Preprocessor Extension
The Preprocessor extension runs prior to the AsciiDoc source being parsed. Specifically, it runs after the Document instance has been created and its PreprocessorReader has been initialized to start reading the lines of the source document.
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If the preprocessor extension reads lines from the reader, it will cause side effects. One of these side effects is that document attributes in the header may not be interpreted correctly. Another is that it may interfere with line number tracking. If the extension adds or removes lines, the parser could produce misleading line number information. Thus, interacting with the reader in a prepreprocessor extension should be avoided, if possible. A preprocessor extension can still be used to modify options on the Document object, in which case there’s no side effect. It may also be possible to avoid side effects by reading lines which are not interpreted as AsciiDoc, as is shown in the example on this page. |
Prior to invoking the preprocessor extension, Asciidoctor splits the source text of the primary document into lines and normalizes them. It then creates an instance of PreprocessorReader from these lines.
Asciidoctor then passes the Document and PreprocessorReader instances to the Asciidoctor::Extensions::Processor#process method of the extension.
If the extension reads lines from reader, those lines will first be preprocessed by Asciidoctor’s internal preprocessor.
The extension can modify the Reader and either return the same Reader (or falsy, which is equivalent) or a new Reader instance to replace it.
However, as stated above, be careful when reading lines from the original PreprocessorReader instance as it can have side effets.
Preprocessor Extension Example
- Purpose
-
Skim off front matter from the top of the document that gets used by site generators like Jekyll.
sample-with-front-matter.adoc
tags: [announcement, website]
---
= Document Title
content
[subs=+attributes]
.Captured front matter
----
---
{front-matter}
---
----FrontMatterPreprocessor
class FrontMatterPreprocessor < Asciidoctor::Extensions::Preprocessor
def process document, reader
lines = reader.lines # get raw lines
return reader if lines.empty?
front_matter = []
if lines.first.chomp == '---'
original_lines = lines.dup
lines.shift
while !lines.empty? && lines.first.chomp != '---'
front_matter << lines.shift
end
if (first = lines.first).nil? || first.chomp != '---'
lines = original_lines
else
lines.shift
document.attributes['front-matter'] = front_matter.join.chomp
# advance the reader by the number of lines taken
(front_matter.length + 2).times { reader.advance }
end
end
reader
end
end