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use std::pin::Pin;
use tokio::sync::watch::Receiver;
use futures_core::Stream;
use tokio_util::sync::ReusableBoxFuture;
use std::fmt;
use std::task::{Context, Poll};
use tokio::sync::watch::error::RecvError;
/// A wrapper around [`tokio::sync::watch::Receiver`] that implements [`Stream`].
///
/// This stream will start by yielding the current value when the WatchStream is polled,
/// regardless of whether it was the initial value or sent afterwards,
/// unless you use [`WatchStream<T>::from_changes`].
///
/// # Examples
///
/// ```
/// # #[tokio::main]
/// # async fn main() {
/// use tokio_stream::{StreamExt, wrappers::WatchStream};
/// use tokio::sync::watch;
///
/// let (tx, rx) = watch::channel("hello");
/// let mut rx = WatchStream::new(rx);
///
/// assert_eq!(rx.next().await, Some("hello"));
///
/// tx.send("goodbye").unwrap();
/// assert_eq!(rx.next().await, Some("goodbye"));
/// # }
/// ```
///
/// ```
/// # #[tokio::main]
/// # async fn main() {
/// use tokio_stream::{StreamExt, wrappers::WatchStream};
/// use tokio::sync::watch;
///
/// let (tx, rx) = watch::channel("hello");
/// let mut rx = WatchStream::new(rx);
///
/// // existing rx output with "hello" is ignored here
///
/// tx.send("goodbye").unwrap();
/// assert_eq!(rx.next().await, Some("goodbye"));
/// # }
/// ```
///
/// Example with [`WatchStream<T>::from_changes`]:
///
/// ```
/// # #[tokio::main]
/// # async fn main() {
/// use futures::future::FutureExt;
/// use tokio::sync::watch;
/// use tokio_stream::{StreamExt, wrappers::WatchStream};
///
/// let (tx, rx) = watch::channel("hello");
/// let mut rx = WatchStream::from_changes(rx);
///
/// // no output from rx is available at this point - let's check this:
/// assert!(rx.next().now_or_never().is_none());
///
/// tx.send("goodbye").unwrap();
/// assert_eq!(rx.next().await, Some("goodbye"));
/// # }
/// ```
///
/// [`tokio::sync::watch::Receiver`]: struct@tokio::sync::watch::Receiver
/// [`Stream`]: trait@crate::Stream
#[cfg_attr(docsrs, doc(cfg(feature = "sync")))]
pub struct WatchStream<T> {
inner: ReusableBoxFuture<'static, (Result<(), RecvError>, Receiver<T>)>,
}
async fn make_future<T: Clone + Send + Sync>(
mut rx: Receiver<T>,
) -> (Result<(), RecvError>, Receiver<T>) {
let result = rx.changed().await;
(result, rx)
}
impl<T: 'static + Clone + Send + Sync> WatchStream<T> {
/// Create a new `WatchStream`.
pub fn new(rx: Receiver<T>) -> Self {
Self {
inner: ReusableBoxFuture::new(async move { (Ok(()), rx) }),
}
}
/// Create a new `WatchStream` that waits for the value to be changed.
pub fn from_changes(rx: Receiver<T>) -> Self {
Self {
inner: ReusableBoxFuture::new(make_future(rx)),
}
}
}
impl<T: Clone + 'static + Send + Sync> Stream for WatchStream<T> {
type Item = T;
fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
let (result, mut rx) = ready!(self.inner.poll(cx));
match result {
Ok(_) => {
let received = (*rx.borrow_and_update()).clone();
self.inner.set(make_future(rx));
Poll::Ready(Some(received))
}
Err(_) => {
self.inner.set(make_future(rx));
Poll::Ready(None)
}
}
}
}
impl<T> Unpin for WatchStream<T> {}
impl<T> fmt::Debug for WatchStream<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("WatchStream").finish()
}
}
impl<T: 'static + Clone + Send + Sync> From<Receiver<T>> for WatchStream<T> {
fn from(recv: Receiver<T>) -> Self {
Self::new(recv)
}
}