Measure container children without intermediate placement

core/src/ui/painter.rs

@@ -139,7 +139,7 @@ impl<'a> Painter<'a> {
     /// around one child wants, since its box is the child's.
     pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
         let own = self.placement;
-        self.widget_at_inner(id, UiRegion::FULL, [Some(own.x), Some(own.y)], true)
+        self.widget_at_inner(id, UiRegion::FULL, [Some(own.x), Some(own.y)], true, false)
     }
 
     /// What a widget's rules declare its lengths to be, which whoever draws
@@ -191,7 +191,7 @@ impl<'a> Painter<'a> {
         region: UiRegion,
         placement: [Option<UiSpan>; 2],
     ) -> DrawResult<'s, 'a, W> {
-        self.widget_at_inner(id, region, placement, false)
+        self.widget_at_inner(id, region, placement, false, false)
     }
 
     fn widget_at_inner<'s, W: ?Sized>(
@@ -200,6 +200,7 @@ impl<'a> Painter<'a> {
         region: UiRegion,
         placement: [Option<UiSpan>; 2],
         inherited: bool,
+        measuring: bool,
     ) -> DrawResult<'s, 'a, W> {
         if inherited {
             if !self.inherited_children.contains(&id.id()) {
@@ -266,6 +267,7 @@ impl<'a> Painter<'a> {
                 placement,
             },
             None,
+            measuring,
             self.rsc,
         );
         // Whatever the child's answer holds for keeps this one to the boxes
@@ -321,32 +323,34 @@ impl<'a> Painter<'a> {
         }
     }
 
-    /// A child's length in the region it is about to be offered, if it can
-    /// be had without drawing it: from its hint, or from a drawing it already
-    /// has that holds for that box.
-    pub fn known_len<W: ?Sized>(
+    /// Measures a child's length from its hint, a retained answer, or `draw`.
+    /// A fresh draw evaluates the offer without placing its answer. The caller
+    /// must later place or undraw the child.
+    pub fn measure_len<W: ?Sized>(
         &mut self,
         child: &StrongWidget<W>,
         axis: Axis,
         region: UiRegion,
         placement: [Option<UiSpan>; 2],
-    ) -> Option<LayoutLen> {
+    ) -> LayoutLen {
+        let offered = placement;
         let declared = self.declared_lens(child);
         let align = self.rsc.widgets().alignment(child.id());
         let (local, placement) = ask_box(region, declared, align, placement);
         let first_ask = self.at_offer && !self.offered.contains(&child.id());
 
         if let Some(hint) = self.size_hint(child, axis) {
-            return Some(hint);
+            return hint;
         }
         let px = local.size().to_px(self.px);
-        let (size, holds) = self.state.retained_size(
-            child.id(),
-            px,
-            placement,
-            self.move_idx,
-            self.rsc.widgets(),
-        )?;
+        let retained =
+            self.state
+                .retained_size(child.id(), px, placement, self.move_idx, self.rsc.widgets());
+        let Some((size, holds)) = retained else {
+            return self
+                .widget_at_inner(child, region, offered, false, true)
+                .len(axis);
+        };
         #[cfg(feature = "layout-diagnostics")]
         diag::bump(Counter::RetainedSizeHits);
         self.depend_on(child);
@@ -364,7 +368,7 @@ impl<'a> Painter<'a> {
         for (axis, under) in AXES.into_iter().zip(self.under.iter_mut()) {
             *under = under.and(holds[axis as usize].through(local.axis(axis).len()));
         }
-        Some(in_parent_frame(size, local.size(), declared).axis(axis))
+        in_parent_frame(size, local.size(), declared).axis(axis)
     }
 
     /// Whether this is the first box a child is asked about in during a draw

core/src/ui/render_state.rs

@@ -193,7 +193,7 @@ impl UiRenderState {
         if let Some(id) = root {
             let region = Self::root_region(id.id(), rsc.widgets());
             let info = self.root_info(region);
-            self.draw_inner(id.id(), region, info, None, rsc);
+            self.draw_inner(id.id(), region, info, None, false, rsc);
         }
     }
 
@@ -213,6 +213,7 @@ impl UiRenderState {
         region: UiRegion,
         info: DrawInfo,
         mut old: Option<ActiveData>,
+        measuring: bool,
         rsc: &mut dyn UiRsc,
     ) -> (Size, LayoutHolds) {
         #[cfg(feature = "layout-diagnostics")]
@@ -239,16 +240,20 @@ impl UiRenderState {
             self.draw_at(id, region, info.offered_placement(), info, old.take(), rsc)
         });
 
-        let declared = declared_lens(rsc.widgets(), id);
         // Where the drawing goes, in the region's own coordinates: what the
         // parent chose, and on any axis it left open, what the answer took of
         // the region placed by the widget's alignment. The region itself does
         // not change, so nothing under it resolves a fraction a second time.
-        let lens = placed_lens(answer.0, declared, info.decided());
-        let own = placed_box(UiRegion::FULL, lens, align);
-        let placement = UiRegion {
-            x: info.placement[0].unwrap_or(own.x),
-            y: info.placement[1].unwrap_or(own.y),
+        let placement = if measuring {
+            info.offered_placement()
+        } else {
+            let declared = declared_lens(rsc.widgets(), id);
+            let lens = placed_lens(answer.0, declared, info.decided());
+            let own = placed_box(UiRegion::FULL, lens, align);
+            UiRegion {
+                x: info.placement[0].unwrap_or(own.x),
+                y: info.placement[1].unwrap_or(own.y),
+            }
         };
         self.place(id, region, placement, info, rsc);
 
@@ -1137,7 +1142,7 @@ impl UiRenderState {
             #[cfg(feature = "layout-diagnostics")]
             diag::bump(Counter::LocalRedraws);
             let old = self.remove(id, false, rsc);
-            self.draw_inner(id, region, info, old, rsc);
+            self.draw_inner(id, region, info, old, false, rsc);
             return true;
         };
         let (given_px, offered_px) = self.asked_px(id);
@@ -1179,9 +1184,9 @@ impl UiRenderState {
             placement: info.offer_placement,
             ..info
         };
-        let answer = self.draw_inner(id, given, offered, old, rsc);
+        let answer = self.draw_inner(id, given, offered, old, false, rsc);
         if info.placement != offered.placement {
-            self.draw_inner(id, given, info, None, rsc);
+            self.draw_inner(id, given, info, None, false, rsc);
         }
         if Some(answer) != was_answer {
             // Its parent chose its box knowing the old answer, so it lays out

src/widget/position/scroll.rs

@@ -17,10 +17,7 @@ impl Widget for Scroll {
         let whole = UiRegion::FULL;
         let own = painter.placement();
         let answer_len =
-            match painter.known_len(&self.inner, self.axis, whole, [Some(own.x), Some(own.y)]) {
-                Some(len) => len,
-                None => painter.widget(&self.inner).size().axis(self.axis),
-            };
+            painter.measure_len(&self.inner, self.axis, whole, [Some(own.x), Some(own.y)]);
         let content = answer_len.apply_leftover();
         self.container_len = container_len;
         self.content_len = content.to_px(container_len);

src/widget/position/span.rs

@@ -37,10 +37,7 @@ impl Widget for Span {
             // from the cursor, because a text has to wrap at the width
             // actually there.
             let room = axis.pair(Some(along(cursor, far)), None);
-            let len = match painter.known_len(child, axis, region, room) {
-                Some(len) => len,
-                None => painter.widget_at(child, region, room).len(axis),
-            };
+            let len = painter.measure_len(child, axis, region, room);
             cursor.px += len.px + self.gap;
             cursor.rel += len.rel;
             lens.push(len);

tests/cases/retained.rs

@@ -914,3 +914,40 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
         assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
     }
 }
+
+#[test]
+fn a_span_does_not_place_its_measurement_before_assigning_the_childs_slot() {
+    struct MeasuredBox(Rc<Cell<usize>>);
+
+    impl Widget for MeasuredBox {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.0.set(self.0.get() + 1);
+            painter.px_size();
+            painter.primitive(RectPrimitive::color(Color::BLUE));
+            Size::from((100, 50))
+        }
+    }
+
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = MeasuredBox(draws.clone()).add(&mut h.rsc);
+    h.set_root((leaf,).span(Dir::RIGHT).width(rel(1.0)).height(rel(1.0)));
+
+    assert_eq!(draws.get(), 3);
+    assert_corners!(h, leaf, (0, 75), (100, 125));
+    assert_eq!(
+        primitive_bounds(&h, leaf.id()),
+        vec![h.region(&leaf.id()).unwrap()]
+    );
+    h.frame();
+    assert_eq!(draws.get(), 3);
+
+    h.resize((600, 300));
+    h.frame();
+    assert_eq!(draws.get(), 6);
+    assert_corners!(h, leaf, (0, 125), (100, 175));
+    assert_eq!(
+        primitive_bounds(&h, leaf.id()),
+        vec![h.region(&leaf.id()).unwrap()]
+    );
+}

tests/layout_diagnostics.rs

@@ -5,11 +5,11 @@
 //!     cargo test --release --features layout-diagnostics \
 //!       --test layout_diagnostics -- --ignored --nocapture
 //!
-//! Uninstrumented hardware totals for one phase:
+//! Build the uninstrumented test with `cargo test --release --test
+//! layout_diagnostics --no-run`, then run the emitted executable directly:
 //!
-//!     IRIS_PHASE=resize IRIS_FRAMES=1000 perf stat \
-//!       -e cycles:u,instructions:u cargo test --release \
-//!       --test layout_diagnostics -- --ignored --nocapture
+//!     IRIS_PHASE=resize IRIS_FRAMES=10000 perf stat -r 7 \
+//!       -e cycles:u,instructions:u /path/to/layout_diagnostics --ignored --nocapture
 //!
 //! `IRIS_PHASE` is `cold`, `repaint`, `many`, `size`, `scroll`, `resize`, or
 //! `all`. `IRIS_SEED`, `IRIS_DEPTH`, and `IRIS_FRAMES` select the load, and
@@ -134,6 +134,9 @@ fn report(label: &str, mut elapsed: Vec<f64>, _harness: &Harness) {
     {
         let diagnostics = iris::core::layout_diagnostics::take();
         print!("{}", diagnostics.per_frame(frames));
+        for event in diagnostics.traces() {
+            println!("    {event:?}");
+        }
         for callsite in diagnostics.hot_text().iter().take(3) {
             let mut ancestry = Vec::new();
             let mut id = Some(callsite.id);