Introduce on_press_maybe helper for Button

examples/screenshot/src/main.rs

@@ -79,12 +79,13 @@ impl Application for Example {
             }
             Message::Png => {
                 if let Some(screenshot) = &self.screenshot {
+                    self.png_saving = true;
+
                     return Command::perform(
                         save_to_png(screenshot.clone()),
                         Message::PngSaved,
                     );
                 }
-                self.png_saving = true;
             }
             Message::PngSaved(res) => {
                 self.png_saving = false;
@@ -185,7 +186,7 @@ impl Application for Example {
             button("Save to png")
                 .style(Button::Secondary)
                 .padding([10, 20, 10, 20])
-                .on_press(Message::Png)
+                .on_press_maybe(self.screenshot.is_some().then(|| Message::Png))
         } else {
             button("Saving...")
                 .style(Button::Secondary)

widget/src/button.rs

@@ -102,8 +102,17 @@ where
     /// Sets the message that will be produced when the [`Button`] is pressed.
     ///
     /// Unless `on_press` is called, the [`Button`] will be disabled.
-    pub fn on_press(mut self, msg: Message) -> Self {
+    pub fn on_press(mut self, on_press: Message) -> Self {
-        self.on_press = Some(msg);
+        self.on_press = Some(on_press);
+        self
+    }
+
+    /// Sets the message that will be produced when the [`Button`] is pressed,
+    /// if `Some`.
+    ///
+    /// If `None`, the [`Button`] will be disabled.
+    pub fn on_press_maybe(mut self, on_press: Option<Message>) -> Self {
+        self.on_press = on_press;
         self
     }