Add custom_widget example

It showcases how to build a simple native custom widget that draws a
circle.

Cargo.toml

@@ -41,6 +41,8 @@ serde_json = "1.0"
 directories = "2.0"
 reqwest = "0.9"
 rand = "0.7"
+iced_native = { version = "0.1", path = "./native" }
+iced_wgpu = { version = "0.1", path = "./wgpu" }
 
 [target.'cfg(target_arch = "wasm32")'.dev-dependencies]
 wasm-bindgen = "0.2.51"

examples/custom_widget.rs

@@ -0,0 +1,149 @@
+//! This example showcases a simple native custom widget that draws a circle.
+mod circle {
+    // For now, to implement a custom native widget you will need to add
+    // `iced_native` and `iced_wgpu` to your dependencies.
+    //
+    // Then, you simply need to define your widget type and implement the
+    // `iced_native::Widget` trait with the `iced_wgpu::Renderer`.
+    //
+    // Of course, you can choose to make the implementation renderer-agnostic,
+    // if you wish to, by creating your own `Renderer` trait, which could be
+    // implemented by `iced_wgpu` and other renderers.
+    use iced_native::{
+        layout, Background, Color, Element, Hasher, Layout, Length,
+        MouseCursor, Point, Size, Widget,
+    };
+    use iced_wgpu::{Primitive, Renderer};
+
+    pub struct Circle {
+        radius: u16,
+    }
+
+    impl Circle {
+        pub fn new(radius: u16) -> Self {
+            Self { radius }
+        }
+    }
+
+    impl<Message> Widget<Message, Renderer> for Circle {
+        fn width(&self) -> Length {
+            Length::Shrink
+        }
+
+        fn height(&self) -> Length {
+            Length::Shrink
+        }
+
+        fn layout(
+            &self,
+            _renderer: &Renderer,
+            limits: &layout::Limits,
+        ) -> layout::Node {
+            let size = limits
+                .width(Length::Units(self.radius * 2))
+                .height(Length::Units(self.radius * 2))
+                .resolve(Size::ZERO);
+
+            layout::Node::new(size)
+        }
+
+        fn hash_layout(&self, state: &mut Hasher) {
+            use std::hash::Hash;
+
+            self.radius.hash(state);
+        }
+
+        fn draw(
+            &self,
+            _renderer: &mut Renderer,
+            layout: Layout<'_>,
+            _cursor_position: Point,
+        ) -> (Primitive, MouseCursor) {
+            let bounds = layout.bounds();
+
+            (
+                Primitive::Quad {
+                    bounds,
+                    background: Background::Color(Color::BLACK),
+                    border_radius: self.radius,
+                },
+                MouseCursor::OutOfBounds,
+            )
+        }
+    }
+
+    impl<'a, Message> Into<Element<'a, Message, Renderer>> for Circle {
+        fn into(self) -> Element<'a, Message, Renderer> {
+            Element::new(self)
+        }
+    }
+}
+
+use circle::Circle;
+use iced::{
+    slider, Align, Column, Container, Element, Length, Sandbox, Settings,
+    Slider, Text,
+};
+
+pub fn main() {
+    Example::run(Settings::default())
+}
+
+struct Example {
+    radius: u16,
+    slider: slider::State,
+}
+
+#[derive(Debug, Clone, Copy)]
+enum Message {
+    RadiusChanged(f32),
+}
+
+impl Sandbox for Example {
+    type Message = Message;
+
+    fn new() -> Self {
+        Example {
+            radius: 50,
+            slider: slider::State::new(),
+        }
+    }
+
+    fn title(&self) -> String {
+        String::from("Custom widget - Iced")
+    }
+
+    fn update(&mut self, message: Message) {
+        match message {
+            Message::RadiusChanged(radius) => {
+                self.radius = radius.round() as u16;
+            }
+        }
+    }
+
+    fn view(&mut self) -> Element<Message> {
+        let content = Column::new()
+            .padding(20)
+            .spacing(20)
+            .max_width(500)
+            .align_items(Align::Center)
+            .push(Circle::new(self.radius))
+            .push(
+                Text::new(format!("Radius: {}", self.radius.to_string()))
+                    .width(Length::Shrink),
+            )
+            .push(Slider::new(
+                &mut self.slider,
+                1.0..=100.0,
+                f32::from(self.radius),
+                Message::RadiusChanged,
+            ));
+
+        Container::new(content)
+            .width(Length::Fill)
+            .height(Length::Fill)
+            .center_x()
+            .center_y()
+            .into()
+    }
+}