refactored window storage;

new helper window events (Destroyed, Created);
clippy + fmt;

runtime/src/multi_window/program.rs

@@ -0,0 +1,32 @@
+//! Build interactive programs using The Elm Architecture.
+use crate::{window, Command};
+
+use crate::core::text;
+use crate::core::{Element, Renderer};
+
+/// The core of a user interface for a multi-window application following The Elm Architecture.
+pub trait Program: Sized {
+    /// The graphics backend to use to draw the [`Program`].
+    type Renderer: Renderer + text::Renderer;
+
+    /// The type of __messages__ your [`Program`] will produce.
+    type Message: std::fmt::Debug + Send;
+
+    /// Handles a __message__ and updates the state of the [`Program`].
+    ///
+    /// This is where you define your __update logic__. All the __messages__,
+    /// produced by either user interactions or commands, will be handled by
+    /// this method.
+    ///
+    /// Any [`Command`] returned will be executed immediately in the
+    /// background by shells.
+    fn update(&mut self, message: Self::Message) -> Command<Self::Message>;
+
+    /// Returns the widgets to display in the [`Program`] for the `window`.
+    ///
+    /// These widgets can produce __messages__ based on user interaction.
+    fn view(
+        &self,
+        window: window::Id,
+    ) -> Element<'_, Self::Message, Self::Renderer>;
+}