closed when `Dispatcher` is dropped")
+        {
+            // SAFETY: The given `Closure` here isn't really `'static`, so we shouldn't do anything
+            // funny with it here. See `Self::queue()`.
+            closure(
+                &self
+                    .wrapper
+                    .value()
+                    .expect("don't call this outside the main thread"),
+            )
+        }
+    }
+}