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.

tests/retained.rs

@@ -0,0 +1,137 @@
+//! What a second frame draws again, and what it keeps.
+
+use std::{cell::Cell, rc::Rc};
+
+use iris::harness::{Harness, assert_corners};
+use iris::prelude::*;
+
+/// A leaf that counts its draws and reports whatever size it is given, so a
+/// test can see what the retained path skipped.
+struct Counted {
+    draws: Rc<Cell<usize>>,
+    size: Size,
+    dependence: SizeDependence,
+}
+
+impl Widget for Counted {
+    fn draw(&mut self, painter: &mut Painter) {
+        self.draws.set(self.draws.get() + 1);
+        painter.set_size(self.size);
+    }
+
+    fn size_dependence(&self, _: Axis) -> SizeDependence {
+        self.dependence
+    }
+}
+
+struct Counts(Rc<Cell<usize>>);
+
+impl Counts {
+    fn get(&self) -> usize {
+        self.0.get()
+    }
+}
+
+fn counted(
+    h: &mut Harness,
+    size: Size,
+    dependence: SizeDependence,
+) -> (WeakWidget<Counted>, Counts) {
+    let draws = Rc::new(Cell::new(0));
+    let id = Counted {
+        draws: draws.clone(),
+        size,
+        dependence,
+    }
+    .add(&mut h.rsc);
+    (id, Counts(draws))
+}
+
+/// A fixed-width leaf beside one that takes the rest, so changing the first
+/// hands the second a different box without the output changing.
+fn pair(h: &mut Harness, rest: SizeDependence) -> (WeakWidget<Counted>, Counts, WidgetId) {
+    let (first, _) = counted(h, Size::from((100, 200)), SizeDependence::Internal);
+    let (second, draws) = counted(h, Size::REST, rest);
+    h.set_root((first, second).span(Dir::RIGHT));
+    (first, draws, second.id())
+}
+
+#[test]
+fn a_leaf_that_ignores_its_box_is_not_drawn_again_when_the_box_changes() {
+    let mut h = Harness::new((400, 200));
+    let (first, draws, second) = pair(&mut h, SizeDependence::None);
+    let settled = draws.get();
+    assert_corners!(h, second, (100, 0), (400, 200));
+
+    h.rsc[first].size = Size::from((150, 200));
+    h.frame();
+
+    assert_eq!(
+        draws.get(),
+        settled,
+        "its box is a field to write, not a reason to draw"
+    );
+    assert_corners!(h, second, (150, 0), (400, 200));
+}
+
+#[test]
+fn a_leaf_that_depends_on_its_box_is_drawn_again_when_the_box_changes() {
+    let mut h = Harness::new((400, 200));
+    let (first, draws, second) = pair(&mut h, SizeDependence::External);
+    let settled = draws.get();
+
+    h.rsc[first].size = Size::from((150, 200));
+    h.frame();
+
+    // Twice: once for the span to measure it, once for its real box. A child
+    // that can hint its length is spared the first, and a smaller number here
+    // means someone has made that cheaper rather than broken it.
+    assert_eq!(draws.get(), settled + 2);
+    assert_corners!(h, second, (150, 0), (400, 200));
+}
+
+#[test]
+fn a_span_child_that_declares_its_length_is_drawn_once() {
+    let mut h = Harness::new((400, 200));
+    let (told, told_draws) = counted(&mut h, Size::from((100, 200)), SizeDependence::Internal);
+    let (asked, asked_draws) = counted(&mut h, Size::from((100, 200)), SizeDependence::Internal);
+    // The span takes one child's length from its hint and has to draw the
+    // other to find out, so only the second is drawn before it is placed.
+    let hinted = told.width(100).add(&mut h.rsc);
+    h.set_root((hinted, asked).span(Dir::RIGHT));
+
+    assert_eq!(told_draws.get(), 1);
+    assert_eq!(
+        asked_draws.get(),
+        2,
+        "drawn to be measured, then again to be placed"
+    );
+}
+
+#[test]
+fn a_span_relays_out_when_a_child_it_measured_changes() {
+    let mut h = Harness::new((400, 200));
+    let (first, _, second) = pair(&mut h, SizeDependence::Internal);
+
+    h.rsc[first].size = Size::from((250, 200));
+    h.frame();
+
+    assert_corners!(h, first, (0, 0), (250, 200));
+    assert_corners!(h, second, (250, 0), (400, 200));
+}
+
+#[test]
+fn a_placed_child_survives_the_next_frame() {
+    let mut h = Harness::new((400, 200));
+    // Both children declare a length, so the span places them from their hints
+    // rather than drawing them to find out.
+    let top = rect(Color::RED).height(80).add(&mut h.rsc);
+    let bottom = rect(Color::BLUE).height(120).add(&mut h.rsc);
+    h.set_root((top, bottom).span(Dir::DOWN));
+
+    h.rsc.widgets_mut().get_dyn_mut(top.id());
+    h.frame();
+
+    assert_corners!(h, top, (0, 0), (400, 80));
+    assert_corners!(h, bottom, (0, 80), (400, 200));
+}