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Re: An observation about "live" NodeLists

From: Miles Sabin <msabin@cromwellmedia.co.uk>
Date: Tue, 20 Oct 1998 18:10:15 +0100
Message-ID: <c=US%a=_%p=Cromwell_Media%l=ODIN-981020171015Z-24762@odin.cromwellmedia.co.uk>
To: "'DOM list'" <www-dom@w3.org>
As requested ... some elaboration on my slightly
cryptic comments.

Other than the absence of nested classes, this is the
the same as the technique mentioned by Ray.

Joe Keshlam wrote,
> Since Java's GC-based, making the Nodes aware of the
> NodeLists means the NodeList objects have to persist as
> long as the Nodes they're based on do; there's no 
> architected way to tell the DOM that they're no longer
> needed.

This isn't quite true, tho' it might as well be.

Nodes don't have to be aware of the object which is
returned to the user ... they only have to be aware of the
entity that maintains the NodeList state, and that can be
a separate object, breaking the cycle between the Node and
the user-level NodeList. So, instead of,

     user-level
      NodeList
     reference
         |
         |
         V
   |-----------|                         |-----------|
   |           |*------------------------|           |
   |  NodeList |                         |   Node    |
   |    Impl   |------------------------*|           |
   |-----------|                         |-----------|

we have,

   |-----------|      |-----------|      |-----------|
   |           |*-----|           |*-----|           |
   |  NodeList |      |  NodeList |      |   Node    | 
   |   Handle  |      |    Body   |-----*|           |
   |-----------|      |-----------|      |-----------|


The handle object returned to the user will be GC'ed when
there are no user-level references to it, and its finalize()
method can invoke a method telling the body object to
detach itself from its node.

For example,

  
  // Handle class, instances of which are returned
  // to DOM users
  public class NodeListHandle
    implements NodeList
  {
    private NodeListBody itsBody;

    public NodeListHandle(NodeListBody body)
    {
      itsBody = body;
    }

    public void finalize()
    {
      itsBody.destroy();
    }

    public Node item(int index)
    {
      return itsBody.item(index);
    }

    // etc.
  }

  // Body class, instances of which are referred to
  // by corresponding handle and Node.  
  class NodeListBody
  {
    private Node itsNode;

    public NodeListBody(Node node)
    {
      itsNode = node;
    }

    public void destroy()
    {
      itsNode.deregisterNodeList(this);
    }

    public Node item(int index)
    {
      // whatever
    }

    // etc.
  }

This doesn't help much tho', because we're still at the
mercy of Java GC ... we've no way of knowing when the handle's
finalize() method will be called, other than by explicitly
invoking GC, which is probably not a good idea.

Cheers,


Miles

-- 
Miles Sabin                          Cromwell Media
Internet Systems Architect           5/6 Glenthorne Mews
+44 (0)181 410 2230                  London, W6 0LJ
msabin@cromwellmedia.co.uk           England
Received on Tuesday, 20 October 1998 13:14:57 GMT

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