Layout API prototype, for discussion

sky/framework/node.dart

+library node;
+
+class Node {
+
+  // nodes always have an order greater than their ancestors'.
+  // there's no guarantee regarding order between siblings
+
+  int _order = 0;
+  int get order => _order;
+  void reorderChild(Node child) { // internal, do not call
+    assert(child._attached == _attached);
+    if (child._order <= _order) {
+      child._order = _order + 1;
+      child.reorderChildren();
+    }
+  }
+  void reorderChildren() { // internal, do not call
+    // override this in subclasses with child nodes
+    // simply call reorderChild(child) for each child
+  }
+
+  bool _attached = false;
+  bool get attached => _attached;
+  void attach() {
+    // override this in subclasses with child nodes
+    // simply call attach() for each child then call your superclass
+    _attached = true;
+    attachChildren();
+  }
+  attachChildren() { } // workaround for lack of inter-class mixins in Dart
+  void detach() {
+    // override this in subclasses with child nodes
+    // simply call detach() for each child then call your superclass
+    _attached = false;
+    detachChildren();
+  }
+  detachChildren() { } // workaround for lack of inter-class mixins in Dart
+
+  void setAsChild(Node child) { // only for use by subclasses
+    assert(child != null);
+    if (attached)
+      child.attach();
+    reorderChild(child);
+  }
+  void dropChild(Node child) { // only for use by subclasses
+    assert(child != null);
+    assert(child.attached == attached);
+    if (attached)
+      child.detach();
+  }
+
+}