Snippet

Manual Polling Pattern for Asynchronous Streams

Creating stream-like primitives in std Rust without futures crate involves returning Poll<Result<T, E>> to express readiness and operational status manually.

snippet.rs

rust

use std::task::{Context, Poll};
 
pub struct RingStream<T> {
    buffer: Vec<T>,
    cursor: usize,
}
 
impl<T: Clone> RingStream<T> {
    pub fn new(items: Vec<T>) -> Self {
        Self { buffer: items, cursor: 0 }
    }
 
    pub fn poll_next_item(&mut self, _cx: &mut Context<'_>) -> Poll<Result<T, &'static str>> {
        if self.buffer.is_empty() {
            Poll::Ready(Err("Stream source buffer is empty"))
        } else {
            let item = self.buffer[self.cursor].clone();
            self.cursor = (self.cursor + 1) % self.buffer.len();
            Poll::Ready(Ok(item))
        }
    }
}

Breakdown

pub struct RingStream<T> {

Encapsulates a cyclic item buffer with an internal reading cursor state.

pub fn poll_next_item(&mut self, _cx: &mut Context<'_>) -> Poll<Result<T, &'static str>>

Defines custom poll method returning Poll enum with Result item payload.

if self.buffer.is_empty() {

Evaluates buffer validity and returns Ready error status if stream cannot produce data.

self.cursor = (self.cursor + 1) % self.buffer.len();

Advances cursor position atomically within fixed circular boundaries.

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Manual Polling Pattern for Asynchronous Streams

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