Retain layout sizes by pixel axis

core/src/ui/painter.rs

@@ -23,7 +23,10 @@ pub struct Painter<'a> {
     pub(super) children: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub(super) size_deps: Vec<WidgetId>,
-    pub(super) reads_output: bool,
+    /// Offered pixel axes which can affect the size this draw reports.
+    pub(super) size_box_inputs: [bool; 2],
+    pub(super) size_output_inputs: [bool; 2],
+    pub(super) reads_output: [bool; 2],
     /// The slot this widget's primitives are positioned through: its own if
     /// its parent placed it, otherwise the nearest ancestor that has one.
     pub(super) move_idx: MoveIdx,
@@ -158,7 +161,7 @@ impl<'a> Painter<'a> {
             Some(hint) => {
                 #[cfg(feature = "layout-diagnostics")]
                 diag::bump(Counter::HintHits);
-                self.depend_on_size(id);
+                self.depend_on_size(id, false);
                 Some(hint)
             }
             None => {
@@ -169,10 +172,66 @@ impl<'a> Painter<'a> {
         }
     }
 
-    fn depend_on_size<W: ?Sized>(&mut self, child: &StrongWidget<W>) {
+    /// A retained child length valid under the region it is about to be
+    /// offered. Unlike a hint, this is contextual: it is kept only when none
+    /// of the offered pixel axes which produced it changed.
+    pub fn known_len<W: ?Sized>(
+        &mut self,
+        child: &StrongWidget<W>,
+        axis: Axis,
+        region: UiRegion,
+    ) -> Option<Len> {
+        if let Some(hint) = self.size_hint(child, axis) {
+            return Some(hint);
+        }
+        self.retained_size(child, region)
+            .map(|size| size.axis(axis))
+    }
+
+    fn retained_size<W: ?Sized>(
+        &mut self,
+        child: &StrongWidget<W>,
+        region: UiRegion,
+    ) -> Option<Size> {
+        let region = region.within(&self.region);
+        let (size, box_inputs, output_inputs) =
+            self.state
+                .retained_size(child.id(), region, self.move_idx, self.rsc.widgets())?;
+        #[cfg(feature = "layout-diagnostics")]
+        diag::bump(Counter::RetainedSizeHits);
+        self.depend_on_size_inputs(child, box_inputs, output_inputs);
+        Some(size)
+    }
+
+    fn depend_on_size<W: ?Sized>(&mut self, child: &StrongWidget<W>, inherit_inputs: bool) {
+        let (box_inputs, output_inputs) = match inherit_inputs {
+            true => self
+                .state
+                .active
+                .get(&child.id())
+                .map_or(([false; 2], [false; 2]), |active| {
+                    (active.size_box_inputs, active.size_output_inputs)
+                }),
+            false => ([false; 2], [false; 2]),
+        };
+        self.depend_on_size_inputs(child, box_inputs, output_inputs);
+    }
+
+    fn depend_on_size_inputs<W: ?Sized>(
+        &mut self,
+        child: &StrongWidget<W>,
+        box_inputs: [bool; 2],
+        output_inputs: [bool; 2],
+    ) {
         if !self.size_deps.contains(&child.id()) {
             self.size_deps.push(child.id());
         }
+        for (own, child) in self.size_box_inputs.iter_mut().zip(box_inputs) {
+            *own |= child;
+        }
+        for (own, child) in self.size_output_inputs.iter_mut().zip(output_inputs) {
+            *own |= child;
+        }
     }
 
     pub fn render_text(
@@ -220,19 +279,41 @@ impl<'a> Painter<'a> {
     /// The output's size in pixels. A widget that reads it draws again when
     /// the output changes, since nothing else can put that right.
     pub fn output_size(&mut self) -> Vec2 {
-        self.reads_output = true;
+        self.reads_output = [true; 2];
+        self.size_output_inputs = [true; 2];
         self.state.output_size
     }
 
+    /// One axis of the output in pixels. Prefer this to [`Self::output_size`]
+    /// when the other axis cannot affect the size this widget reports.
+    pub fn output_len(&mut self, axis: Axis) -> f32 {
+        self.reads_output[axis as usize] = true;
+        self.size_output_inputs[axis as usize] = true;
+        self.state.output_size.axis(axis)
+    }
+
     /// This widget's box in pixels. Resolved against the output's size and
     /// the boxes it sits within, so a widget that reads it draws again when
     /// the output changes.
     pub fn px_size(&mut self) -> Vec2 {
-        self.reads_output = true;
+        self.reads_output = [true; 2];
+        self.size_box_inputs = [true; 2];
         let region = self.state.moves.resolve(self.move_idx, self.region);
         region.size().to_abs(self.state.output_size)
     }
 
+    /// One axis of this widget's box in pixels. Prefer this to
+    /// [`Self::px_size`] when the other axis cannot affect the reported size.
+    pub fn px_len(&mut self, axis: Axis) -> f32 {
+        self.reads_output[axis as usize] = true;
+        self.size_box_inputs[axis as usize] = true;
+        let region = self.state.moves.resolve(self.move_idx, self.region);
+        region
+            .size()
+            .axis(axis)
+            .to_abs(self.state.output_size.axis(axis))
+    }
+
     pub fn text_data(&mut self) -> &mut TextData {
         &mut self.rsc.ui_mut().text
     }
@@ -270,7 +351,7 @@ impl<W: ?Sized> DrawResult<'_, '_, W> {
             diag::bump(Counter::SizeReads);
             diag::size_read(self.child.id(), self.painter.id, self.size);
         }
-        self.painter.depend_on_size(self.child);
+        self.painter.depend_on_size(self.child, true);
         self.size
     }