[ expat-Bugs-544682 ] Support Pull operation

Bugs item #544682, was opened at 2002-04-16 10:34
Message generated for change (Comment added) made by fdrake
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Category: None
Group: Feature Request
>Status: Closed
Resolution: None
Priority: 5
Submitted By: Karl Waclawek (kwaclaw)
Assigned to: Karl Waclawek (kwaclaw)
Summary: Support Pull operation

Initial Comment:
Would it be hard to add the following functions?
 - suspend() ...suspends parsing, remembers state
 - resume()  ...resumes parsing from where it was 
suspended
 - abort()   ...cancels parsing
 
This looks like the concept of coroutines, and could 
have the following benefit: One could implement a
"pull" based  parser on top of the "push" based model 
of Expat.
 
This has recently been chosen as Microsoft's new 
approach,  i.e. in XML.NET, SAX has been replaced by
a pull based parser("XMLReader").
 
Typical pull code would look like:
 while parser.nextNode() do
 begin
  //process current node
 end;
 
It seems to me such a parser could be implemented on
top of SAX or Expat, if there was functionality as 
described above. Microsoft's MSXML3 implementation
has the IMXReaderControl interface, which allows 
exactly that. Makes me think that (maybe) MS's "pull"
implementation is implemented on top of their own SAX 
parser.
 
So, my question again: How hard would it be to add
such coroutine-like functionality to Expat?
 
Karl
 


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>Comment By: Fred L. Drake, Jr. (fdrake)
Date: 2004-07-15 20:48

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The basic requirements are now supported, so I'm closing
this.  Implementing specific APIs on top of the current
implementation is an exercise for the reader; Expat itself
should remain small.

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Comment By: Karl Waclawek (kwaclaw)
Date: 2004-01-20 11:44

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Yes, in principle Expat now has the capability to be
used like a Pull API. However, I als came across the new
XMLReader pull API implemented in libxml2 (in C), which
is modelled after the same API in .NET.

And, as I am working now (slowly) on writing a SAX wrapper
in C#, I realized that an XMLReader wrapper might be nice too.
One thing I am not sure about: Implement the Pull API 
wrapper in C#, or implement it in C, and call from C#.
As far as I can tell, call-backs from C into .NET are quite
expensive, and calls from .NET into C are not cheap either,
so a less "chatty" API is better - which would mean that it 
would be better to implement XMLReader in C instead of C#.

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Comment By: Fred L. Drake, Jr. (fdrake)
Date: 2004-01-20 11:27

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Note that patch #835123 has been applied to current CVS.

Karl, it's probably safe to close this.

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Comment By: Karl Waclawek (kwaclaw)
Date: 2003-11-10 13:21

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Check out patch #835123. If accepted, this should be
a major step towards fulfilling this feature request.

The question even is if patch #835123 would not be sufficient,
as it may be a better approach to implement the actual
Pull API in the target language that wraps the Expat parser.

A C-style Pull API would then be only targeted at the
C-language itself.

Karl

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Comment By: Karl Waclawek (kwaclaw)
Date: 2002-07-27 23:04

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It looks as if I am going to try to port the block/unblock
functionality from the Mozilla version of Expat.
Assigned to me because of that.

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