Events and Handlers (Perl and XML)

4.2. Events and Handlers

Why do we call it an event stream and not an element stream or a markup object stream? The fact that XML is hierarchical (elements contain other elements) makes it impossible to package individual elements and serve them up as tokens in the stream. In a well-formed document, all elements are contained in one root element. A root element that contains the whole document is not a stream. Thus, we really can't expect a stream to give a complete element in a token, unless it's an empty element.

Instead, XML streams are composed of events. An event is a signal that the state of the document (as we've seen it so far in the stream) has changed. For example, when the parser comes across the start tag for an element, it indicates that another element was opened and the state of parsing has changed. An end tag affects the state by closing the most recently opened element. An XML processor can keep track of open elements in a stack data structure, pushing newly opened elements and popping off closed ones. At any given moment during parsing, the processor knows how deep it is in the document by the size of the stack.

Though parsers support a variety of events, there is a lot of overlap. For example, one parser may distinguish between a start tag and an empty element, while another may not, but all will signal the presence of that element. Let's look more closely at how a parser might dole out tokens, as shown Example 4-1.

Example 4-1. XML fragment

<recipe>
  <name>peanut butter and jelly sandwich</name>
  <!-- add picture of sandwich here -->
  <ingredients>
    <ingredient>Gloppy&trade; brand peanut butter</ingredient>
    <ingredient>bread</ingredient>
    <ingredient>jelly</ingredient>
  </ingredients>
  <instructions>
    <step>Spread peanutbutter on one slice of bread.</step>
    <step>Spread jelly on the other slice of bread.</step>
    <step>Put bread slices together, with peanut butter and
  jelly touching.</step>
  </instructions>
</recipe>

Apply a parser to the preceding example and it might generate this list of events:

A document start (if this is the beginning of a document and not a fragment)
A start tag for the <recipe> element
A start tag for the <name> element
The piece of text "peanut butter and jelly sandwich"
An end tag for the <name> element
A comment with the text "add picture of sandwich here"
A start tag for the <ingredients> element
A start tag for the <ingredient> element
The text "Gloppy"
A reference to the entity trade
The text "brand peanut butter"
An end tag for the <ingredient> element

. . . and so on, until the final event -- the end of the document -- is reached.

Somewhere between chopping up a stream into tokens and processing the tokens is a layer one might call a dispatcher. It branches the processing depending on the type of token. The code that deals with a particular token type is called a handler. There could be a handler for start tags, another for character data, and so on. It could be a compound if statement, switching to a subroutine to handle each case. Or, it could be built into the parser as a callback dispatcher, as is the case with XML::Parser's stream mode. If you register a set of subroutines, one to an event type, the parser calls the appropriate one for each token as it's generated. Which strategy you use depends on the parser.


4. Event Streams		4.3. The Parser as Commodity