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Trait tracing_subscriber::fmt::FormatEvent

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pub trait FormatEvent<S, N> where
    S: Subscriber + for<'a> LookupSpan<'a>,
    N: for<'a> FormatFields<'a> + 'static, {
    fn format_event(
        &self, 
        ctx: &FmtContext<'_, S, N>, 
        writer: &mut dyn Write, 
        event: &Event<'_>
    ) -> Result;
}
Expand description

A type that can format a tracing Event for a fmt::Write.

FormatEvent is primarily used in the context of fmt::Subscriber or fmt::Layer. Each time an event is dispatched to fmt::Subscriber or fmt::Layer, the subscriber or layer forwards it to its associated FormatEvent to emit a log message.

This trait is already implemented for function pointers with the same signature as format_event.

Examples

use std::fmt::{self, Write};
use tracing_core::{Subscriber, Event};
use tracing_subscriber::fmt::{FormatEvent, FormatFields, FmtContext, FormattedFields};
use tracing_subscriber::registry::LookupSpan;

struct MyFormatter;

impl<S, N> FormatEvent<S, N> for MyFormatter
where
    S: Subscriber + for<'a> LookupSpan<'a>,
    N: for<'a> FormatFields<'a> + 'static,
{
    fn format_event(
        &self,
        ctx: &FmtContext<'_, S, N>,
        writer: &mut dyn fmt::Write,
        event: &Event<'_>,
    ) -> fmt::Result {
        // Write level and target
        let level = *event.metadata().level();
        let target = event.metadata().target();
        write!(
            writer,
            "{} {}: ",
            level,
            target,
        )?;

        // Write spans and fields of each span
        ctx.visit_spans(|span| {
            write!(writer, "{}", span.name())?;

            let ext = span.extensions();

            // `FormattedFields` is a a formatted representation of the span's
            // fields, which is stored in its extensions by the `fmt` layer's
            // `new_span` method. The fields will have been formatted
            // by the same field formatter that's provided to the event
            // formatter in the `FmtContext`.
            let fields = &ext
                .get::<FormattedFields<N>>()
                .expect("will never be `None`");

            if !fields.is_empty() {
                write!(writer, "{{{}}}", fields)?;
            }
            write!(writer, ": ")?;

            Ok(())
        })?;

        // Write fields on the event
        ctx.field_format().format_fields(writer, event)?;

        writeln!(writer)
    }
}

This formatter will print events like this:

DEBUG yak_shaving::shaver: some-span{field-on-span=foo}: started shaving yak

Required methods

Write a log message for Event in Context to the given Write.

Implementations on Foreign Types

Implementors