Redesign span layout around retained placement

core/src/widget/mod.rs

@@ -1,4 +1,4 @@
-use crate::{Painter, Size};
+use crate::{Axis, Len, Painter, Size};
 use std::any::Any;
 
 mod data;
@@ -16,46 +16,28 @@ pub use view::*;
 pub use widgets::*;
 
 pub trait Widget: Any {
-    /// Draw within `painter.region()` (the space the parent offered) and
-    /// report how much of it was actually used, per axis.
     fn draw(&mut self, painter: &mut Painter) -> Size;
 
-    /// True if `draw`'s output (both the primitives it writes and the
-    /// `Size` it returns) is the same for any `painter.region()` of the
-    /// same *content* -- an icon, a fixed-size rect, an already-decoded
-    /// image at its natural size. Default `false` (redraw on any change to
-    /// the offered region) because assuming independence wrongly produces
-    /// a stale draw; a widget must opt in. See LAYOUT.md.
+    /// An exact, context-free length known without drawing or inspecting children.
+    fn size_hint(&self, _axis: Axis) -> Option<Len> {
+        None
+    }
+
+    /// Whether the draw result is independent of the offered region.
     fn is_size_independent(&self) -> bool {
         false
     }
 
-    /// What kind of control this is, for the AccessKit tree `ui::access`
-    /// builds (RUST.md's I4). Only consulted for a widget that also has an
-    /// explicit `.label()` -- an unnamed widget is never visited by that
-    /// tree at all, named or not, so the default here costs nothing except
-    /// at the handful of call sites that opt in. Default `Unknown` (a
-    /// generic control with no more specific semantics); a widget with a
-    /// real platform equivalent -- `TextEdit`'s `MultilineTextInput` --
-    /// overrides it.
+    fn requires_exact_region(&self) -> bool {
+        false
+    }
+
+    /// The AccessKit role for a labelled widget.
     fn access_role(&self) -> accesskit::Role {
         accesskit::Role::Unknown
     }
 
-    /// Advance whatever this widget is animating to `now`, and say whether
-    /// it is still animating afterwards. Default: nothing is, so a widget
-    /// opts in by overriding this *and* by something calling
-    /// [`crate::UiData::animate`] with its id when the animation starts --
-    /// which is that animation's path out, since the driver
-    /// ([`crate::UiData::tick_animations`]) drops every id whose `tick`
-    /// answers `false`.
-    ///
-    /// Called once per frame, before the frame's draw, by whichever
-    /// backend owns the surface; a `true` answer is what makes that
-    /// backend ask for another frame. So this is the only thing in iris
-    /// that moves without an input event, and a widget that animates
-    /// without registering simply never moves -- which is exactly how a
-    /// finger fling looked on Iris's phone before this existed.
+    /// Advance an animation and report whether it needs another frame.
     #[allow(unused_variables)]
     fn tick(&mut self, now: std::time::Instant) -> bool {
         false
@@ -70,6 +52,10 @@ impl Widget for () {
     fn is_size_independent(&self) -> bool {
         true
     }
+
+    fn size_hint(&self, _axis: Axis) -> Option<Len> {
+        Some(Len::ZERO)
+    }
 }
 
 impl dyn Widget {