Introduce `multi_window` in `iced_winit`

4 files changed, 958 insertions, 0 deletions

diff --git a/winit/Cargo.toml b/winit/Cargo.toml
index 94aaa2ca..2152e7da 100644
--- a/winit/Cargo.toml
+++ b/winit/Cargo.toml
@@ -16,6 +16,7 @@ chrome-trace = ["trace", "tracing-chrome"]
 debug = ["iced_native/debug"]
 system = ["sysinfo"]
 application = []
+multi_window = []
 
 [dependencies]
 window_clipboard = "0.2"
diff --git a/winit/src/lib.rs b/winit/src/lib.rs
index 06674109..9b3c0a02 100644
--- a/winit/src/lib.rs
+++ b/winit/src/lib.rs
@@ -35,6 +35,9 @@
 pub use iced_native::*;
 pub use winit;
 
+#[cfg(feature = "multi_window")]
+pub mod multi_window;
+
 #[cfg(feature = "application")]
 pub mod application;
 pub mod clipboard;
diff --git a/winit/src/multi_window.rs b/winit/src/multi_window.rs
new file mode 100644
index 00000000..4ea7383a
--- /dev/null
+++ b/winit/src/multi_window.rs
@@ -0,0 +1,742 @@
+//! Create interactive, native cross-platform applications.
+mod state;
+
+pub use state::State;
+
+use crate::clipboard::{self, Clipboard};
+use crate::conversion;
+use crate::mouse;
+use crate::renderer;
+use crate::widget::operation;
+use crate::{
+    Command, Debug, Error, Executor, Proxy, Runtime, Settings, Size,
+    Subscription,
+};
+
+use iced_futures::futures;
+use iced_futures::futures::channel::mpsc;
+use iced_graphics::compositor;
+use iced_graphics::window;
+use iced_native::program::Program;
+use iced_native::user_interface::{self, UserInterface};
+
+pub use iced_native::application::{Appearance, StyleSheet};
+
+use std::mem::ManuallyDrop;
+
+/// An interactive, native cross-platform application.
+///
+/// This trait is the main entrypoint of Iced. Once implemented, you can run
+/// your GUI application by simply calling [`run`]. It will run in
+/// its own window.
+///
+/// An [`Application`] can execute asynchronous actions by returning a
+/// [`Command`] in some of its methods.
+///
+/// When using an [`Application`] with the `debug` feature enabled, a debug view
+/// can be toggled by pressing `F12`.
+pub trait Application: Program
+where
+    <Self::Renderer as crate::Renderer>::Theme: StyleSheet,
+{
+    /// The data needed to initialize your [`Application`].
+    type Flags;
+
+    /// Initializes the [`Application`] with the flags provided to
+    /// [`run`] as part of the [`Settings`].
+    ///
+    /// Here is where you should return the initial state of your app.
+    ///
+    /// Additionally, you can return a [`Command`] if you need to perform some
+    /// async action in the background on startup. This is useful if you want to
+    /// load state from a file, perform an initial HTTP request, etc.
+    fn new(flags: Self::Flags) -> (Self, Command<Self::Message>);
+
+    /// Returns the current title of the [`Application`].
+    ///
+    /// This title can be dynamic! The runtime will automatically update the
+    /// title of your application when necessary.
+    fn title(&self) -> String;
+
+    /// Returns the current [`Theme`] of the [`Application`].
+    fn theme(&self) -> <Self::Renderer as crate::Renderer>::Theme;
+
+    /// Returns the [`Style`] variation of the [`Theme`].
+    fn style(
+        &self,
+    ) -> <<Self::Renderer as crate::Renderer>::Theme as StyleSheet>::Style {
+        Default::default()
+    }
+
+    /// Returns the event `Subscription` for the current state of the
+    /// application.
+    ///
+    /// The messages produced by the `Subscription` will be handled by
+    /// [`update`](#tymethod.update).
+    ///
+    /// A `Subscription` will be kept alive as long as you keep returning it!
+    ///
+    /// By default, it returns an empty subscription.
+    fn subscription(&self) -> Subscription<Self::Message> {
+        Subscription::none()
+    }
+
+    /// Returns the scale factor of the [`Application`].
+    ///
+    /// It can be used to dynamically control the size of the UI at runtime
+    /// (i.e. zooming).
+    ///
+    /// For instance, a scale factor of `2.0` will make widgets twice as big,
+    /// while a scale factor of `0.5` will shrink them to half their size.
+    ///
+    /// By default, it returns `1.0`.
+    fn scale_factor(&self) -> f64 {
+        1.0
+    }
+
+    /// Returns whether the [`Application`] should be terminated.
+    ///
+    /// By default, it returns `false`.
+    fn should_exit(&self) -> bool {
+        false
+    }
+}
+
+/// Runs an [`Application`] with an executor, compositor, and the provided
+/// settings.
+pub fn run<A, E, C>(
+    settings: Settings<A::Flags>,
+    compositor_settings: C::Settings,
+) -> Result<(), Error>
+where
+    A: Application + 'static,
+    E: Executor + 'static,
+    C: window::Compositor<Renderer = A::Renderer> + 'static,
+    <A::Renderer as crate::Renderer>::Theme: StyleSheet,
+{
+    use futures::task;
+    use futures::Future;
+    use winit::event_loop::EventLoopBuilder;
+
+    let mut debug = Debug::new();
+    debug.startup_started();
+
+    let event_loop = EventLoopBuilder::with_user_event().build();
+    let proxy = event_loop.create_proxy();
+
+    let runtime = {
+        let proxy = Proxy::new(event_loop.create_proxy());
+        let executor = E::new().map_err(Error::ExecutorCreationFailed)?;
+
+        Runtime::new(executor, proxy)
+    };
+
+    let (application, init_command) = {
+        let flags = settings.flags;
+
+        runtime.enter(|| A::new(flags))
+    };
+
+    let builder = settings.window.into_builder(
+        &application.title(),
+        event_loop.primary_monitor(),
+        settings.id,
+    );
+
+    log::info!("Window builder: {:#?}", builder);
+
+    let window = builder
+        .build(&event_loop)
+        .map_err(Error::WindowCreationFailed)?;
+
+    #[cfg(target_arch = "wasm32")]
+    {
+        use winit::platform::web::WindowExtWebSys;
+
+        let canvas = window.canvas();
+
+        let window = web_sys::window().unwrap();
+        let document = window.document().unwrap();
+        let body = document.body().unwrap();
+
+        let _ = body
+            .append_child(&canvas)
+            .expect("Append canvas to HTML body");
+    }
+
+    let (compositor, renderer) = C::new(compositor_settings, Some(&window))?;
+
+    let (mut sender, receiver) = mpsc::unbounded();
+
+    let mut instance = Box::pin(run_instance::<A, E, C>(
+        application,
+        compositor,
+        renderer,
+        runtime,
+        proxy,
+        debug,
+        receiver,
+        init_command,
+        window,
+        settings.exit_on_close_request,
+    ));
+
+    let mut context = task::Context::from_waker(task::noop_waker_ref());
+
+    platform::run(event_loop, move |event, _, control_flow| {
+        use winit::event_loop::ControlFlow;
+
+        if let ControlFlow::ExitWithCode(_) = control_flow {
+            return;
+        }
+
+        let event = match event {
+            winit::event::Event::WindowEvent {
+                event:
+                    winit::event::WindowEvent::ScaleFactorChanged {
+                        new_inner_size,
+                        ..
+                    },
+                window_id,
+            } => Some(winit::event::Event::WindowEvent {
+                event: winit::event::WindowEvent::Resized(*new_inner_size),
+                window_id,
+            }),
+            _ => event.to_static(),
+        };
+
+        if let Some(event) = event {
+            sender.start_send(event).expect("Send event");
+
+            let poll = instance.as_mut().poll(&mut context);
+
+            *control_flow = match poll {
+                task::Poll::Pending => ControlFlow::Wait,
+                task::Poll::Ready(_) => ControlFlow::Exit,
+            };
+        }
+    })
+}
+
+async fn run_instance<A, E, C>(
+    mut application: A,
+    mut compositor: C,
+    mut renderer: A::Renderer,
+    mut runtime: Runtime<E, Proxy<A::Message>, A::Message>,
+    mut proxy: winit::event_loop::EventLoopProxy<A::Message>,
+    mut debug: Debug,
+    mut receiver: mpsc::UnboundedReceiver<winit::event::Event<'_, A::Message>>,
+    init_command: Command<A::Message>,
+    window: winit::window::Window,
+    exit_on_close_request: bool,
+) where
+    A: Application + 'static,
+    E: Executor + 'static,
+    C: window::Compositor<Renderer = A::Renderer> + 'static,
+    <A::Renderer as crate::Renderer>::Theme: StyleSheet,
+{
+    use iced_futures::futures::stream::StreamExt;
+    use winit::event;
+
+    let mut clipboard = Clipboard::connect(&window);
+    let mut cache = user_interface::Cache::default();
+    let mut surface = compositor.create_surface(&window);
+
+    let mut state = State::new(&application, &window);
+    let mut viewport_version = state.viewport_version();
+
+    let physical_size = state.physical_size();
+
+    compositor.configure_surface(
+        &mut surface,
+        physical_size.width,
+        physical_size.height,
+    );
+
+    run_command(
+        &application,
+        &mut cache,
+        &state,
+        &mut renderer,
+        init_command,
+        &mut runtime,
+        &mut clipboard,
+        &mut proxy,
+        &mut debug,
+        &window,
+        || compositor.fetch_information(),
+    );
+    runtime.track(application.subscription());
+
+    let mut user_interface = ManuallyDrop::new(build_user_interface(
+        &application,
+        user_interface::Cache::default(),
+        &mut renderer,
+        state.logical_size(),
+        &mut debug,
+    ));
+
+    let mut mouse_interaction = mouse::Interaction::default();
+    let mut events = Vec::new();
+    let mut messages = Vec::new();
+
+    debug.startup_finished();
+
+    while let Some(event) = receiver.next().await {
+        match event {
+            event::Event::MainEventsCleared => {
+                if events.is_empty() && messages.is_empty() {
+                    continue;
+                }
+
+                debug.event_processing_started();
+
+                let (interface_state, statuses) = user_interface.update(
+                    &events,
+                    state.cursor_position(),
+                    &mut renderer,
+                    &mut clipboard,
+                    &mut messages,
+                );
+
+                debug.event_processing_finished();
+
+                for event in events.drain(..).zip(statuses.into_iter()) {
+                    runtime.broadcast(event);
+                }
+
+                if !messages.is_empty()
+                    || matches!(
+                        interface_state,
+                        user_interface::State::Outdated,
+                    )
+                {
+                    let mut cache =
+                        ManuallyDrop::into_inner(user_interface).into_cache();
+
+                    // Update application
+                    update(
+                        &mut application,
+                        &mut cache,
+                        &state,
+                        &mut renderer,
+                        &mut runtime,
+                        &mut clipboard,
+                        &mut proxy,
+                        &mut debug,
+                        &mut messages,
+                        &window,
+                        || compositor.fetch_information(),
+                    );
+
+                    // Update window
+                    state.synchronize(&application, &window);
+
+                    let should_exit = application.should_exit();
+
+                    user_interface = ManuallyDrop::new(build_user_interface(
+                        &application,
+                        cache,
+                        &mut renderer,
+                        state.logical_size(),
+                        &mut debug,
+                    ));
+
+                    if should_exit {
+                        break;
+                    }
+                }
+
+                debug.draw_started();
+                let new_mouse_interaction = user_interface.draw(
+                    &mut renderer,
+                    state.theme(),
+                    &renderer::Style {
+                        text_color: state.text_color(),
+                    },
+                    state.cursor_position(),
+                );
+                debug.draw_finished();
+
+                if new_mouse_interaction != mouse_interaction {
+                    window.set_cursor_icon(conversion::mouse_interaction(
+                        new_mouse_interaction,
+                    ));
+
+                    mouse_interaction = new_mouse_interaction;
+                }
+
+                window.request_redraw();
+            }
+            event::Event::PlatformSpecific(event::PlatformSpecific::MacOS(
+                event::MacOS::ReceivedUrl(url),
+            )) => {
+                use iced_native::event;
+
+                events.push(iced_native::Event::PlatformSpecific(
+                    event::PlatformSpecific::MacOS(event::MacOS::ReceivedUrl(
+                        url,
+                    )),
+                ));
+            }
+            event::Event::UserEvent(message) => {
+                messages.push(message);
+            }
+            event::Event::RedrawRequested(_) => {
+                let physical_size = state.physical_size();
+
+                if physical_size.width == 0 || physical_size.height == 0 {
+                    continue;
+                }
+
+                debug.render_started();
+                let current_viewport_version = state.viewport_version();
+
+                if viewport_version != current_viewport_version {
+                    let logical_size = state.logical_size();
+
+                    debug.layout_started();
+                    user_interface = ManuallyDrop::new(
+                        ManuallyDrop::into_inner(user_interface)
+                            .relayout(logical_size, &mut renderer),
+                    );
+                    debug.layout_finished();
+
+                    debug.draw_started();
+                    let new_mouse_interaction = user_interface.draw(
+                        &mut renderer,
+                        state.theme(),
+                        &renderer::Style {
+                            text_color: state.text_color(),
+                        },
+                        state.cursor_position(),
+                    );
+
+                    if new_mouse_interaction != mouse_interaction {
+                        window.set_cursor_icon(conversion::mouse_interaction(
+                            new_mouse_interaction,
+                        ));
+
+                        mouse_interaction = new_mouse_interaction;
+                    }
+                    debug.draw_finished();
+
+                    compositor.configure_surface(
+                        &mut surface,
+                        physical_size.width,
+                        physical_size.height,
+                    );
+
+                    viewport_version = current_viewport_version;
+                }
+
+                match compositor.present(
+                    &mut renderer,
+                    &mut surface,
+                    state.viewport(),
+                    state.background_color(),
+                    &debug.overlay(),
+                ) {
+                    Ok(()) => {
+                        debug.render_finished();
+
+                        // TODO: Handle animations!
+                        // Maybe we can use `ControlFlow::WaitUntil` for this.
+                    }
+                    Err(error) => match error {
+                        // This is an unrecoverable error.
+                        compositor::SurfaceError::OutOfMemory => {
+                            panic!("{:?}", error);
+                        }
+                        _ => {
+                            debug.render_finished();
+
+                            // Try rendering again next frame.
+                            window.request_redraw();
+                        }
+                    },
+                }
+            }
+            event::Event::WindowEvent {
+                event: window_event,
+                ..
+            } => {
+                if requests_exit(&window_event, state.modifiers())
+                    && exit_on_close_request
+                {
+                    break;
+                }
+
+                state.update(&window, &window_event, &mut debug);
+
+                if let Some(event) = conversion::window_event(
+                    &window_event,
+                    state.scale_factor(),
+                    state.modifiers(),
+                ) {
+                    events.push(event);
+                }
+            }
+            _ => {}
+        }
+    }
+
+    // Manually drop the user interface
+    drop(ManuallyDrop::into_inner(user_interface));
+}
+
+/// Returns true if the provided event should cause an [`Application`] to
+/// exit.
+pub fn requests_exit(
+    event: &winit::event::WindowEvent<'_>,
+    _modifiers: winit::event::ModifiersState,
+) -> bool {
+    use winit::event::WindowEvent;
+
+    match event {
+        WindowEvent::CloseRequested => true,
+        #[cfg(target_os = "macos")]
+        WindowEvent::KeyboardInput {
+            input:
+                winit::event::KeyboardInput {
+                    virtual_keycode: Some(winit::event::VirtualKeyCode::Q),
+                    state: winit::event::ElementState::Pressed,
+                    ..
+                },
+            ..
+        } if _modifiers.logo() => true,
+        _ => false,
+    }
+}
+
+/// Builds a [`UserInterface`] for the provided [`Application`], logging
+/// [`struct@Debug`] information accordingly.
+pub fn build_user_interface<'a, A: Application>(
+    application: &'a A,
+    cache: user_interface::Cache,
+    renderer: &mut A::Renderer,
+    size: Size,
+    debug: &mut Debug,
+) -> UserInterface<'a, A::Message, A::Renderer>
+where
+    <A::Renderer as crate::Renderer>::Theme: StyleSheet,
+{
+    debug.view_started();
+    let view = application.view();
+    debug.view_finished();
+
+    debug.layout_started();
+    let user_interface = UserInterface::build(view, size, cache, renderer);
+    debug.layout_finished();
+
+    user_interface
+}
+
+/// Updates an [`Application`] by feeding it the provided messages, spawning any
+/// resulting [`Command`], and tracking its [`Subscription`].
+pub fn update<A: Application, E: Executor>(
+    application: &mut A,
+    cache: &mut user_interface::Cache,
+    state: &State<A>,
+    renderer: &mut A::Renderer,
+    runtime: &mut Runtime<E, Proxy<A::Message>, A::Message>,
+    clipboard: &mut Clipboard,
+    proxy: &mut winit::event_loop::EventLoopProxy<A::Message>,
+    debug: &mut Debug,
+    messages: &mut Vec<A::Message>,
+    window: &winit::window::Window,
+    graphics_info: impl FnOnce() -> compositor::Information + Copy,
+) where
+    <A::Renderer as crate::Renderer>::Theme: StyleSheet,
+{
+    for message in messages.drain(..) {
+        debug.log_message(&message);
+
+        debug.update_started();
+        let command = runtime.enter(|| application.update(message));
+        debug.update_finished();
+
+        run_command(
+            application,
+            cache,
+            state,
+            renderer,
+            command,
+            runtime,
+            clipboard,
+            proxy,
+            debug,
+            window,
+            graphics_info,
+        );
+    }
+
+    let subscription = application.subscription();
+    runtime.track(subscription);
+}
+
+/// Runs the actions of a [`Command`].
+pub fn run_command<A, E>(
+    application: &A,
+    cache: &mut user_interface::Cache,
+    state: &State<A>,
+    renderer: &mut A::Renderer,
+    command: Command<A::Message>,
+    runtime: &mut Runtime<E, Proxy<A::Message>, A::Message>,
+    clipboard: &mut Clipboard,
+    proxy: &mut winit::event_loop::EventLoopProxy<A::Message>,
+    debug: &mut Debug,
+    window: &winit::window::Window,
+    _graphics_info: impl FnOnce() -> compositor::Information + Copy,
+) where
+    A: Application,
+    E: Executor,
+    <A::Renderer as crate::Renderer>::Theme: StyleSheet,
+{
+    use iced_native::command;
+    use iced_native::system;
+    use iced_native::window;
+
+    for action in command.actions() {
+        match action {
+            command::Action::Future(future) => {
+                runtime.spawn(future);
+            }
+            command::Action::Clipboard(action) => match action {
+                clipboard::Action::Read(tag) => {
+                    let message = tag(clipboard.read());
+
+                    proxy
+                        .send_event(message)
+                        .expect("Send message to event loop");
+                }
+                clipboard::Action::Write(contents) => {
+                    clipboard.write(contents);
+                }
+            },
+            command::Action::Window(action) => match action {
+                window::Action::Resize { width, height } => {
+                    window.set_inner_size(winit::dpi::LogicalSize {
+                        width,
+                        height,
+                    });
+                }
+                window::Action::Move { x, y } => {
+                    window.set_outer_position(winit::dpi::LogicalPosition {
+                        x,
+                        y,
+                    });
+                }
+                window::Action::SetMode(mode) => {
+                    window.set_visible(conversion::visible(mode));
+                    window.set_fullscreen(conversion::fullscreen(
+                        window.primary_monitor(),
+                        mode,
+                    ));
+                }
+                window::Action::FetchMode(tag) => {
+                    let mode = if window.is_visible().unwrap_or(true) {
+                        conversion::mode(window.fullscreen())
+                    } else {
+                        window::Mode::Hidden
+                    };
+
+                    proxy
+                        .send_event(tag(mode))
+                        .expect("Send message to event loop");
+                }
+            },
+            command::Action::System(action) => match action {
+                system::Action::QueryInformation(_tag) => {
+                    #[cfg(feature = "system")]
+                    {
+                        let graphics_info = _graphics_info();
+                        let proxy = proxy.clone();
+
+                        let _ = std::thread::spawn(move || {
+                            let information =
+                                crate::system::information(graphics_info);
+
+                            let message = _tag(information);
+
+                            proxy
+                                .send_event(message)
+                                .expect("Send message to event loop")
+                        });
+                    }
+                }
+            },
+            command::Action::Widget(action) => {
+                let mut current_cache = std::mem::take(cache);
+                let mut current_operation = Some(action.into_operation());
+
+                let mut user_interface = build_user_interface(
+                    application,
+                    current_cache,
+                    renderer,
+                    state.logical_size(),
+                    debug,
+                );
+
+                while let Some(mut operation) = current_operation.take() {
+                    user_interface.operate(renderer, operation.as_mut());
+
+                    match operation.finish() {
+                        operation::Outcome::None => {}
+                        operation::Outcome::Some(message) => {
+                            proxy
+                                .send_event(message)
+                                .expect("Send message to event loop");
+                        }
+                        operation::Outcome::Chain(next) => {
+                            current_operation = Some(next);
+                        }
+                    }
+                }
+
+                current_cache = user_interface.into_cache();
+                *cache = current_cache;
+            }
+        }
+    }
+}
+
+#[cfg(not(target_arch = "wasm32"))]
+mod platform {
+    pub fn run<T, F>(
+        mut event_loop: winit::event_loop::EventLoop<T>,
+        event_handler: F,
+    ) -> Result<(), super::Error>
+    where
+        F: 'static
+            + FnMut(
+                winit::event::Event<'_, T>,
+                &winit::event_loop::EventLoopWindowTarget<T>,
+                &mut winit::event_loop::ControlFlow,
+            ),
+    {
+        use winit::platform::run_return::EventLoopExtRunReturn;
+
+        let _ = event_loop.run_return(event_handler);
+
+        Ok(())
+    }
+}
+
+#[cfg(target_arch = "wasm32")]
+mod platform {
+    pub fn run<T, F>(
+        event_loop: winit::event_loop::EventLoop<T>,
+        event_handler: F,
+    ) -> !
+    where
+        F: 'static
+            + FnMut(
+                winit::event::Event<'_, T>,
+                &winit::event_loop::EventLoopWindowTarget<T>,
+                &mut winit::event_loop::ControlFlow,
+            ),
+    {
+        event_loop.run(event_handler)
+    }
+}
diff --git a/winit/src/multi_window/state.rs b/winit/src/multi_window/state.rs
new file mode 100644
index 00000000..2d120ca1
--- /dev/null
+++ b/winit/src/multi_window/state.rs
@@ -0,0 +1,212 @@
+use crate::application::{self, StyleSheet as _};
+use crate::conversion;
+use crate::multi_window::Application;
+use crate::{Color, Debug, Point, Size, Viewport};
+
+use std::marker::PhantomData;
+use winit::event::{Touch, WindowEvent};
+use winit::window::Window;
+
+/// The state of a windowed [`Application`].
+#[allow(missing_debug_implementations)]
+pub struct State<A: Application>
+where
+    <A::Renderer as crate::Renderer>::Theme: application::StyleSheet,
+{
+    title: String,
+    scale_factor: f64,
+    viewport: Viewport,
+    viewport_version: usize,
+    cursor_position: winit::dpi::PhysicalPosition<f64>,
+    modifiers: winit::event::ModifiersState,
+    theme: <A::Renderer as crate::Renderer>::Theme,
+    appearance: application::Appearance,
+    application: PhantomData<A>,
+}
+
+impl<A: Application> State<A>
+where
+    <A::Renderer as crate::Renderer>::Theme: application::StyleSheet,
+{
+    /// Creates a new [`State`] for the provided [`Application`] and window.
+    pub fn new(application: &A, window: &Window) -> Self {
+        let title = application.title();
+        let scale_factor = application.scale_factor();
+        let theme = application.theme();
+        let appearance = theme.appearance(application.style());
+
+        let viewport = {
+            let physical_size = window.inner_size();
+
+            Viewport::with_physical_size(
+                Size::new(physical_size.width, physical_size.height),
+                window.scale_factor() * scale_factor,
+            )
+        };
+
+        Self {
+            title,
+            scale_factor,
+            viewport,
+            viewport_version: 0,
+            // TODO: Encode cursor availability in the type-system
+            cursor_position: winit::dpi::PhysicalPosition::new(-1.0, -1.0),
+            modifiers: winit::event::ModifiersState::default(),
+            theme,
+            appearance,
+            application: PhantomData,
+        }
+    }
+
+    /// Returns the current [`Viewport`] of the [`State`].
+    pub fn viewport(&self) -> &Viewport {
+        &self.viewport
+    }
+
+    /// Returns the version of the [`Viewport`] of the [`State`].
+    ///
+    /// The version is incremented every time the [`Viewport`] changes.
+    pub fn viewport_version(&self) -> usize {
+        self.viewport_version
+    }
+
+    /// Returns the physical [`Size`] of the [`Viewport`] of the [`State`].
+    pub fn physical_size(&self) -> Size<u32> {
+        self.viewport.physical_size()
+    }
+
+    /// Returns the logical [`Size`] of the [`Viewport`] of the [`State`].
+    pub fn logical_size(&self) -> Size<f32> {
+        self.viewport.logical_size()
+    }
+
+    /// Returns the current scale factor of the [`Viewport`] of the [`State`].
+    pub fn scale_factor(&self) -> f64 {
+        self.viewport.scale_factor()
+    }
+
+    /// Returns the current cursor position of the [`State`].
+    pub fn cursor_position(&self) -> Point {
+        conversion::cursor_position(
+            self.cursor_position,
+            self.viewport.scale_factor(),
+        )
+    }
+
+    /// Returns the current keyboard modifiers of the [`State`].
+    pub fn modifiers(&self) -> winit::event::ModifiersState {
+        self.modifiers
+    }
+
+    /// Returns the current theme of the [`State`].
+    pub fn theme(&self) -> &<A::Renderer as crate::Renderer>::Theme {
+        &self.theme
+    }
+
+    /// Returns the current background [`Color`] of the [`State`].
+    pub fn background_color(&self) -> Color {
+        self.appearance.background_color
+    }
+
+    /// Returns the current text [`Color`] of the [`State`].
+    pub fn text_color(&self) -> Color {
+        self.appearance.text_color
+    }
+
+    /// Processes the provided window event and updates the [`State`]
+    /// accordingly.
+    pub fn update(
+        &mut self,
+        window: &Window,
+        event: &WindowEvent<'_>,
+        _debug: &mut Debug,
+    ) {
+        match event {
+            WindowEvent::Resized(new_size) => {
+                let size = Size::new(new_size.width, new_size.height);
+
+                self.viewport = Viewport::with_physical_size(
+                    size,
+                    window.scale_factor() * self.scale_factor,
+                );
+
+                self.viewport_version = self.viewport_version.wrapping_add(1);
+            }
+            WindowEvent::ScaleFactorChanged {
+                scale_factor: new_scale_factor,
+                new_inner_size,
+            } => {
+                let size =
+                    Size::new(new_inner_size.width, new_inner_size.height);
+
+                self.viewport = Viewport::with_physical_size(
+                    size,
+                    new_scale_factor * self.scale_factor,
+                );
+
+                self.viewport_version = self.viewport_version.wrapping_add(1);
+            }
+            WindowEvent::CursorMoved { position, .. }
+            | WindowEvent::Touch(Touch {
+                location: position, ..
+            }) => {
+                self.cursor_position = *position;
+            }
+            WindowEvent::CursorLeft { .. } => {
+                // TODO: Encode cursor availability in the type-system
+                self.cursor_position =
+                    winit::dpi::PhysicalPosition::new(-1.0, -1.0);
+            }
+            WindowEvent::ModifiersChanged(new_modifiers) => {
+                self.modifiers = *new_modifiers;
+            }
+            #[cfg(feature = "debug")]
+            WindowEvent::KeyboardInput {
+                input:
+                    winit::event::KeyboardInput {
+                        virtual_keycode: Some(winit::event::VirtualKeyCode::F12),
+                        state: winit::event::ElementState::Pressed,
+                        ..
+                    },
+                ..
+            } => _debug.toggle(),
+            _ => {}
+        }
+    }
+
+    /// Synchronizes the [`State`] with its [`Application`] and its respective
+    /// window.
+    ///
+    /// Normally an [`Application`] should be synchronized with its [`State`]
+    /// and window after calling [`Application::update`].
+    ///
+    /// [`Application::update`]: crate::Program::update
+    pub fn synchronize(&mut self, application: &A, window: &Window) {
+        // Update window title
+        let new_title = application.title();
+
+        if self.title != new_title {
+            window.set_title(&new_title);
+
+            self.title = new_title;
+        }
+
+        // Update scale factor
+        let new_scale_factor = application.scale_factor();
+
+        if self.scale_factor != new_scale_factor {
+            let size = window.inner_size();
+
+            self.viewport = Viewport::with_physical_size(
+                Size::new(size.width, size.height),
+                window.scale_factor() * new_scale_factor,
+            );
+
+            self.scale_factor = new_scale_factor;
+        }
+
+        // Update theme and appearance
+        self.theme = application.theme();
+        self.appearance = self.theme.appearance(application.style());
+    }
+}