Make alignment a widget property

core/src/widget/widgets.rs

@@ -1,7 +1,8 @@
 use std::sync::mpsc::{Receiver, Sender, channel};
 
 use crate::{
-    Axis, IdLike, SizeRule, SizeRules, StrongWidget, WeakWidget, Widget, WidgetData, WidgetId,
+    Axis, AxisAlign, IdLike, RegionAlign, SizeRule, SizeRules, StrongWidget, WeakWidget, Widget,
+    WidgetData, WidgetId,
     util::{DynBorrower, HashSet, SlotVec, forget_mut, to_mut},
 };
 
@@ -136,6 +137,23 @@ impl Widgets {
         self.needs_redraw.insert(id);
     }
 
+    /// Where this widget sits in a box longer than the length it takes.
+    pub fn alignment(&self, id: impl IdLike) -> RegionAlign {
+        self.data(id).unwrap().align
+    }
+
+    /// Sets one axis's alignment. Which box a widget ends up in is its
+    /// parent's to decide, so this is escalated the way a length rule is.
+    pub fn set_alignment(&mut self, id: impl IdLike, axis: Axis, align: AxisAlign) {
+        let id = id.id();
+        let data = self.data_mut(id).unwrap();
+        if *data.align.axis_mut(axis) == align {
+            return;
+        }
+        *data.align.axis_mut(axis) = align;
+        self.needs_redraw.insert(id);
+    }
+
     /// Both axes at once, for a caller holding a pair.
     pub fn set_size_rules(
         &mut self,