Merge pull request #1480 from pauliyobo/preprocessor_docs

Improving documentation on implementation of preprocessors.
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Eric Huss 2021-09-05 18:46:09 -07:00 committed by GitHub
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@ -34,8 +34,15 @@ command = "python3 /path/to/foo.py"
renderer = ["html", "epub"] renderer = ["html", "epub"]
``` ```
In typical unix style, all inputs to the plugin will be written to `stdin` as Once the preprocessor has been defined and the build process starts, mdBook executes the command defined in the `preprocessor.foo.command` key twice.
JSON and `mdbook` will read from `stdout` if it is expecting output. The first time it runs the preprocessor to determine if it supports the given renderer.
mdBook passes two arguments to the process: the first argument is the string `supports` and the second argument is the renderer name.
The preprocessor should exit with a status code 0 if it supports the given renderer, or return a non-zero exit code if it does not.
If the preprocessor supports the renderer, then mdbook runs it a second time, passing JSON data into stdin.
The JSON consists of an array of `[context, book]` where `context` is the serialized object [`PreprocessorContext`] and `book` is a [`Book`] object containing the content of the book.
The preprocessor should return the JSON format of the [`Book`] object to stdout, with any modifications it wishes to perform.
The easiest way to get started is by creating your own implementation of the The easiest way to get started is by creating your own implementation of the
`Preprocessor` trait (e.g. in `lib.rs`) and then creating a shell binary which `Preprocessor` trait (e.g. in `lib.rs`) and then creating a shell binary which
@ -106,6 +113,33 @@ fn remove_emphasis(
For everything else, have a look [at the complete example][example]. For everything else, have a look [at the complete example][example].
## Implementing a preprocessor with a different language
The fact that mdBook utilizes stdin and stdout to communicate with the preprocessors makes it easy to implement them in a language other than Rust.
The following code shows how to implement a simple preprocessor in Python, which will modify the content of the first chapter.
The example below follows the configuration shown above with `preprocessor.foo.command` actually pointing to a Python script.
```python
import json
import sys
if __name__ == '__main__':
if len(sys.argv) > 1: # we check if we received any argument
if sys.argv[1] == "supports":
# then we are good to return an exit status code of 0, since the other argument will just be the renderer's name
sys.exit(0)
# load both the context and the book representations from stdin
context, book = json.load(sys.stdin)
# and now, we can just modify the content of the first chapter
book['sections'][0]['Chapter']['content'] = '# Hello'
# we are done with the book's modification, we can just print it to stdout,
print(json.dumps(book))
```
[preprocessor-docs]: https://docs.rs/mdbook/latest/mdbook/preprocess/trait.Preprocessor.html [preprocessor-docs]: https://docs.rs/mdbook/latest/mdbook/preprocess/trait.Preprocessor.html
[pc]: https://crates.io/crates/pulldown-cmark [pc]: https://crates.io/crates/pulldown-cmark
[pctc]: https://crates.io/crates/pulldown-cmark-to-cmark [pctc]: https://crates.io/crates/pulldown-cmark-to-cmark
@ -113,3 +147,5 @@ For everything else, have a look [at the complete example][example].
[an example no-op preprocessor]: https://github.com/rust-lang/mdBook/blob/master/examples/nop-preprocessor.rs [an example no-op preprocessor]: https://github.com/rust-lang/mdBook/blob/master/examples/nop-preprocessor.rs
[`CmdPreprocessor::parse_input()`]: https://docs.rs/mdbook/latest/mdbook/preprocess/trait.Preprocessor.html#method.parse_input [`CmdPreprocessor::parse_input()`]: https://docs.rs/mdbook/latest/mdbook/preprocess/trait.Preprocessor.html#method.parse_input
[`Book::for_each_mut()`]: https://docs.rs/mdbook/latest/mdbook/book/struct.Book.html#method.for_each_mut [`Book::for_each_mut()`]: https://docs.rs/mdbook/latest/mdbook/book/struct.Book.html#method.for_each_mut
[`PreprocessorContext`]: https://docs.rs/mdbook/latest/mdbook/preprocess/struct.PreprocessorContext.html
[`Book`]: https://docs.rs/mdbook/latest/mdbook/book/struct.Book.html