Introduce Shell type in iced_native

Widgets now can invalidate the current layout of the application on demand.

native/src/shell.rs

@@ -0,0 +1,54 @@
+/// A connection to the state of a shell.
+///
+/// A [`Widget`] can leverage a [`Shell`] to trigger changes in an application,
+/// like publishing messages or invalidating the current layout.
+///
+/// [`Widget`]: crate::Widget
+#[derive(Debug)]
+pub struct Shell<'a, Message> {
+    messages: &'a mut Vec<Message>,
+    is_layout_invalid: bool,
+}
+
+impl<'a, Message> Shell<'a, Message> {
+    /// Creates a new [`Shell`] with the provided buffer of messages.
+    pub fn new(messages: &'a mut Vec<Message>) -> Self {
+        Self {
+            messages,
+            is_layout_invalid: false,
+        }
+    }
+
+    /// Triggers the given function if the layout is invalid, cleaning it in the
+    /// process.
+    pub fn with_invalid_layout(&mut self, f: impl FnOnce()) {
+        if self.is_layout_invalid {
+            self.is_layout_invalid = false;
+
+            f()
+        }
+    }
+
+    /// Publish the given `Message` for an application to process it.
+    pub fn publish(&mut self, message: Message) {
+        self.messages.push(message);
+    }
+
+    /// Invalidates the current application layout.
+    ///
+    /// The shell will relayout the application widgets.
+    pub fn invalidate_layout(&mut self) {
+        self.is_layout_invalid = true;
+    }
+
+    /// Merges the current [`Shell`] with another one by applying the given
+    /// function to the messages of the latter.
+    ///
+    /// This method is useful for composition.
+    pub fn merge<B>(&mut self, other: Shell<'_, B>, f: impl Fn(B) -> Message) {
+        self.messages.extend(other.messages.drain(..).map(f));
+
+        self.is_layout_invalid =
+            self.is_layout_invalid || other.is_layout_invalid;
+    }
+}