Implement Empty widget

It can be useful if you want to fill some space with nothing.

native/src/widget/empty.rs

@@ -0,0 +1,103 @@
+//! Distribute content vertically.
+use std::hash::Hash;
+
+use crate::{
+    layout, Element, Hasher, Layout, Length, Point, Rectangle, Size, Widget,
+};
+
+/// An amount of empty space.
+///
+/// It can be useful if you want to fill some space with nothing.
+///
+/// [`Empty`]: struct.Empty.html
+#[derive(Debug)]
+pub struct Empty {
+    width: Length,
+    height: Length,
+}
+
+impl Empty {
+    /// Creates an amount of [`Empty`] space.
+    ///
+    /// [`Empty`]: struct.Empty.html
+    pub fn new() -> Self {
+        Empty {
+            width: Length::Shrink,
+            height: Length::Shrink,
+        }
+    }
+
+    /// Sets the width of the [`Empty`] space.
+    ///
+    /// [`Empty`]: struct..html
+    pub fn width(mut self, width: Length) -> Self {
+        self.width = width;
+        self
+    }
+
+    /// Sets the height of the [`Empty`] space.
+    ///
+    /// [`Empty`]: struct.Column.html
+    pub fn height(mut self, height: Length) -> Self {
+        self.height = height;
+        self
+    }
+}
+
+impl<'a, Message, Renderer> Widget<Message, Renderer> for Empty
+where
+    Renderer: self::Renderer,
+{
+    fn width(&self) -> Length {
+        self.width
+    }
+
+    fn height(&self) -> Length {
+        self.height
+    }
+
+    fn layout(
+        &self,
+        _renderer: &Renderer,
+        limits: &layout::Limits,
+    ) -> layout::Node {
+        let limits = limits.width(self.width).height(self.height);
+
+        layout::Node::new(limits.resolve(Size::ZERO))
+    }
+
+    fn draw(
+        &self,
+        renderer: &mut Renderer,
+        layout: Layout<'_>,
+        _cursor_position: Point,
+    ) -> Renderer::Output {
+        renderer.draw(layout.bounds())
+    }
+
+    fn hash_layout(&self, state: &mut Hasher) {
+        std::any::TypeId::of::<Empty>().hash(state);
+        self.width.hash(state);
+        self.height.hash(state);
+    }
+}
+
+/// The renderer of an amount of [`Empty`] space.
+///
+/// [`Empty`]: struct.Empty.html
+pub trait Renderer: crate::Renderer {
+    /// Draws an amount of [`Empty`] space.
+    ///
+    /// You should most likely return an empty primitive here.
+    fn draw(&mut self, bounds: Rectangle) -> Self::Output;
+}
+
+impl<'a, Message, Renderer> From<Empty> for Element<'a, Message, Renderer>
+where
+    Renderer: self::Renderer,
+    Message: 'static,
+{
+    fn from(empty: Empty) -> Element<'a, Message, Renderer> {
+        Element::new(empty)
+    }
+}