Move `winit` logic from `iced` to `iced_winit`

- Added new `renderer::Windowed` trait. This shoud allow users to easily
  try different renderers by simply changing one line.
- Renamed `UserInterface` traits to `Application`, as the `run` method
  takes total control of the current thread.
- Moved `MouseCursor` back to `iced_native`. The new
  `renderer::Windowed` trait returns one on `draw`.
- Split `iced_native` renderer in multiple modules, for consistency.

13 files changed, 245 insertions, 155 deletions

diff --git a/examples/tour.rs b/examples/tour.rs
index 542efc66..6ce1e9f1 100644
--- a/examples/tour.rs
+++ b/examples/tour.rs
@@ -1,7 +1,7 @@
 use iced::{
-    button, slider, text::HorizontalAlignment, Align, Background, Button,
-    Checkbox, Color, Column, Element, Image, Justify, Length, Radio, Row,
-    Slider, Text, UserInterface,
+    button, slider, text::HorizontalAlignment, Align, Application, Background,
+    Button, Checkbox, Color, Column, Element, Image, Justify, Length, Radio,
+    Row, Slider, Text,
 };
 
 pub fn main() {
@@ -30,7 +30,7 @@ impl Tour {
     }
 }
 
-impl UserInterface for Tour {
+impl Application for Tour {
     type Message = Message;
 
     fn update(&mut self, event: Message) {
diff --git a/native/Cargo.toml b/native/Cargo.toml
index 5f7e5e41..07c14535 100644
--- a/native/Cargo.toml
+++ b/native/Cargo.toml
@@ -14,6 +14,7 @@ features = ["winit"]
 iced_core = { version = "0.1.0-alpha", path = "../core" }
 stretch = "0.2"
 twox-hash = "1.5"
+raw-window-handle = "0.1"
 
 # Enable to obtain conversion traits
 winit = { version = "0.20.0-alpha3", optional = true }
diff --git a/native/src/lib.rs b/native/src/lib.rs
index 18ce3a37..cada56f9 100644
--- a/native/src/lib.rs
+++ b/native/src/lib.rs
@@ -206,6 +206,7 @@ mod element;
 mod event;
 mod hasher;
 mod layout;
+mod mouse_cursor;
 mod node;
 mod style;
 mod user_interface;
@@ -223,6 +224,7 @@ pub use element::Element;
 pub use event::Event;
 pub use hasher::Hasher;
 pub use layout::Layout;
+pub use mouse_cursor::MouseCursor;
 pub use node::Node;
 pub use renderer::Renderer;
 pub use style::Style;
diff --git a/wgpu/src/mouse_cursor.rs b/native/src/mouse_cursor.rs
index 4ef6361a..4ef6361a 100644
--- a/wgpu/src/mouse_cursor.rs
+++ b/native/src/mouse_cursor.rs
diff --git a/native/src/renderer.rs b/native/src/renderer.rs
index d16a0289..a56d7b76 100644
--- a/native/src/renderer.rs
+++ b/native/src/renderer.rs
@@ -1,8 +1,10 @@
 //! Write your own renderer.
 //!
-//! There is not a common entrypoint or trait for a __renderer__ in Iced.
-//! Instead, every [`Widget`] constrains its generic `Renderer` type as
-//! necessary.
+//! You will need to implement the `Renderer` trait first. It simply contains
+//! a `Primitive` associated type.
+//!
+//! There is no common trait to draw all the widgets. Instead, every [`Widget`]
+//! constrains its generic `Renderer` type as necessary.
 //!
 //! This approach is flexible and composable. For instance, the
 //! [`Text`] widget only needs a [`text::Renderer`] while a [`Checkbox`] widget
@@ -17,32 +19,13 @@
 //! [`text::Renderer`]: ../widget/text/trait.Renderer.html
 //! [`Checkbox`]: ../widget/checkbox/struct.Checkbox.html
 //! [`checkbox::Renderer`]: ../widget/checkbox/trait.Renderer.html
-use crate::{Color, Layout, Point, Widget};
+
+mod debugger;
+mod windowed;
+
+pub use debugger::Debugger;
+pub use windowed::Windowed;
 
 pub trait Renderer {
     type Primitive;
 }
-
-/// A renderer able to graphically explain a [`Layout`].
-///
-/// [`Layout`]: ../struct.Layout.html
-pub trait Debugger: Renderer {
-    /// Explains the [`Layout`] of an [`Element`] for debugging purposes.
-    ///
-    /// This will be called when [`Element::explain`] has been used. It should
-    /// _explain_ the given [`Layout`] graphically.
-    ///
-    /// A common approach consists in recursively rendering the bounds of the
-    /// [`Layout`] and its children.
-    ///
-    /// [`Layout`]: struct.Layout.html
-    /// [`Element`]: struct.Element.html
-    /// [`Element::explain`]: struct.Element.html#method.explain
-    fn explain<Message>(
-        &mut self,
-        widget: &dyn Widget<Message, Self>,
-        layout: Layout<'_>,
-        cursor_position: Point,
-        color: Color,
-    ) -> Self::Primitive;
-}
diff --git a/native/src/renderer/debugger.rs b/native/src/renderer/debugger.rs
new file mode 100644
index 00000000..cb472dd4
--- /dev/null
+++ b/native/src/renderer/debugger.rs
@@ -0,0 +1,25 @@
+use crate::{Color, Layout, Point, Widget};
+
+/// A renderer able to graphically explain a [`Layout`].
+///
+/// [`Layout`]: ../struct.Layout.html
+pub trait Debugger: super::Renderer {
+    /// Explains the [`Layout`] of an [`Element`] for debugging purposes.
+    ///
+    /// This will be called when [`Element::explain`] has been used. It should
+    /// _explain_ the given [`Layout`] graphically.
+    ///
+    /// A common approach consists in recursively rendering the bounds of the
+    /// [`Layout`] and its children.
+    ///
+    /// [`Layout`]: struct.Layout.html
+    /// [`Element`]: struct.Element.html
+    /// [`Element::explain`]: struct.Element.html#method.explain
+    fn explain<Message>(
+        &mut self,
+        widget: &dyn Widget<Message, Self>,
+        layout: Layout<'_>,
+        cursor_position: Point,
+        color: Color,
+    ) -> Self::Primitive;
+}
diff --git a/native/src/renderer/windowed.rs b/native/src/renderer/windowed.rs
new file mode 100644
index 00000000..f89da40b
--- /dev/null
+++ b/native/src/renderer/windowed.rs
@@ -0,0 +1,17 @@
+use crate::MouseCursor;
+
+use raw_window_handle::HasRawWindowHandle;
+
+pub trait Windowed: super::Renderer {
+    type Target;
+
+    fn new<W: HasRawWindowHandle>(window: &W) -> Self;
+
+    fn target(&self, width: u16, height: u16) -> Self::Target;
+
+    fn draw(
+        &mut self,
+        target: &mut Self::Target,
+        primitive: &Self::Primitive,
+    ) -> MouseCursor;
+}
diff --git a/src/lib.rs b/src/lib.rs
index 2e65ed6c..46574285 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -9,123 +9,35 @@ pub type Row<'a, Message> = iced_winit::Row<'a, Message, Renderer>;
 pub type Column<'a, Message> = iced_winit::Column<'a, Message, Renderer>;
 pub type Button<'a, Message> = iced_winit::Button<'a, Message, Renderer>;
 
-pub trait UserInterface {
+pub trait Application {
     type Message;
 
     fn update(&mut self, message: Self::Message);
 
     fn view(&mut self) -> Element<Self::Message>;
 
-    fn run(mut self)
+    fn run(self)
     where
         Self: 'static + Sized,
     {
-        use winit::{
-            event::{Event, WindowEvent},
-            event_loop::{ControlFlow, EventLoop},
-            window::WindowBuilder,
-        };
-
-        let event_loop = EventLoop::new();
-
-        // TODO: Ask for window settings and configure this properly
-        let window = WindowBuilder::new()
-            .build(&event_loop)
-            .expect("Open window");
-
-        let size = window.inner_size().to_physical(window.hidpi_factor());;
-        let (width, height) = (size.width as u16, size.height as u16);
-
-        let mut renderer = Renderer::new(&window);
-        let mut target = renderer.target(width, height);
-
-        let user_interface = iced_winit::UserInterface::build(
-            document(&mut self, width, height),
-            iced_winit::Cache::default(),
-            &mut renderer,
-        );
-
-        let mut primitive = user_interface.draw(&mut renderer);
-        let mut cache = Some(user_interface.into_cache());
-        let mut events = Vec::new();
-
-        window.request_redraw();
-
-        event_loop.run(move |event, _, control_flow| match event {
-            Event::EventsCleared => {
-                // TODO: We should be able to keep a user interface alive
-                // between events once we remove state references.
-                //
-                // This will allow us to rebuild it only when a message is
-                // handled.
-                let mut user_interface = iced_winit::UserInterface::build(
-                    document(&mut self, width, height),
-                    cache.take().unwrap(),
-                    &mut renderer,
-                );
-
-                let messages = user_interface.update(events.drain(..));
-
-                if messages.is_empty() {
-                    primitive = user_interface.draw(&mut renderer);
-
-                    cache = Some(user_interface.into_cache());
-                } else {
-                    // When there are messages, we are forced to rebuild twice
-                    // for now :^)
-                    let temp_cache = user_interface.into_cache();
-
-                    for message in messages {
-                        self.update(message);
-                    }
-
-                    let user_interface = iced_winit::UserInterface::build(
-                        document(&mut self, width, height),
-                        temp_cache,
-                        &mut renderer,
-                    );
-
-                    primitive = user_interface.draw(&mut renderer);
-
-                    cache = Some(user_interface.into_cache());
-                }
-
-                window.request_redraw();
-            }
-            Event::WindowEvent {
-                event: WindowEvent::RedrawRequested,
-                ..
-            } => {
-                renderer.draw(&mut target, &primitive);
-
-                // TODO: Handle animations!
-                // Maybe we can use `ControlFlow::WaitUntil` for this.
-            }
-            Event::WindowEvent {
-                event: WindowEvent::CloseRequested,
-                ..
-            } => {
-                *control_flow = ControlFlow::Exit;
-            }
-            _ => {
-                *control_flow = ControlFlow::Wait;
-            }
-        })
+        iced_winit::Application::run(Instance(self))
     }
 }
 
-fn document<UserInterface>(
-    user_interface: &mut UserInterface,
-    width: u16,
-    height: u16,
-) -> Element<UserInterface::Message>
+struct Instance<A: Application>(A);
+
+impl<A> iced_winit::Application for Instance<A>
 where
-    UserInterface: self::UserInterface,
-    UserInterface::Message: 'static,
+    A: Application,
 {
-    Column::new()
-        .width(Length::Units(width))
-        .height(Length::Units(height))
-        .push(user_interface.view())
-        .into()
+    type Renderer = Renderer;
+    type Message = A::Message;
+
+    fn update(&mut self, message: Self::Message) {
+        self.0.update(message);
+    }
+
+    fn view(&mut self) -> Element<Self::Message> {
+        self.0.view()
+    }
 }
diff --git a/web/src/bus.rs b/web/src/bus.rs
index d76466f5..b4fd67c7 100644
--- a/web/src/bus.rs
+++ b/web/src/bus.rs
@@ -1,4 +1,4 @@
-use crate::Application;
+use crate::Instance;
 
 use std::rc::Rc;
 
@@ -14,7 +14,7 @@ where
     pub fn new() -> Self {
         Self {
             publish: Rc::new(Box::new(|message, root| {
-                let app = root.unwrap_mut::<Application<Message>>();
+                let app = root.unwrap_mut::<Instance<Message>>();
 
                 app.update(message)
             })),
diff --git a/web/src/lib.rs b/web/src/lib.rs
index caf17df5..04848d07 100644
--- a/web/src/lib.rs
+++ b/web/src/lib.rs
@@ -11,7 +11,7 @@ pub use element::Element;
 pub use iced_core::{Align, Color, Justify, Length};
 pub use widget::*;
 
-pub trait UserInterface {
+pub trait Application {
     type Message;
 
     fn update(
@@ -25,23 +25,23 @@ pub trait UserInterface {
     where
         Self: 'static + Sized,
     {
+        let app = Instance::new(self);
+
         let window = web_sys::window().unwrap();
         let document = window.document().unwrap();
         let body = document.body().unwrap();
-
-        let app = Application::new(self);
-
         let vdom = dodrio::Vdom::new(&body, app);
+
         vdom.forget();
     }
 }
 
-struct Application<Message> {
-    ui: RefCell<Box<dyn UserInterface<Message = Message>>>,
+struct Instance<Message> {
+    ui: RefCell<Box<dyn Application<Message = Message>>>,
 }
 
-impl<Message> Application<Message> {
-    fn new(ui: impl UserInterface<Message = Message> + 'static) -> Self {
+impl<Message> Instance<Message> {
+    fn new(ui: impl Application<Message = Message> + 'static) -> Self {
         Self {
             ui: RefCell::new(Box::new(ui)),
         }
@@ -55,7 +55,7 @@ impl<Message> Application<Message> {
     }
 }
 
-impl<Message> dodrio::Render for Application<Message>
+impl<Message> dodrio::Render for Instance<Message>
 where
     Message: 'static,
 {
diff --git a/wgpu/src/lib.rs b/wgpu/src/lib.rs
index 8f8d50e9..46849aab 100644
--- a/wgpu/src/lib.rs
+++ b/wgpu/src/lib.rs
@@ -1,4 +1,3 @@
-mod mouse_cursor;
 mod primitive;
 mod quad;
 mod renderer;
@@ -7,6 +6,5 @@ mod transformation;
 pub(crate) use quad::Quad;
 pub(crate) use transformation::Transformation;
 
-pub use mouse_cursor::MouseCursor;
 pub use primitive::Primitive;
 pub use renderer::{Renderer, Target};
diff --git a/wgpu/src/renderer.rs b/wgpu/src/renderer.rs
index ae5692e3..036efd27 100644
--- a/wgpu/src/renderer.rs
+++ b/wgpu/src/renderer.rs
@@ -1,6 +1,7 @@
 use crate::{quad, Primitive, Quad, Transformation};
 use iced_native::{
-    renderer::Debugger, Background, Color, Layout, Point, Widget,
+    renderer::Debugger, renderer::Windowed, Background, Color, Layout,
+    MouseCursor, Point, Widget,
 };
 
 use raw_window_handle::HasRawWindowHandle;
@@ -41,7 +42,7 @@ pub struct Target {
 }
 
 impl Renderer {
-    pub fn new<W: HasRawWindowHandle>(window: &W) -> Self {
+    fn new<W: HasRawWindowHandle>(window: &W) -> Self {
         let adapter = Adapter::request(&RequestAdapterOptions {
             power_preference: PowerPreference::LowPower,
             backends: BackendBit::all(),
@@ -79,7 +80,7 @@ impl Renderer {
         }
     }
 
-    pub fn target(&self, width: u16, height: u16) -> Target {
+    fn target(&self, width: u16, height: u16) -> Target {
         Target {
             width,
             height,
@@ -97,7 +98,11 @@ impl Renderer {
         }
     }
 
-    pub fn draw(&mut self, target: &mut Target, primitive: &Primitive) {
+    fn draw(
+        &mut self,
+        target: &mut Target,
+        primitive: &Primitive,
+    ) -> MouseCursor {
         log::debug!("Drawing");
 
         let frame = target.swap_chain.get_next_texture();
@@ -146,8 +151,9 @@ impl Renderer {
             .expect("Draw text");
 
         self.queue.submit(&[encoder.finish()]);
-    }
 
+        MouseCursor::OutOfBounds
+    }
     fn draw_primitive(&mut self, primitive: &Primitive) {
         match primitive {
             Primitive::None => {}
@@ -240,6 +246,26 @@ impl iced_native::Renderer for Renderer {
     type Primitive = Primitive;
 }
 
+impl Windowed for Renderer {
+    type Target = Target;
+
+    fn new<W: HasRawWindowHandle>(window: &W) -> Self {
+        Self::new(window)
+    }
+
+    fn target(&self, width: u16, height: u16) -> Target {
+        self.target(width, height)
+    }
+
+    fn draw(
+        &mut self,
+        target: &mut Target,
+        primitive: &Primitive,
+    ) -> MouseCursor {
+        self.draw(target, primitive)
+    }
+}
+
 impl Debugger for Renderer {
     fn explain<Message>(
         &mut self,
diff --git a/winit/src/lib.rs b/winit/src/lib.rs
index 54a0bd9a..93e922d5 100644
--- a/winit/src/lib.rs
+++ b/winit/src/lib.rs
@@ -1,2 +1,128 @@
 pub use iced_native::*;
 pub use winit;
+
+pub use iced_native::renderer::Windowed;
+
+pub trait Application {
+    type Renderer: iced_native::renderer::Windowed
+        + iced_native::column::Renderer;
+
+    type Message;
+
+    fn update(&mut self, message: Self::Message);
+
+    fn view(&mut self) -> Element<Self::Message, Self::Renderer>;
+
+    fn run(mut self)
+    where
+        Self: 'static + Sized,
+    {
+        use winit::{
+            event::{Event, WindowEvent},
+            event_loop::{ControlFlow, EventLoop},
+            window::WindowBuilder,
+        };
+
+        let event_loop = EventLoop::new();
+
+        // TODO: Ask for window settings and configure this properly
+        let window = WindowBuilder::new()
+            .build(&event_loop)
+            .expect("Open window");
+
+        let size = window.inner_size().to_physical(window.hidpi_factor());;
+        let (width, height) = (size.width as u16, size.height as u16);
+
+        let mut renderer = Self::Renderer::new(&window);
+        let mut target = renderer.target(width, height);
+
+        let user_interface = UserInterface::build(
+            document(&mut self, width, height),
+            Cache::default(),
+            &mut renderer,
+        );
+
+        let mut primitive = user_interface.draw(&mut renderer);
+        let mut cache = Some(user_interface.into_cache());
+        let mut events = Vec::new();
+
+        window.request_redraw();
+
+        event_loop.run(move |event, _, control_flow| match event {
+            Event::EventsCleared => {
+                // TODO: We should be able to keep a user interface alive
+                // between events once we remove state references.
+                //
+                // This will allow us to rebuild it only when a message is
+                // handled.
+                let mut user_interface = UserInterface::build(
+                    document(&mut self, width, height),
+                    cache.take().unwrap(),
+                    &mut renderer,
+                );
+
+                let messages = user_interface.update(events.drain(..));
+
+                if messages.is_empty() {
+                    primitive = user_interface.draw(&mut renderer);
+
+                    cache = Some(user_interface.into_cache());
+                } else {
+                    // When there are messages, we are forced to rebuild twice
+                    // for now :^)
+                    let temp_cache = user_interface.into_cache();
+
+                    for message in messages {
+                        self.update(message);
+                    }
+
+                    let user_interface = UserInterface::build(
+                        document(&mut self, width, height),
+                        temp_cache,
+                        &mut renderer,
+                    );
+
+                    primitive = user_interface.draw(&mut renderer);
+
+                    cache = Some(user_interface.into_cache());
+                }
+
+                window.request_redraw();
+            }
+            Event::WindowEvent {
+                event: WindowEvent::RedrawRequested,
+                ..
+            } => {
+                renderer.draw(&mut target, &primitive);
+
+                // TODO: Handle animations!
+                // Maybe we can use `ControlFlow::WaitUntil` for this.
+            }
+            Event::WindowEvent {
+                event: WindowEvent::CloseRequested,
+                ..
+            } => {
+                *control_flow = ControlFlow::Exit;
+            }
+            _ => {
+                *control_flow = ControlFlow::Wait;
+            }
+        })
+    }
+}
+
+fn document<Application>(
+    application: &mut Application,
+    width: u16,
+    height: u16,
+) -> Element<Application::Message, Application::Renderer>
+where
+    Application: self::Application,
+    Application::Message: 'static,
+{
+    Column::new()
+        .width(Length::Units(width))
+        .height(Length::Units(height))
+        .push(application.view())
+        .into()
+}