Trait ContextField

Source

pub trait ContextField: Default {
    type Context;
    type Value: Send + 'static;

    // Required method
    fn read(self, src: &Self::Context) -> Self::Value;
}

Expand description

A Field-style key that also names its context type and yields an owned value.

It powers the Ctx extractor: because the key carries its context as an associated type, a handler taking Ctx(value): Ctx<Key> needs no &mut Context parameter at all - the #[subscriber] macro projects the subscription’s context type from the first Ctx key it sees. The value is owned ('static) on purpose: extractor values bind before the handler body runs, so borrowing from the context is not an option. Borrowed keys keep working through Field and ctx.context(KEY).

Broker crates implement it next to Field on the same zero-sized keys; a key typically implements both, so it works as a context read and as an extractor.

§Examples

use ruststream::ContextField;

struct Delivery {
    sequence: u64,
}

#[derive(Clone, Copy, Default)]
struct Sequence;

impl ContextField for Sequence {
    type Context = Delivery;
    type Value = u64;
    fn read(self, src: &Delivery) -> u64 {
        src.sequence
    }
}

let delivery = Delivery { sequence: 7 };
assert_eq!(Sequence.read(&delivery), 7);