Struct synchronoise::CountdownEvent

pub struct CountdownEvent { /* fields omitted */ }

A synchronization primitive that signals when its count reaches zero.

With a CountdownEvent, it's possible to cause one thread to wait on a set of computations occurring in other threads by making the other threads interact with the counter as they perform their work.

The main limitation of a CountdownEvent is that once its counter reaches zero (even by starting there), any attempts to update the counter will return CountdownError::AlreadySet until the counter is reset by calling reset or reset_to_count.

CountdownEvent is a port of System.Threading.CountdownEvent from .NET (also called CountDownLatch in Java).

Example

This example uses a CountdownEvent to make the "coordinator" thread sleep until all of its "worker" threads have finished. Each thread calls signal.decrement() to signal to the Event that its work has completed. When the last thread does this (and brings the counter to zero), the "coordinator" thread wakes up and prints all done!.

use synchronoise::CountdownEvent;
use std::sync::Arc;
use std::thread;
use std::time::Duration;

let thread_count = 5;
let counter = Arc::new(CountdownEvent::new(thread_count));

for i in 0..thread_count {
    let signal = counter.clone();
    thread::spawn(move || {
        thread::sleep(Duration::from_secs(i as u64));
        println!("thread {} activated!", i);
        signal.decrement().unwrap();
    });
}

counter.wait();

println!("all done!");

Methods

impl CountdownEvent

fn new(count: usize) -> CountdownEvent

Creates a new CountdownEvent, initialized to the given count.

Remember that once the counter reaches zero, calls to add or signal will fail, so passing zero to this function will create a CountdownEvent that is permanently signaled.

fn reset(&mut self)

Resets the counter to the count given to new.

This function is safe because the &mut self enforces that no other references or locks exist.

fn reset_to_count(&mut self, count: usize)

Resets the counter to the given count.

This function is safe because the &mut self enforces that no other references or locks exist.

fn count(&self) -> usize

Returns the current counter value.

fn add(&self, count: usize) -> Result<(), CountdownError>

Adds the given count to the counter.

Errors

If the counter is already at zero, this function will return CountdownError::AlreadySet.

If the given count would cause the counter to overflow usize, this function will return CountdownError::SaturatedCounter.

fn signal(&self, count: usize) -> Result<bool, CountdownError>

Subtracts the given count to the counter, and returns whether this caused any waiting threads to wake up.

Errors

If the counter is already at zero, this function will return CountdownError::AlreadySet.

If the given count would cause the counter to go below zero (instead of reaching zero), this function will return CountdownError::TooManySignals.

fn increment(&self) -> Result<(), CountdownError>

Adds one to the count.

Errors

See add for the situations where this function will return an error.

fn decrement(&self) -> Result<bool, CountdownError>

Subtracts one from the counter, and returns whether this caused any waiting threads to wake up.

Errors

See signal for the situations where this function will return an error.

fn guard(&self) -> Result<CountdownGuard, CountdownError>

Increments the counter, then returns a guard object that will decrement the counter upon drop.

Errors

This function will return the same errors as add. If the event has already signaled by the time the guard is dropped (and would cause its decrement call to return an error), then the error will be silently ignored.

Example

Here's the sample from the main docs, using CountdownGuards instead of manually decrementing:

use synchronoise::CountdownEvent;
use std::sync::Arc;
use std::thread;
use std::time::Duration;

let thread_count = 5;
//counter can't start from zero, but the guard increments on its own, so start at one and
//just decrement once when we're ready to wait
let counter = Arc::new(CountdownEvent::new(1));

for i in 0..thread_count {
    let signal = counter.clone();
    thread::spawn(move || {
        let _guard = signal.guard().unwrap();
        thread::sleep(Duration::from_secs(i));
        println!("thread {} activated!", i);
    });
}

//give all the threads time to increment the counter before continuing
thread::sleep(Duration::from_millis(100));
counter.decrement().unwrap();
counter.wait();

println!("all done!");

fn wait(&self)

Blocks the current thread until the counter reaches zero.

This function will block indefinitely until the counter reaches zero. It will return immediately if it is already at zero.

fn wait_timeout(&self, timeout: Duration) -> usize

Blocks the current thread until the timer reaches zero, or until the given timeout elapses, returning the count at the time of wakeup.

This function will return immediately if the counter was already at zero. Otherwise, it will block for roughly no longer than timeout, or when the counter reaches zero, whichever comes first.