1 files changed, 69 insertions, 11 deletions

diff --git a/src/layout/frame.rs b/src/layout/frame.rs
index 5e5bcfe8..b6cf4645 100644
--- a/src/layout/frame.rs
+++ b/src/layout/frame.rs
@@ -1,3 +1,5 @@
+use std::rc::Rc;
+
 use serde::{Deserialize, Serialize};
 
 use super::{Constrained, Constraints};
@@ -14,37 +16,93 @@ pub struct Frame {
     /// The baseline of the frame measured from the top.
     pub baseline: Length,
     /// The elements composing this layout.
-    pub elements: Vec<(Point, Element)>,
+    children: Vec<(Point, Child)>,
+}
+
+/// An iterator over all elements in a frame, alongside with their positions.
+#[derive(Debug, Clone)]
+pub struct ElementIter<'a> {
+    stack: Vec<(usize, Point, &'a Frame)>,
+}
+
+impl<'a> Iterator for ElementIter<'a> {
+    type Item = (Point, &'a Element);
+
+    /// Get the next element, if any.
+    fn next(&mut self) -> Option<Self::Item> {
+        let (cursor, offset, frame) = self.stack.last_mut()?;
+        match frame.children.get(*cursor) {
+            Some((pos, Child::Frame(f))) => {
+                let new_offset = *offset + *pos;
+                self.stack.push((0, new_offset, f.as_ref()));
+                self.next()
+            }
+            Some((pos, Child::Element(e))) => {
+                *cursor += 1;
+                Some((*offset + *pos, e))
+            }
+            None => {
+                self.stack.pop();
+                if let Some((cursor, _, _)) = self.stack.last_mut() {
+                    *cursor += 1;
+                }
+                self.next()
+            }
+        }
+    }
 }
 
 impl Frame {
     /// Create a new, empty frame.
     pub fn new(size: Size, baseline: Length) -> Self {
         assert!(size.is_finite());
-        Self { size, baseline, elements: vec![] }
+        Self { size, baseline, children: vec![] }
     }
 
-    /// Add an element at a position.
+    /// Add an element at a position in the foreground.
     pub fn push(&mut self, pos: Point, element: Element) {
-        self.elements.push((pos, element));
+        self.children.push((pos, Child::Element(element)));
+    }
+
+    /// Add an element at a position in the background.
+    pub fn prepend(&mut self, pos: Point, element: Element) {
+        self.children.insert(0, (pos, Child::Element(element)))
+    }
+
+    /// Add a frame element.
+    pub fn push_frame(&mut self, pos: Point, subframe: Rc<Self>) {
+        self.children.push((pos, Child::Frame(subframe)))
     }
 
     /// Add all elements of another frame, placing them relative to the given
     /// position.
-    pub fn push_frame(&mut self, pos: Point, subframe: Self) {
-        if pos == Point::zero() && self.elements.is_empty() {
-            self.elements = subframe.elements;
+    pub fn merge_frame(&mut self, pos: Point, subframe: Self) {
+        if pos == Point::zero() && self.children.is_empty() {
+            self.children = subframe.children;
         } else {
-            for (subpos, element) in subframe.elements {
-                self.push(pos + subpos, element);
+            for (subpos, child) in subframe.children {
+                self.children.push((pos + subpos, child));
             }
         }
     }
 
     /// Wraps the frame with constraints.
-    pub fn constrain(self, constraints: Constraints) -> Constrained<Self> {
-        Constrained { item: self, constraints }
+    pub fn constrain(self, constraints: Constraints) -> Constrained<Rc<Self>> {
+        Constrained { item: Rc::new(self), constraints }
     }
+
+    /// Returns an iterator over all elements in the frame and its children.
+    pub fn elements(&self) -> ElementIter {
+        ElementIter { stack: vec![(0, Point::zero(), self)] }
+    }
+}
+
+/// A frame can contain multiple children: elements or other frames, complete
+/// with their children.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+enum Child {
+    Element(Element),
+    Frame(Rc<Frame>),
 }
 
 /// The building block frames are composed of.