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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: Reusing, Refactoring, Reinventing??? (was Re: SA
 Fredrik Lindgren wrote: > Please explain how the parser would both give the > TypedContentHandler it's typed data and fulfill the > expectations of the ContentHandler interface with regard > to elements and attributes. Actually, I can't explain how this would work because it wouldn't. I was wrong when I wrote: "TypedContentHandler works exactly like ContentHandler except that it can do just a little bit more." The problem is that the "little bit more" that I mentioned would sometimes mean "just a little bit less." As I finally came to understand after a three hour long, but very enjoyable, dinner with Elliote Rusty Harold (excellent chicken BTW!), the core problem with my proposal is the part about using the "callbackCharacters" feature to "turn off" callbacks to ContentHandler.characters(). This changes the behavior of the interface in such a way that classes that derive from TypedContentHandler don't see the expected behavior of the ContentHandler.characters() method. This is not good. The contracts of the ContentHandler interface should be met by interfaces that extend it. Thus, TypedContentHandler.characters() should be called whenever the SAX 2 specification says that ContentHandler.characters() should be called. My proposal was syntactically legal in Java, it was simple, it was many things... but it wasn't good design. So, to do this "correctly" one could still extend ContentHandler and create TypedContentHandler, however, the feature flags would not be used. The result would be exactly what I proposed but with both features turned on. A program that wanted characters would catch the character() callback, and one that wanted values would catch the values() callback. The XMLReader would make both callbacks. Of course, this would be somewhat inefficient... But, it would keep things working properly and would allow programmers to "speak in types", which has some value. The result would probably be just about the same as what people do today, calling an appropriate conversion routine on the data passed by each call to characters. However, the double callback would be very ugly, so, it would probably be better to provide a new interface with a "getValueFromCharacters" method that would return a typed equivalent of whatever returned by characters(). This routine would rely on information in the schema, etc. to make sure that conversions were done properly and thus would be less error prone than what people do today. Also, it wouldn't have to be called on every callback since sometimes strings are what one wants. Doing it this way would mean some improvements in performance over some of the alternatives and should be considered "non-polluting" in any case. Of course, you could get back to exactly the behaviour that I was proposing simply by making TypedContentHandler a peer of ContentHandler rather than an extension of it. You would copy ContentHandler and add the values() method. This would serve many situations well, and by *not* being an extension of ContentHandler would prevent subclasses or extensions from getting confused. But, then, you would be complicating the construction of SAX pipelines and filter chains. Not good -- thus, probably shouldn't be pursued. Anyway, enjoy and thanks for all the fish. Sorry 'bout the diversion. I was wrong. bob wyman 
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