Add on_open handler to combo_box widget

widget/src/combo_box.rs

@@ -41,6 +41,7 @@ pub struct ComboBox<
     selection: text_input::Value,
     on_selected: Box<dyn Fn(T) -> Message>,
     on_option_hovered: Option<Box<dyn Fn(T) -> Message>>,
+    on_open: Option<Message>,
     on_close: Option<Message>,
     on_input: Option<Box<dyn Fn(String) -> Message>>,
     menu_class: <Theme as menu::Catalog>::Class<'a>,
@@ -77,6 +78,7 @@ where
             on_selected: Box::new(on_selected),
             on_option_hovered: None,
             on_input: None,
+            on_open: None,
             on_close: None,
             menu_class: <Theme as Catalog>::default_menu(),
             padding: text_input::DEFAULT_PADDING,
@@ -104,6 +106,13 @@ where
         self
     }
 
+    /// Sets the message that will be produced when the  [`ComboBox`] is
+    /// opened.
+    pub fn on_open(mut self, message: Message) -> Self {
+        self.on_open = Some(message);
+        self
+    }
+
     /// Sets the message that will be produced when the outside area
     /// of the [`ComboBox`] is pressed.
     pub fn on_close(mut self, message: Message) -> Self {
@@ -632,15 +641,19 @@ where
             text_input_state.is_focused()
         };
 
-        if started_focused && !is_focused && !published_message_to_shell {
-            if let Some(message) = self.on_close.take() {
-                shell.publish(message);
-            }
-        }
-
-        // Focus changed, invalidate widget tree to force a fresh `view`
         if started_focused != is_focused {
+            // Focus changed, invalidate widget tree to force a fresh `view`
             shell.invalidate_widgets();
+
+            if !published_message_to_shell {
+                if is_focused {
+                    if let Some(on_open) = self.on_open.take() {
+                        shell.publish(on_open);
+                    }
+                } else if let Some(on_close) = self.on_close.take() {
+                    shell.publish(on_close);
+                }
+            }
         }
 
         event_status