2 files changed, 193 insertions, 0 deletions
diff --git a/native/src/program.rs b/native/src/program.rs
new file mode 100644
index 00000000..2652dee9
--- /dev/null
+++ b/native/src/program.rs
@@ -0,0 +1,39 @@
+//! Build interactive programs using The Elm Architecture.
+use crate::{Command, Element, Renderer};
+
+mod state;
+
+pub use state::State;
+
+/// An interactive, native cross-platform program.
+pub trait Program: Sized {
+    /// The graphics backend to use to draw the [`Program`].
+    ///
+    /// [`Program`]: trait.Program.html
+    type Renderer: Renderer;
+
+    /// The type of __messages__ your [`Program`] will produce.
+    ///
+    /// [`Application`]: trait.Program.html
+    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.
+    ///
+    /// [`Program`]: trait.Application.html
+    /// [`Command`]: struct.Command.html
+    fn update(&mut self, message: Self::Message) -> Command<Self::Message>;
+
+    /// Returns the widgets to display in the [`Program`].
+    ///
+    /// These widgets can produce __messages__ based on user interaction.
+    ///
+    /// [`Program`]: trait.Application.html
+    fn view(&mut self) -> Element<'_, Self::Message, Self::Renderer>;
+}
diff --git a/native/src/program/state.rs b/native/src/program/state.rs
new file mode 100644
index 00000000..bcb7212d
--- /dev/null
+++ b/native/src/program/state.rs
@@ -0,0 +1,154 @@
+use crate::{
+    Cache, Clipboard, Command, Debug, Event, Program, Renderer, Size,
+    UserInterface,
+};
+
+#[allow(missing_debug_implementations)]
+pub struct State<P>
+where
+    P: Program + 'static,
+{
+    program: P,
+    cache: Option<Cache>,
+    primitive: <P::Renderer as Renderer>::Output,
+    queued_events: Vec<Event>,
+    queued_messages: Vec<P::Message>,
+}
+
+impl<P> State<P>
+where
+    P: Program + 'static,
+{
+    pub fn new(
+        mut program: P,
+        bounds: Size,
+        renderer: &mut P::Renderer,
+        debug: &mut Debug,
+    ) -> Self {
+        let user_interface = build_user_interface(
+            &mut program,
+            Cache::default(),
+            renderer,
+            bounds,
+            debug,
+        );
+
+        debug.draw_started();
+        let primitive = user_interface.draw(renderer);
+        debug.draw_finished();
+
+        let cache = Some(user_interface.into_cache());
+
+        State {
+            program,
+            cache,
+            primitive,
+            queued_events: Vec::new(),
+            queued_messages: Vec::new(),
+        }
+    }
+
+    pub fn program(&self) -> &P {
+        &self.program
+    }
+
+    pub fn primitive(&self) -> &<P::Renderer as Renderer>::Output {
+        &self.primitive
+    }
+
+    pub fn queue_event(&mut self, event: Event) {
+        self.queued_events.push(event);
+    }
+
+    pub fn queue_message(&mut self, message: P::Message) {
+        self.queued_messages.push(message);
+    }
+
+    pub fn update(
+        &mut self,
+        clipboard: Option<&dyn Clipboard>,
+        bounds: Size,
+        renderer: &mut P::Renderer,
+        debug: &mut Debug,
+    ) -> Option<Command<P::Message>> {
+        if self.queued_events.is_empty() && self.queued_messages.is_empty() {
+            return None;
+        }
+
+        let mut user_interface = build_user_interface(
+            &mut self.program,
+            self.cache.take().unwrap(),
+            renderer,
+            bounds,
+            debug,
+        );
+
+        debug.event_processing_started();
+        let mut messages = user_interface.update(
+            self.queued_events.drain(..),
+            clipboard,
+            renderer,
+        );
+        messages.extend(self.queued_messages.drain(..));
+        debug.event_processing_finished();
+
+        if messages.is_empty() {
+            debug.draw_started();
+            self.primitive = user_interface.draw(renderer);
+            debug.draw_finished();
+
+            self.cache = Some(user_interface.into_cache());
+
+            None
+        } else {
+            // When there are messages, we are forced to rebuild twice
+            // for now :^)
+            let temp_cache = user_interface.into_cache();
+
+            let commands =
+                Command::batch(messages.into_iter().map(|message| {
+                    debug.log_message(&message);
+
+                    debug.update_started();
+                    let command = self.program.update(message);
+                    debug.update_finished();
+
+                    command
+                }));
+
+            let user_interface = build_user_interface(
+                &mut self.program,
+                temp_cache,
+                renderer,
+                bounds,
+                debug,
+            );
+
+            debug.draw_started();
+            self.primitive = user_interface.draw(renderer);
+            debug.draw_finished();
+
+            self.cache = Some(user_interface.into_cache());
+
+            Some(commands)
+        }
+    }
+}
+
+fn build_user_interface<'a, P: Program>(
+    program: &'a mut P,
+    cache: Cache,
+    renderer: &mut P::Renderer,
+    size: Size,
+    debug: &mut Debug,
+) -> UserInterface<'a, P::Message, P::Renderer> {
+    debug.view_started();
+    let view = program.view();
+    debug.view_finished();
+
+    debug.layout_started();
+    let user_interface = UserInterface::build(view, size, cache, renderer);
+    debug.layout_finished();
+
+    user_interface
+}