Saving and loading Gaphor diagrams

Everything interesting is an ancestor of the Gaphor root tag.

The idea is to keep the file format as simple and extensible as possible: UML elements (including Diagram) are at toplevel, no nesting. A UML element can have two tags: Reference and Value. Reference is used to point to other UML elements, Value has a value inside (an integer or astring).

Diagram is a special case. Since this element contains a diagram canvas inside, it may become pretty big (with lots of nested elements). This is handled by the load and save function of the Diagram class. All elements inside a canvas have a tag Item.

<?xml version="1.0" ?>
<Gaphor version="1.0" gaphor_version="0.3">
  <Package id="1">
        <ref refid="2"/>
        <ref refid="3"/>
        <ref refid="4"/>
  <Diagram id="2">
      <ref refid="1"/>
    <canvas extents="(9.0, 9.0, 189.0, 247.0)" grid_bg="0xFFFFFFFF"
         grid_color="0x80ff" grid_int_x="10.0" grid_int_y="10.0"
         grid_ofs_x="0.0" grid_ofs_y="0.0" snap_to_grid="0"
         static_extents="0" affine="(1.0, 0.0, 0.0, 1.0, 0.0, 0.0)"
      <item affine="(1.0, 0.0, 0.0, 1.0, 150.0, 50.0)" cid="0x8293e74"
             height="78.0" subject="3" type="ActorItem" width="38.0"/>
      <item affine="(1.0, 0.0, 0.0, 1.0, 10.0, 10.0)" cid="0x82e7d74"
             height="26.0" subject="5" type="CommentItem" width="100.0"/>
  <Actor id="3">
      <ref refid="1"/>
  <UseCase id="4">
      <ref refid="1"/>
  <Comment id="5"/>