Convert Callback to be an enum to support Fn and FnOnce.

yew/src/callback.rs

@@ -71,6 +71,15 @@ impl<IN> Callback<IN> {
         };
     }
 
+    /// Creates a callback from a FnOnce. You are responsible for ensuring
+    /// the callback is only called once otherwise it will panic.
+    pub fn callback_once<F>(func: F) -> Self
+    where
+        F: FnOnce(IN) + 'static
+    {
+        Callback::CallbackOnce(Rc::new(RefCell::new(Some(Box::new(func)))))
+    }
+
     /// Creates a no-op callback which can be used when it is not suitable to use an
     /// `Option<Callback>`.
     pub fn noop() -> Self {

yew/src/html/scope.rs

@@ -119,7 +119,7 @@ impl<COMP: Component> Scope<COMP> {
         self.update(ComponentUpdate::MessageBatch(messages));
     }
 
-    /// This method creates a `Callback` which will send a message to the linked component's
+    /// Creates a `Callback` which will send a message to the linked component's
     /// update method when invoked.
     pub fn callback<F, IN, M>(&self, function: F) -> Callback<IN>
     where
@@ -134,8 +134,23 @@ impl<COMP: Component> Scope<COMP> {
         closure.into()
     }
 
-    /// This method creates a `Callback` which will send a batch of messages back to the linked
-    /// component's update method when called.
+    /// Creates a `Callback` from a FnOnce which will send a message to the linked
+    /// component's update method when invoked.
+    pub fn callback_once<F, IN, M>(&self, function: F) -> Callback<IN>
+    where
+        M: Into<COMP::Message>,
+        F: FnOnce(IN) -> M + 'static,
+    {
+        let scope = self.clone();
+        let closure = move |input| {
+            let output = function(input);
+            scope.send_message(output);
+        };
+        Callback::callback_once(closure)
+    }
+
+    /// Creates a `Callback` which will send a batch of messages back to the linked
+    /// component's update method when invoked.
     pub fn batch_callback<F, IN>(&self, function: F) -> Callback<IN>
     where
         F: Fn(IN) -> Vec<COMP::Message> + 'static,