mistakes were fixed and sins were committed

src/layout/widget.rs

@@ -1,16 +1,21 @@
-use crate::layout::{Painter, Size, SizeCtx, StaticWidgetId, Ui, WidgetId, WidgetIdFn};
+use crate::layout::{Len, Painter, SizeCtx, StaticWidgetId, Ui, WidgetId, WidgetIdFn};
 
 use std::{any::Any, marker::PhantomData};
 
 pub trait Widget: Any {
     fn draw(&mut self, painter: &mut Painter);
-    fn desired_size(&mut self, _: &mut SizeCtx) -> Size {
-        Size::default()
-    }
+    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len;
+    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len;
 }
 
 impl Widget for () {
     fn draw(&mut self, _: &mut Painter) {}
+    fn desired_width(&mut self, _: &mut SizeCtx) -> Len {
+        Len::ZERO
+    }
+    fn desired_height(&mut self, _: &mut SizeCtx) -> Len {
+        Len::ZERO
+    }
 }
 
 pub struct WidgetTag;