Add a has_changed function to tokio::sync::watch::Receiver

tokio/src/sync/watch.rs

@@ -318,6 +318,41 @@ impl<T> Receiver<T> {
         Ref { inner }
     }
 
+    /// Checks if the value had changed, without marking the newest value as seen.
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::watch;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx) = watch::channel("hello");
+    ///
+    ///     tx.send("goodbye").unwrap();
+    ///
+    ///     assert!(rx.has_changed().unwrap());
+    ///     assert_eq!(*rx.borrow_and_update(), "goodbye");
+    ///
+    ///     // The value has been marked as seen
+    ///     assert!(!rx.has_changed().unwrap());
+    ///
+    ///     drop(tx);
+    ///     // The `tx` handle has been dropped
+    ///     assert!(rx.has_changed().is_err());
+    /// }
+    /// ```
+    pub fn has_changed(&self) -> Result<bool, error::RecvError> {
+        // Load the version from the state
+        let state = self.shared.state.load();
+        if state.is_closed() {
+            // All receivers have dropped.
+            return Err(error::RecvError(()));
+        }
+        let new_version = state.version();
+
+        Ok(self.version != new_version)
+    }
+
     /// Waits for a change notification, then marks the newest value as seen.
     ///
     /// If the newest value in the channel has not yet been marked seen when