refactor engine and layout into input and output

output/src/transform_xy.rs

@@ -0,0 +1,40 @@
+use crate::*;
+
+/// Defines an enum that transforms its content
+/// along either the X axis, the Y axis, or both.
+///
+/// The `_Unused` variant wraps the `Output` type
+/// using `PhantomData` to permit the double generic.
+macro_rules! transform_xy {
+    ($self:ident : $Enum:ident |$to:ident|$area:expr) => {
+        pub enum $Enum<T> { X(T), Y(T), XY(T) }
+        impl<T> $Enum<T> {
+            pub fn x (item: T)  -> Self { Self::X(item) }
+            pub fn y (item: T)  -> Self { Self::Y(item) }
+            pub fn xy (item: T) -> Self { Self::XY(item) }
+        }
+        impl<E: Output, T: Content<E>> Content<E> for $Enum<T> {
+            fn content (&self) -> impl Render<E> {
+                match self {
+                    Self::X(item)  => item,
+                    Self::Y(item)  => item,
+                    Self::XY(item) => item
+                }
+            }
+            fn layout (&$self, $to: <E as Output>::Area) -> <E as Output>::Area {
+                use $Enum::*;
+                $area
+            }
+        }
+    }
+}
+
+transform_xy!(self: Fill |to|{
+    let [x0, y0, wmax, hmax] = to.xywh();
+    let [x, y, w, h] = self.content().layout(to).xywh();
+    match self {
+        X(_)  => [x0, y, wmax, h],
+        Y(_)  => [x, y0, w, hmax],
+        XY(_) => [x0, y0, wmax, hmax],
+    }.into()
+});