mdBook/examples/nop-preprocessor.rs

//! A basic example of a preprocessor that does nothing.

use crate::nop_lib::Nop;
use clap::{Arg, ArgMatches, Command};
use mdbook_preprocessor::book::Book;
use mdbook_preprocessor::errors::Result;
use mdbook_preprocessor::{Preprocessor, PreprocessorContext};
use semver::{Version, VersionReq};
use std::io;
use std::process;

fn make_app() -> Command {
    Command::new("nop-preprocessor")
        .about("A mdbook preprocessor which does precisely nothing")
        .subcommand(
            Command::new("supports")
                .arg(Arg::new("renderer").required(true))
                .about("Check whether a renderer is supported by this preprocessor"),
        )
}

fn main() {
    let matches = make_app().get_matches();

    // Users will want to construct their own preprocessor here
    let preprocessor = Nop::new();

    if let Some(sub_args) = matches.subcommand_matches("supports") {
        handle_supports(&preprocessor, sub_args);
    } else if let Err(e) = handle_preprocessing(&preprocessor) {
        eprintln!("{e:?}");
        process::exit(1);
    }
}

fn handle_preprocessing(pre: &dyn Preprocessor) -> Result<()> {
    let (ctx, book) = mdbook_preprocessor::parse_input(io::stdin())?;

    let book_version = Version::parse(&ctx.mdbook_version)?;
    let version_req = VersionReq::parse(mdbook_preprocessor::MDBOOK_VERSION)?;

    if !version_req.matches(&book_version) {
        eprintln!(
            "Warning: The {} plugin was built against version {} of mdbook, \
             but we're being called from version {}",
            pre.name(),
            mdbook_preprocessor::MDBOOK_VERSION,
            ctx.mdbook_version
        );
    }

    let processed_book = pre.run(&ctx, book)?;
    serde_json::to_writer(io::stdout(), &processed_book)?;

    Ok(())
}

fn handle_supports(pre: &dyn Preprocessor, sub_args: &ArgMatches) -> ! {
    let renderer = sub_args
        .get_one::<String>("renderer")
        .expect("Required argument");
    let supported = pre.supports_renderer(renderer).unwrap();

    // Signal whether the renderer is supported by exiting with 1 or 0.
    if supported {
        process::exit(0);
    } else {
        process::exit(1);
    }
}

/// The actual implementation of the `Nop` preprocessor. This would usually go
/// in your main `lib.rs` file.
#[allow(unreachable_pub, reason = "wouldn't be a problem in a proper lib.rs")]
mod nop_lib {
    use super::*;

    /// A no-op preprocessor.
    pub struct Nop;

    impl Nop {
        pub fn new() -> Nop {
            Nop
        }
    }

    impl Preprocessor for Nop {
        fn name(&self) -> &str {
            "nop-preprocessor"
        }

        fn run(&self, ctx: &PreprocessorContext, book: Book) -> Result<Book> {
            // In testing we want to tell the preprocessor to blow up by setting a
            // particular config value
            match ctx
                .config
                .get::<bool>("preprocessor.nop-preprocessor.blow-up")
            {
                Ok(Some(true)) => anyhow::bail!("Boom!!1!"),
                Ok(_) => {}
                Err(e) => anyhow::bail!("expect bool for blow-up: {e}"),
            }

            // we *are* a no-op preprocessor after all
            Ok(book)
        }

        fn supports_renderer(&self, renderer: &str) -> Result<bool> {
            Ok(renderer != "not-supported")
        }
    }

    #[cfg(test)]
    mod test {
        use super::*;

        #[test]
        fn nop_preprocessor_run() {
            let input_json = r##"[
                {
                    "root": "/path/to/book",
                    "config": {
                        "book": {
                            "authors": ["AUTHOR"],
                            "language": "en",
                            "src": "src",
                            "title": "TITLE"
                        },
                        "preprocessor": {
                            "nop": {}
                        }
                    },
                    "renderer": "html",
                    "mdbook_version": "0.4.21"
                },
                {
                    "items": [
                        {
                            "Chapter": {
                                "name": "Chapter 1",
                                "content": "# Chapter 1\n",
                                "number": [1],
                                "sub_items": [],
                                "path": "chapter_1.md",
                                "source_path": "chapter_1.md",
                                "parent_names": []
                            }
                        }
                    ]
                }
            ]"##;
            let input_json = input_json.as_bytes();

            let (ctx, book) = mdbook_preprocessor::parse_input(input_json).unwrap();
            let expected_book = book.clone();
            let result = Nop::new().run(&ctx, book);
            assert!(result.is_ok());

            // The nop-preprocessor should not have made any changes to the book content.
            let actual_book = result.unwrap();
            assert_eq!(actual_book, expected_book);
        }
    }
}