Size a widget while drawing it, not in a pass of its own

`desired_width`/`desired_height`, `WidgetAxisFns`, `SizeCtx` and the size
cache are gone. A widget states what it used with `Painter::set_size`
while it draws, and `Painter::widget` hands back a `DrawResult` whose
`size()` both reads the child and records that this widget's size depends
on it. Reading nothing keeps the parent independent of what the child came
to.

`Span` is what the change is for. It takes each child's `size_hint` where
there is one, draws only the children that cannot answer, allocates the
flexible space, then places everything -- which deletes `desired_ortho`,
whose own comment said it "literally copies draw ... which makes this slow
and not cool".

Invalidation follows the dependency edges the draw recorded: a widget that
needs redrawing hands off to the highest ancestor that read its size,
instead of re-running a measurement to find out whether anything changed.

`Widget::size_dependence(axis)` says how much of its box a widget's
drawing depends on -- none of it, its own extent, or the whole box -- so
the retained path can keep a drawing and write a new box into it. Asked
per axis, because wrapped text depends on the width it is offered and not
on the height. `Internal` does not yet buy more than `External`: keeping a
drawing when only the room around it changed is a translation, which waits
for the move chain.

`tests/retained.rs` covers the second frame rather than the first, which
is where the bugs were: a placed child that was not recorded as one got
pruned as departed on the next draw.

`examples/text.rs` is new, since wrapping was the one thing here with no
way to see it on its own.

core/src/ui/active.rs

@@ -1,14 +1,18 @@
-use crate::{LayerId, MaskIdx, PrimitiveHandle, TextureHandle, UiRegion, WidgetId};
+use crate::{LayerId, MaskIdx, PrimitiveHandle, Size, TextureHandle, UiRegion, WidgetId};
 
 /// important non rendering data for retained drawing
 #[derive(Debug)]
 pub struct ActiveData {
     pub id: WidgetId,
     pub region: UiRegion,
+    /// What the widget said it used of `region`, the last time it drew.
+    pub size: Size,
     pub parent: Option<WidgetId>,
     pub textures: Vec<TextureHandle>,
     pub primitives: Vec<PrimitiveHandle>,
     pub children: Vec<WidgetId>,
+    /// The children whose size this widget read while drawing.
+    pub size_deps: Vec<WidgetId>,
     pub mask: MaskIdx,
     pub layer: LayerId,
 }

core/src/ui/cache.rs

@@ -1,18 +0,0 @@
-use crate::{BothAxis, Len, UiVec2, WidgetId, util::HashMap};
-
-#[derive(Default)]
-pub struct Cache {
-    pub size: BothAxis<HashMap<WidgetId, (UiVec2, Len)>>,
-}
-
-impl Cache {
-    pub fn remove(&mut self, id: WidgetId) {
-        self.size.x.remove(&id);
-        self.size.y.remove(&id);
-    }
-
-    pub fn clear(&mut self) {
-        self.size.x.clear();
-        self.size.y.clear();
-    }
-}

core/src/ui/mod.rs

@@ -3,15 +3,12 @@ use crate::{
 };
 
 mod active;
-mod cache;
 mod painter;
 mod render_state;
-mod size;
 
 pub use active::*;
 pub use painter::{Painter, PrimitiveLike};
 pub use render_state::*;
-pub use size::*;
 
 #[derive(Default)]
 pub struct UiData {

core/src/ui/painter.rs

@@ -1,6 +1,6 @@
 use crate::{
-    Axis, Len, RenderedText, Size, SizeCtx, StrongWidget, TextAttrs, TextBuffer, TextData,
-    TextureHandle, UiRegion, UiRenderState, UiRsc, UiScalar, UiVec2, Widget, WidgetId,
+    Axis, Len, RenderedText, Size, StrongWidget, TextAttrs, TextBuffer, TextData, TextureHandle,
+    UiRegion, UiRenderState, UiRsc, UiScalar, UiVec2, WidgetId,
     render::{
         GlyphPrimitive, Mask, MaskIdx, Primitive, PrimitiveHandle, PrimitiveInst, PrimitiveKind,
         TexturePrimitive,
@@ -18,6 +18,9 @@ pub struct Painter<'a> {
     pub(super) textures: Vec<TextureHandle>,
     pub(super) primitives: Vec<PrimitiveHandle>,
     pub(super) children: Vec<WidgetId>,
+    /// The children whose size this widget read while drawing.
+    pub(super) size_deps: Vec<WidgetId>,
+    pub(super) size: Option<Size>,
     pub layer: usize,
     pub(super) id: WidgetId,
 }
@@ -67,20 +70,40 @@ impl<'a> Painter<'a> {
         self.mask = self.rsc.ui_mut().masks.push(Mask { region });
     }
 
+    /// States what this widget uses of the box it was handed, once per draw.
+    /// This is the size a parent reads.
+    pub fn set_size(&mut self, size: impl Into<Size>) {
+        debug_assert!(
+            self.size.is_none(),
+            "{} set its size twice in one draw",
+            self.label(),
+        );
+        self.size = Some(size.into());
+    }
+
     /// Draws a widget within this widget's region.
-    pub fn widget<W: ?Sized>(&mut self, id: &StrongWidget<W>) {
-        self.widget_at(id, self.region);
+    pub fn widget<W: ?Sized>(&mut self, id: &StrongWidget<W>) -> DrawResult<'_, 'a> {
+        self.widget_at(id, self.region)
     }
 
     /// Draws a widget somewhere within this one.
     /// Useful for drawing child widgets in select areas.
-    pub fn widget_within<W: ?Sized>(&mut self, id: &StrongWidget<W>, region: UiRegion) {
-        self.widget_at(id, region.within(&self.region));
+    pub fn widget_within<W: ?Sized>(
+        &mut self,
+        id: &StrongWidget<W>,
+        region: UiRegion,
+    ) -> DrawResult<'_, 'a> {
+        let region = region.within(&self.region);
+        self.widget_at(id, region)
     }
 
-    fn widget_at<W: ?Sized>(&mut self, id: &StrongWidget<W>, region: UiRegion) {
+    fn widget_at<W: ?Sized>(
+        &mut self,
+        id: &StrongWidget<W>,
+        region: UiRegion,
+    ) -> DrawResult<'_, 'a> {
         self.children.push(id.id());
-        self.state.draw_inner(
+        let size = self.state.draw_inner(
             self.layer,
             id.id(),
             region,
@@ -89,6 +112,44 @@ impl<'a> Painter<'a> {
             None,
             self.rsc,
         );
+        DrawResult {
+            child: id.id(),
+            painter: self,
+            size,
+        }
+    }
+
+    /// What a child says its length is without being drawn, if it can say.
+    pub fn size_hint<W: ?Sized>(&self, id: &StrongWidget<W>, axis: Axis) -> Option<Len> {
+        self.rsc.widgets().get_dyn(id.id())?.size_hint(axis)
+    }
+
+    /// Gives a child its final box once this widget knows what that is,
+    /// keeping the drawing it already has where it can.
+    pub fn place<W: ?Sized>(&mut self, id: &StrongWidget<W>, region: UiRegion) -> Size {
+        let region = region.within(&self.region);
+        let id = id.id();
+        // Choosing a child's box depends on its size, from a draw or a hint.
+        // Claiming it is separate: one left out of `children` is removed.
+        if !self.size_deps.contains(&id) {
+            self.size_deps.push(id);
+        }
+        if !self.children.contains(&id) {
+            self.children.push(id);
+        }
+        if self.state.active.contains_key(&id) {
+            return self.state.place(id, region, self.rsc);
+        }
+        // Not drawn yet: its length came from a hint, so this is its one draw.
+        self.state.draw_inner(
+            self.layer,
+            id,
+            region,
+            Some(self.id),
+            self.mask,
+            None,
+            self.rsc,
+        )
     }
 
     pub fn render_text(
@@ -129,17 +190,6 @@ impl<'a> Painter<'a> {
         self.region
     }
 
-    pub fn size<W: ?Sized + Widget>(&mut self, id: &StrongWidget<W>) -> Size {
-        self.size_ctx().size(id)
-    }
-
-    pub fn len_axis<W: ?Sized + Widget>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Len {
-        match axis {
-            Axis::X => self.size_ctx().width(id),
-            Axis::Y => self.size_ctx().height(id),
-        }
-    }
-
     pub fn output_size(&self) -> Vec2 {
         self.state.output_size
     }
@@ -167,9 +217,25 @@ impl<'a> Painter<'a> {
     pub fn id(&self) -> &WidgetId {
         &self.id
     }
+}
 
-    pub fn size_ctx(&mut self) -> SizeCtx<'_> {
-        self.state.size_ctx(self.id, self.region.size(), self.rsc)
+/// A child that has just been drawn. Reading its size records that this
+/// widget's own size depends on it; dropping it without reading draws the
+/// child and leaves the parent independent of what it came to.
+pub struct DrawResult<'p, 'a> {
+    painter: &'p mut Painter<'a>,
+    child: WidgetId,
+    size: Size,
+}
+
+impl DrawResult<'_, '_> {
+    pub fn size(self) -> Size {
+        self.painter.size_deps.push(self.child);
+        self.size
+    }
+
+    pub fn len(self, axis: Axis) -> Len {
+        self.size().axis(axis)
     }
 }
 

core/src/ui/render_state.rs

@@ -1,7 +1,6 @@
 use crate::{
-    ActiveData, Axis, DrawLayers, IdLike, MaskIdx, Painter, PixelRegion, SizeCtx, StrongWidget,
-    UiRegion, UiRsc, UiVec2, WidgetId, Widgets,
-    ui::cache::Cache,
+    ActiveData, Axis, DrawLayers, IdLike, MaskIdx, Painter, PixelRegion, Size, SizeDependence,
+    StrongWidget, UiRegion, UiRsc, WidgetId, Widgets,
     util::{HashMap, HashSet, Vec2, forget_ref},
 };
 
@@ -9,7 +8,6 @@ pub struct UiRenderState {
     pub active: HashMap<WidgetId, ActiveData>,
     pub layers: DrawLayers,
     pub(super) output_size: Vec2,
-    pub cache: Cache,
 
     old_root: Option<WidgetId>,
     resized: bool,
@@ -21,7 +19,6 @@ impl UiRenderState {
         Self {
             active: Default::default(),
             layers: Default::default(),
-            cache: Default::default(),
             output_size: Vec2::ZERO,
             old_root: None,
             resized: false,
@@ -84,19 +81,11 @@ impl UiRenderState {
         mask: MaskIdx,
         old_children: Option<Vec<WidgetId>>,
         rsc: &mut dyn UiRsc,
-    ) {
+    ) -> Size {
         let mut old_children = old_children.unwrap_or_default();
-        if let Some(active) = self.active.get_mut(&id)
-            && !rsc.widgets().needs_redraw.contains(&id)
-        {
-            // check to see if we can skip drawing first
-            if active.region == region {
-                return;
-            } else if active.region.size() == region.size() {
-                // TODO: epsilon?
-                let from = active.region;
-                self.mov(id, from, region);
-                return;
+        if self.active.contains_key(&id) {
+            if let Some(size) = self.try_reuse(id, region, rsc) {
+                return size;
             }
             // if not, then maintain resize and track old children to remove unneeded
             let active = self.remove(id, false, rsc).unwrap();
@@ -115,6 +104,8 @@ impl UiRenderState {
             textures: Vec::new(),
             primitives: Vec::new(),
             children: Vec::new(),
+            size_deps: Vec::new(),
+            size: None,
             rsc,
         };
 
@@ -130,18 +121,34 @@ impl UiRenderState {
             textures,
             primitives,
             children,
+            size_deps,
+            size,
             layer,
             id,
         } = painter;
 
+        let size = size.unwrap_or_else(|| {
+            panic!(
+                "'{}' ({id:?}) drew without a size; every widget calls Painter::set_size",
+                rsc.widgets().label(id)
+            )
+        });
+        debug_assert!(
+            Self::hints_agree(id, size, rsc),
+            "'{}' ({id:?}) drew a size its size_hint disagrees with",
+            rsc.widgets().label(id)
+        );
+
         // add to active
         let active = ActiveData {
             id,
             region,
+            size,
             parent,
             textures,
             primitives,
             children,
+            size_deps,
             mask,
             layer,
         };
@@ -155,6 +162,86 @@ impl UiRenderState {
 
         rsc.on_draw(&active);
         self.active.insert(id, active);
+        size
+    }
+
+    /// Gives an already drawn widget a new box, keeping its drawing if it can
+    /// and drawing it again if it cannot.
+    pub(super) fn place(&mut self, id: WidgetId, region: UiRegion, rsc: &mut dyn UiRsc) -> Size {
+        if let Some(size) = self.try_reuse(id, region, rsc) {
+            return size;
+        }
+        let Some(active) = self.remove(id, false, rsc) else {
+            return Size::ZERO;
+        };
+        self.draw_inner(
+            active.layer,
+            id,
+            region,
+            active.parent,
+            active.mask,
+            Some(active.children),
+            rsc,
+        )
+    }
+
+    /// The drawing a widget already has, kept for a new box if the box has not
+    /// changed in a way it depends on.
+    fn try_reuse(&mut self, id: WidgetId, region: UiRegion, rsc: &dyn UiRsc) -> Option<Size> {
+        if rsc.widgets().needs_redraw.contains(&id) {
+            return None;
+        }
+        let active = self.active.get(&id)?;
+        let (size, was) = (active.size, active.region);
+        if was == region {
+            return Some(size);
+        }
+        // TODO: epsilon?
+        if was.size() == region.size() {
+            self.mov(id, was, region);
+            return Some(size);
+        }
+        if self.reusable(id, region, rsc) {
+            // Its drawing stands; the box is written into the primitives.
+            self.mov(id, was, region);
+            return Some(size);
+        }
+        None
+    }
+
+    /// Whether the widget can keep the drawing it has and be given `region`
+    /// instead, asked one axis at a time: a change on an axis it does not
+    /// depend on costs nothing, whatever it depends on elsewhere.
+    fn reusable(&self, id: WidgetId, mut region: UiRegion, rsc: &dyn UiRsc) -> bool {
+        let Some(active) = self.active.get(&id) else {
+            return false;
+        };
+        let Some(widget) = rsc.widgets().get_dyn(id) else {
+            return false;
+        };
+        let mut was = active.region;
+        [Axis::X, Axis::Y].into_iter().all(|axis| {
+            let offered = region.axis_mut(axis).len();
+            let had = was.axis_mut(axis).len();
+            match widget.size_dependence(axis) {
+                SizeDependence::None => true,
+                // `Internal` could also keep its drawing when only the room
+                // around it changed, but that is a translation rather than a
+                // remap, so it waits for the move chain.
+                SizeDependence::Internal | SizeDependence::External => offered == had,
+            }
+        })
+    }
+
+    fn hints_agree(id: WidgetId, size: Size, rsc: &dyn UiRsc) -> bool {
+        let Some(widget) = rsc.widgets().get_dyn(id) else {
+            return true;
+        };
+        [Axis::X, Axis::Y].into_iter().all(|axis| {
+            widget
+                .size_hint(axis)
+                .is_none_or(|hint| hint == size.axis(axis))
+        })
     }
 
     fn mov(&mut self, id: WidgetId, from: UiRegion, to: UiRegion) {
@@ -191,7 +278,6 @@ impl UiRenderState {
     }
 
     fn remove_rec(&mut self, id: WidgetId, rsc: &mut dyn UiRsc) -> Option<ActiveData> {
-        self.cache.remove(id);
         let inst = self.remove(id, true, rsc);
         if let Some(inst) = &inst {
             for c in &inst.children {
@@ -205,7 +291,6 @@ impl UiRenderState {
         for (_, active) in self.active.drain() {
             rsc.on_undraw(&active);
         }
-        self.cache.clear();
         self.layers.clear();
         rsc.widgets_mut().needs_redraw.clear();
         rsc.free();
@@ -267,19 +352,11 @@ impl UiRenderState {
     pub fn redraw(&mut self, id: WidgetId, rsc: &mut dyn UiRsc) {
         rsc.widgets_mut().needs_redraw.remove(&id);
         self.draw_started.remove(&id);
-        // check if parent depends on the desired size of this, if so then redraw it first
-        for axis in [Axis::X, Axis::Y] {
-            if let Some(&(outer, old)) = self.cache.size.axis_dyn(axis).get(&id)
-                && let Some(current) = self.active.get(&id)
-                && let Some(pid) = current.parent
-            {
-                self.cache.size.axis_dyn(axis).remove(&id);
-                let new = self.size_ctx(id, outer, rsc).len_axis(id, axis);
-                self.cache.size.axis_dyn(axis).insert(id, (outer, new));
-                if new != old {
-                    self.redraw(pid, rsc);
-                }
-            }
+        // Whoever read this widget's size may be a different size now, so the
+        // highest reader is what draws; it reaches this one on the way down.
+        if let Some(top) = self.highest_reader(id) {
+            self.redraw(top, rsc);
+            return;
         }
 
         if self.draw_started.contains(&id) {
@@ -301,22 +378,22 @@ impl UiRenderState {
         );
     }
 
-    pub(super) fn size_ctx<'b>(
-        &'b mut self,
-        source: WidgetId,
-        outer: UiVec2,
-        rsc: &'b mut dyn UiRsc,
-    ) -> SizeCtx<'b> {
-        let ui = rsc.ui_mut();
-        SizeCtx {
-            source,
-            cache: &mut self.cache,
-            text: &mut ui.text,
-            widgets: &ui.widgets,
-            outer,
-            output_size: self.output_size,
-            id: source,
+    /// The furthest ancestor that read this widget's size, directly or through
+    /// widgets that did the same.
+    fn highest_reader(&self, id: WidgetId) -> Option<WidgetId> {
+        let mut top = None;
+        let mut at = id;
+        while let Some(active) = self.active.get(&at)
+            && let Some(parent) = active.parent
+            && self
+                .active
+                .get(&parent)
+                .is_some_and(|p| p.size_deps.contains(&at))
+        {
+            top = Some(parent);
+            at = parent;
         }
+        top
     }
 }
 

core/src/ui/size.rs

@@ -1,89 +0,0 @@
-use crate::{
-    Axis, AxisT, IdLike, Len, RenderedText, Size, TextAttrs, TextBuffer, TextData, UiVec2,
-    WidgetAxisFns, WidgetId, Widgets, XAxis, YAxis, ui::cache::Cache, util::Vec2,
-};
-
-pub struct SizeCtx<'a> {
-    pub text: &'a mut TextData,
-    pub(super) source: WidgetId,
-    pub(super) widgets: &'a Widgets,
-    pub(super) cache: &'a mut Cache,
-    /// TODO: should this be pub? rn used for sized
-    pub outer: UiVec2,
-    pub(super) output_size: Vec2,
-    pub(super) id: WidgetId,
-}
-
-impl SizeCtx<'_> {
-    pub fn id(&self) -> &WidgetId {
-        &self.id
-    }
-
-    pub fn source(&self) -> &WidgetId {
-        &self.source
-    }
-
-    pub(super) fn len_inner<A: const AxisT>(&mut self, id: WidgetId) -> Len {
-        if let Some((_, len)) = self.cache.size.axis::<A>().get(&id) {
-            return *len;
-        }
-        let len = self
-            .widgets
-            .get_dyn_dynamic(id)
-            .desired_len::<A>(&mut SizeCtx {
-                text: self.text,
-                source: self.source,
-                widgets: self.widgets,
-                cache: self.cache,
-                outer: self.outer,
-                output_size: self.output_size,
-                id,
-            });
-        self.cache.size.axis::<A>().insert(id, (self.outer, len));
-        len
-    }
-
-    pub fn width(&mut self, id: impl IdLike) -> Len {
-        self.len_inner::<XAxis>(id.id())
-    }
-
-    pub fn height(&mut self, id: impl IdLike) -> Len {
-        self.len_inner::<YAxis>(id.id())
-    }
-
-    pub fn len_axis(&mut self, id: impl IdLike, axis: Axis) -> Len {
-        match axis {
-            Axis::X => self.width(id),
-            Axis::Y => self.height(id),
-        }
-    }
-
-    pub fn size(&mut self, id: impl IdLike) -> Size {
-        let id = id.id();
-        Size {
-            x: self.width(id),
-            y: self.height(id),
-        }
-    }
-
-    pub fn px_size(&mut self) -> Vec2 {
-        self.outer.to_abs(self.output_size)
-    }
-
-    pub fn output_size(&mut self) -> Vec2 {
-        self.output_size
-    }
-
-    pub fn draw_text(
-        &mut self,
-        buffer: &mut TextBuffer,
-        attrs: &TextAttrs,
-        width: Option<f32>,
-    ) -> RenderedText {
-        self.text.render(buffer, attrs, width)
-    }
-
-    pub fn label(&self, id: WidgetId) -> &String {
-        self.widgets.label(id)
-    }
-}

core/src/widget/mod.rs

@@ -1,4 +1,4 @@
-use crate::{Axis, AxisT, Len, Painter, SizeCtx};
+use crate::{Axis, Len, Painter, Size};
 use std::any::Any;
 
 mod data;
@@ -15,32 +15,49 @@ pub use tag::*;
 pub use view::*;
 pub use widgets::*;
 
+/// How much of the box a widget was handed its drawing depends on, and so
+/// what has to change before it must be drawn again. Asked per axis, because
+/// wrapped text depends on the width it is offered and not on the height.
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+pub enum SizeDependence {
+    /// None of it: the box only says where the primitives go, so a new one is
+    /// written into them instead of drawn.
+    None,
+    /// Its own extent, whatever box that sits in. Reusable in any box that
+    /// leaves that extent unchanged, including a larger one it does not fill.
+    #[default]
+    Internal,
+    /// The extent of the box itself, used or not.
+    External,
+}
+
 pub trait Widget: Any {
+    /// Draws the widget, and states what it used with `Painter::set_size`.
     fn draw(&mut self, painter: &mut Painter);
-    fn desired_width(&mut self, ctx: &mut SizeCtx) -> Len;
-    fn desired_height(&mut self, ctx: &mut SizeCtx) -> Len;
-}
 
-pub trait WidgetAxisFns {
-    fn desired_len<A: AxisT>(&mut self, ctx: &mut SizeCtx) -> Len;
-}
+    /// An exact length the widget can give without a painter or its children.
+    /// Optional, and saves a draw rather than changing one: a hint that
+    /// disagrees with the eventual draw fails a debug assertion.
+    fn size_hint(&self, _axis: Axis) -> Option<Len> {
+        None
+    }
 
-impl<W: Widget + ?Sized> WidgetAxisFns for W {
-    fn desired_len<A: AxisT>(&mut self, ctx: &mut SizeCtx) -> Len {
-        match A::get() {
-            Axis::X => self.desired_width(ctx),
-            Axis::Y => self.desired_height(ctx),
-        }
+    fn size_dependence(&self, _axis: Axis) -> SizeDependence {
+        SizeDependence::Internal
     }
 }
 
 impl Widget for () {
-    fn draw(&mut self, _: &mut Painter) {}
-    fn desired_width(&mut self, _: &mut SizeCtx) -> Len {
-        Len::ZERO
+    fn draw(&mut self, painter: &mut Painter) {
+        painter.set_size(Size::ZERO);
     }
-    fn desired_height(&mut self, _: &mut SizeCtx) -> Len {
-        Len::ZERO
+
+    fn size_hint(&self, _axis: Axis) -> Option<Len> {
+        Some(Len::ZERO)
+    }
+
+    fn size_dependence(&self, _axis: Axis) -> SizeDependence {
+        SizeDependence::None
     }
 }