feat: Add a way to get the current number of listeners

Author: beckend

src/lib.rs

@@ -479,6 +479,42 @@ impl<T> Event<T> {
 
         inner
     }
+
+    /// Return the listener count by acquiring a lock.
+    ///
+    /// This is just a snapshot of the number of listeners at this point in time.
+    /// It is possible for the actual number to change at any point.
+    /// The number should only ever be used as a hint.
+    /// This is only available when `std` feature is enabled.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use event_listener::Event;
+    ///
+    /// let event = Event::new();
+    ///
+    /// assert_eq!(event.total_listeners(), 0);
+    ///
+    /// let listener1 = event.listen();
+    /// assert_eq!(event.total_listeners(), 1);    
+    ///
+    /// let listener2 = event.listen();
+    /// assert_eq!(event.total_listeners(), 2);        
+    ///
+    /// drop(listener1);
+    /// drop(listener2);
+    /// assert_eq!(event.total_listeners(), 0);        
+    /// ```
+    #[cfg(feature = "std")]
+    #[inline]
+    pub fn total_listeners(&self) -> usize {
+        if let Some(inner) = self.try_inner() {
+            inner.list.total_listeners_wait()
+        } else {
+            0
+        }
+    }
 }
 
 impl Event<()> {

src/std.rs

@@ -44,7 +44,7 @@ impl<T> List<T> {
             notified: 0,
         }))
     }
-    // Accessor method because fields are private, not sure how to go around it
+    // Accessor method because fields are private, not sure how to go around it.
     pub fn total_listeners(&self) -> Result<usize, &str> {
         match self.0.try_lock() {
             Ok(mutex) => {
@@ -54,6 +54,14 @@ impl<T> List<T> {
             Err(_) => Err("<locked>"),
         }
     }
+
+    // Get the listener count by blocking.
+    pub(crate) fn total_listeners_wait(&self) -> usize {
+        match self.0.lock() {
+            Ok(mutex) => mutex.len,
+            Err(err) => panic!("{err}"),
+        }
+    }
 }
 
 impl<T> crate::Inner<T> {