Web: improve custom cursor handling and add animated cursors (#3384)

src/platform_impl/web/async/notifier.rs

@@ -0,0 +1,78 @@
+use std::future::Future;
+use std::pin::Pin;
+use std::sync::Arc;
+use std::sync::OnceLock;
+use std::task::Context;
+use std::task::Poll;
+use std::task::Waker;
+
+use super::{ConcurrentQueue, PushError};
+
+#[derive(Debug)]
+pub struct Notifier<T: Clone>(Arc<Inner<T>>);
+
+impl<T: Clone> Notifier<T> {
+    pub fn new() -> Self {
+        Self(Arc::new(Inner {
+            queue: ConcurrentQueue::unbounded(),
+            value: OnceLock::new(),
+        }))
+    }
+
+    pub fn notify(self, value: T) {
+        if self.0.value.set(value).is_err() {
+            unreachable!("value set before")
+        }
+
+        self.0.queue.close();
+
+        while let Ok(waker) = self.0.queue.pop() {
+            waker.wake()
+        }
+    }
+
+    pub fn notified(&self) -> Notified<T> {
+        Notified(Some(Arc::clone(&self.0)))
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct Notified<T: Clone>(Option<Arc<Inner<T>>>);
+
+impl<T: Clone> Future for Notified<T> {
+    type Output = T;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = self.0.take().expect("`Receiver` polled after completion");
+
+        if this.value.get().is_none() {
+            match this.queue.push(cx.waker().clone()) {
+                Ok(()) => {
+                    if this.value.get().is_none() {
+                        self.0 = Some(this);
+                        return Poll::Pending;
+                    }
+                }
+                Err(PushError::Closed(_)) => (),
+                Err(PushError::Full(_)) => {
+                    unreachable!("found full queue despite using unbounded queue")
+                }
+            }
+        }
+
+        let (Ok(Some(value)) | Err(Some(value))) = Arc::try_unwrap(this)
+            .map(|mut inner| inner.value.take())
+            .map_err(|this| this.value.get().cloned())
+        else {
+            unreachable!("found no value despite being ready")
+        };
+
+        Poll::Ready(value)
+    }
+}
+
+#[derive(Debug)]
+struct Inner<T> {
+    queue: ConcurrentQueue<Waker>,
+    value: OnceLock<T>,
+}