Custom Writer

Finally, let's implement a custom Writer which simply outputs cucumber events to STDOUT in the order of receiving.

Writer represents anything that consumes a Stream of cucumber events.

extern crate async_trait;
extern crate cucumber;
extern crate futures;
extern crate once_cell;
extern crate tokio;

use std::{
    panic::{self, AssertUnwindSafe},
    path::PathBuf,
    sync::Arc,
    time::Duration,
};

use async_trait::async_trait;
use cucumber::{
    cli, event, gherkin, given, parser, step, then, when, Event, World,
    WriterExt as _,
};
use futures::{
    future::{self, FutureExt as _},
    stream::{self, LocalBoxStream, Stream, StreamExt as _, TryStreamExt as _},
};
use once_cell::sync::Lazy;
use tokio::time::sleep;

#[derive(Clone, Copy, Debug, Default)]
struct Animal {
    pub hungry: bool,
}

impl Animal {
    fn feed(&mut self) {
        self.hungry = false;
    }
}

#[derive(Clone, Debug, Default, World)]
pub struct AnimalWorld {
    cat: Animal,
}

#[given(regex = r"^a (hungry|satiated) cat$")]
async fn hungry_cat(world: &mut AnimalWorld, state: String) {
    sleep(Duration::from_secs(2)).await;

    match state.as_str() {
        "hungry" => world.cat.hungry = true,
        "satiated" => world.cat.hungry = false,
        _ => unreachable!(),
    }
}

#[when("I feed the cat")]
async fn feed_cat(world: &mut AnimalWorld) {
    sleep(Duration::from_secs(2)).await;

    world.cat.feed();
}

#[then("the cat is not hungry")]
async fn cat_is_fed(world: &mut AnimalWorld) {
    sleep(Duration::from_secs(2)).await;

    assert!(!world.cat.hungry);
}

struct CustomParser;

impl<I> cucumber::Parser<I> for CustomParser {
    type Cli = cli::Empty;
    type Output = stream::Once<future::Ready<parser::Result<gherkin::Feature>>>;

    fn parse(self, _: I, _: Self::Cli) -> Self::Output {
        let keyword = "Feature";
        let name = "Animal feature";
        stream::once(future::ok(gherkin::Feature {
            keyword: keyword.into(),
            name: name.into(),
            description: None,
            background: None,
            scenarios: vec![gherkin::Scenario {
                keyword: "Scenario".into(),
                name: "If we feed a hungry cat it won't be hungry".into(),
                description: None,
                steps: vec![
                    gherkin::Step {
                        keyword: "Given".into(),
                        ty: gherkin::StepType::Given,
                        value: "a hungry cat".into(),
                        docstring: None,
                        table: None,
                        span: gherkin::Span { start: 5, end: 18 },
                        position: gherkin::LineCol { line: 3, col: 5 },
                    },
                    gherkin::Step {
                        keyword: "When".into(),
                        ty: gherkin::StepType::When,
                        value: "I feed the cat".into(),
                        docstring: None,
                        table: None,
                        span: gherkin::Span { start: 5, end: 19 },
                        position: gherkin::LineCol { line: 4, col: 5 },
                    },
                    gherkin::Step {
                        keyword: "Then".into(),
                        ty: gherkin::StepType::Then,
                        value: "the cat is not hungry".into(),
                        docstring: None,
                        table: None,
                        span: gherkin::Span { start: 5, end: 26 },
                        position: gherkin::LineCol { line: 5, col: 5 },
                    },
                ],
                examples: vec![],
                tags: vec![],
                span: gherkin::Span { start: 3, end: 52 },
                position: gherkin::LineCol { line: 2, col: 3 },
            }],
            rules: vec![],
            tags: vec![],
            span: gherkin::Span { start: 1, end: 23 },
            position: gherkin::LineCol { line: 1, col: 1 },
            path: Some(PathBuf::from(file!())),
        }))
    }
}

struct CustomRunner;

impl CustomRunner {
    fn steps_fns() -> &'static step::Collection<AnimalWorld> {
        static STEPS: Lazy<step::Collection<AnimalWorld>> =
            Lazy::new(AnimalWorld::collection);
        &STEPS
    }

    async fn execute_step(
        mut world: AnimalWorld,
        step: gherkin::Step,
    ) -> (AnimalWorld, event::Step<AnimalWorld>) {
        let ev = if let Some((step_fn, captures, loc, ctx)) =
            Self::steps_fns().find(&step).expect("Ambiguous match")
        {
            match AssertUnwindSafe(step_fn(&mut world, ctx))
                .catch_unwind()
                .await
            {
                Ok(()) => event::Step::Passed(captures, loc),
                Err(e) => event::Step::Failed(
                    Some(captures),
                    loc,
                    Some(Arc::new(world.clone())),
                    event::StepError::Panic(e.into()),
                ),
            }
        } else {
            event::Step::Skipped
        };
        (world, ev)
    }

    async fn execute_scenario(
        scenario: gherkin::Scenario,
    ) -> impl Stream<Item = event::Feature<AnimalWorld>> {
        let hook = panic::take_hook();
        panic::set_hook(Box::new(|_| {}));

        let mut world = AnimalWorld::new().await.unwrap();
        let mut steps = Vec::with_capacity(scenario.steps.len());

        for step in scenario.steps.clone() {
            let (w, ev) = Self::execute_step(world, step.clone()).await;
            world = w;
            let should_stop = matches!(ev, event::Step::Failed(..));
            steps.push((step, ev));
            if should_stop {
                break;
            }
        }

        panic::set_hook(hook);

        let scenario = Arc::new(scenario);
        stream::once(future::ready(event::Scenario::Started))
            .chain(stream::iter(steps.into_iter().flat_map(|(step, ev)| {
                let step = Arc::new(step);
                [
                    event::Scenario::Step(step.clone(), event::Step::Started),
                    event::Scenario::Step(step, ev),
                ]
            })))
            .chain(stream::once(future::ready(event::Scenario::Finished)))
            .map(move |event| event::Feature::Scenario(
                scenario.clone(), 
                event::RetryableScenario { event, retries: None },
            ))
    }

    fn execute_feature(
        feature: gherkin::Feature,
    ) -> impl Stream<Item = event::Cucumber<AnimalWorld>> {
        let feature = Arc::new(feature);
        stream::once(future::ready(event::Feature::Started))
            .chain(
                stream::iter(feature.scenarios.clone())
                    .then(Self::execute_scenario)
                    .flatten(),
            )
            .chain(stream::once(future::ready(event::Feature::Finished)))
            .map(move |ev| event::Cucumber::Feature(feature.clone(), ev))
    }
}

impl cucumber::Runner<AnimalWorld> for CustomRunner {
    type Cli = cli::Empty;
    type EventStream = LocalBoxStream<
        'static,
        parser::Result<Event<event::Cucumber<AnimalWorld>>>,
    >;

    fn run<S>(self, features: S, _: Self::Cli) -> Self::EventStream
    where
        S: Stream<Item = parser::Result<gherkin::Feature>> + 'static,
    {
        stream::once(future::ok(event::Cucumber::Started))
            .chain(
                features
                    .map_ok(|f| Self::execute_feature(f).map(Ok))
                    .try_flatten(),
            )
            .chain(stream::once(future::ok(event::Cucumber::Finished)))
            .map_ok(Event::new)
            .boxed_local()
    }
}

struct CustomWriter;

#[async_trait(?Send)]
impl<W: 'static> cucumber::Writer<W> for CustomWriter {
    type Cli = cli::Empty; // we provide no CLI options

    async fn handle_event(
        &mut self,
        ev: parser::Result<Event<event::Cucumber<W>>>,
        _: &Self::Cli,
    ) {
        match ev {
            Ok(Event { value, .. }) => match value {
                event::Cucumber::Feature(feature, ev) => match ev {
                    event::Feature::Started => {
                        println!("{}: {}", feature.keyword, feature.name)
                    }
                    event::Feature::Scenario(scenario, ev) => match ev.event {
                        event::Scenario::Started => {
                            println!("{}: {}", scenario.keyword, scenario.name)
                        }
                        event::Scenario::Step(step, ev) => match ev {
                            event::Step::Started => {
                                print!("{} {}...", step.keyword, step.value)
                            }
                            event::Step::Passed(..) => println!("ok"),
                            event::Step::Skipped => println!("skip"),
                            event::Step::Failed(_, _, _, err) => {
                                println!("failed: {err}")
                            }
                        },
                        _ => {}
                    },
                    _ => {}
                },
                _ => {}
            },
            Err(e) => println!("Error: {e}"),
        }
    }
}

#[tokio::main]
async fn main() {
    AnimalWorld::cucumber::<&str>() // aiding type inference for `CustomParser`
        .with_parser(CustomParser)
        .with_runner(CustomRunner)
        .with_writer(CustomWriter.assert_normalized()) // OK because of `CustomRunner`
        .run("tests/features/book")
        .await;
}

TIP: CustomWriter will print trash if we feed unordered event::Cucumbers into it. Though, we shouldn't care about order normalization in our implementations. Instead, we may just wrap CustomWriter into writer::Normalize, which will do that for us.

extern crate async_trait;
extern crate cucumber;
extern crate futures;
extern crate tokio;

use std::{path::PathBuf, time::Duration};

use async_trait::async_trait;
use cucumber::{
    cli, event, gherkin, given, parser, then, when, Event, World, 
    WriterExt as _,
};
use futures::{future, stream};
use tokio::time::sleep;

#[derive(Clone, Copy, Debug, Default)]
struct Animal {
    pub hungry: bool,
}

impl Animal {
    fn feed(&mut self) {
        self.hungry = false;
    }
}

#[derive(Clone, Debug, Default, World)]
pub struct AnimalWorld {
    cat: Animal,
}

#[given(regex = r"^a (hungry|satiated) cat$")]
async fn hungry_cat(world: &mut AnimalWorld, state: String) {
    sleep(Duration::from_secs(2)).await;

    match state.as_str() {
        "hungry" => world.cat.hungry = true,
        "satiated" => world.cat.hungry = false,
        _ => unreachable!(),
    }
}

#[when("I feed the cat")]
async fn feed_cat(world: &mut AnimalWorld) {
    sleep(Duration::from_secs(2)).await;

    world.cat.feed();
}

#[then("the cat is not hungry")]
async fn cat_is_fed(world: &mut AnimalWorld) {
    sleep(Duration::from_secs(2)).await;

    assert!(!world.cat.hungry);
}

struct CustomParser;

impl<I> cucumber::Parser<I> for CustomParser {
    type Cli = cli::Empty;
    type Output = stream::Once<future::Ready<parser::Result<gherkin::Feature>>>;

    fn parse(self, _: I, _: Self::Cli) -> Self::Output {
        let keyword = "Feature";
        let name = "Animal feature";
        stream::once(future::ok(gherkin::Feature {
            keyword: keyword.into(),
            name: name.into(),
            description: None,
            background: None,
            scenarios: vec![gherkin::Scenario {
                keyword: "Scenario".into(),
                name: "If we feed a hungry cat it won't be hungry".into(),
                description: None,
                steps: vec![
                    gherkin::Step {
                        keyword: "Given".into(),
                        ty: gherkin::StepType::Given,
                        value: "a hungry cat".into(),
                        docstring: None,
                        table: None,
                        span: gherkin::Span { start: 5, end: 18 },
                        position: gherkin::LineCol { line: 3, col: 5 },
                    },
                    gherkin::Step {
                        keyword: "When".into(),
                        ty: gherkin::StepType::When,
                        value: "I feed the cat".into(),
                        docstring: None,
                        table: None,
                        span: gherkin::Span { start: 5, end: 19 },
                        position: gherkin::LineCol { line: 4, col: 5 },
                    },
                    gherkin::Step {
                        keyword: "Then".into(),
                        ty: gherkin::StepType::Then,
                        value: "the cat is not hungry".into(),
                        docstring: None,
                        table: None,
                        span: gherkin::Span { start: 5, end: 26 },
                        position: gherkin::LineCol { line: 5, col: 5 },
                    },
                ],
                examples: vec![],
                tags: vec![],
                span: gherkin::Span { start: 3, end: 52 },
                position: gherkin::LineCol { line: 2, col: 3 },
            }],
            rules: vec![],
            tags: vec![],
            span: gherkin::Span { start: 1, end: 23 },
            position: gherkin::LineCol { line: 1, col: 1 },
            path: Some(PathBuf::from(file!())),
        }))
    }
}

struct CustomWriter;

#[async_trait(?Send)]
impl<W: 'static> cucumber::Writer<W> for CustomWriter {
    type Cli = cli::Empty; // we provide no CLI options

    async fn handle_event(
        &mut self,
        ev: parser::Result<Event<event::Cucumber<W>>>,
        _: &Self::Cli,
    ) {
        match ev {
            Ok(Event { value, .. }) => match value {
                event::Cucumber::Feature(feature, ev) => match ev {
                    event::Feature::Started => {
                        println!("{}: {}", feature.keyword, feature.name)
                    }
                    event::Feature::Scenario(scenario, ev) => match ev.event {
                        event::Scenario::Started => {
                            println!("{}: {}", scenario.keyword, scenario.name)
                        }
                        event::Scenario::Step(step, ev) => match ev {
                            event::Step::Started => {
                                print!("{} {}...", step.keyword, step.value)
                            }
                            event::Step::Passed(..) => println!("ok"),
                            event::Step::Skipped => println!("skip"),
                            event::Step::Failed(_, _, _, err) => {
                                println!("failed: {err}", )
                            }
                        },
                        _ => {}
                    },
                    _ => {}
                },
                _ => {}
            },
            Err(e) => println!("Error: {e}"),
        }
    }
}

#[tokio::main]
async fn main() {
    AnimalWorld::cucumber::<&str>() // aiding type inference for `CustomParser`
        .with_parser(CustomParser)
        .with_writer(CustomWriter.normalized()) // wrapping into `writer::Normalize`,
        .run("tests/features/book")             // so it works OK with the default
        .await;                                 // concurrent `Runner`
}

NOTE: Writers are easily pipelined. See WriterExt trait and writer module for more Writer machinery "included batteries".