CountdownLatch

A synchronization aid that allows one or more fibers to wait until a set of operations being performed in other fibers completes.

A CountDownLatch is initialized with a given count. The await method block until the current count reaches zero due to invocations of the countDown method, after which all waiting fibers are released and any subsequent invocations of await return immediately. This is a one-shot phenomenon -- the count cannot be reset. If you need a version that resets the count, consider using a [[CyclicBarrier]].

A CountDownLatch is a versatile synchronization tool and can be used for a number of purposes. A CountDownLatch initialized with a count of one serves as a simple on/off latch, or gate: all fibers invoking await wait at the gate until it is opened by a fiber invoking countDown. A CountDownLatchinitialized to N can be used to make one fiber wait until N fibers have completed some action, or some action has been completed N times.

A useful property of a CountDownLatch is that it doesn't require that fibers calling countDown wait for the count to reach zero before proceeding, it simply prevents any fiber from proceeding past an awaituntil all fibers could pass.

Operations

Creation

Method	Definition
make(n: Int): IO[Option[Nothing], CountdownLatch]	Makes a new CountdownLatch.

Use

Method	Definition
await: UIO[Unit]	Causes the current fiber to wait until the latch has counted down to zero.
countDown: UIO[Unit]	Decrements the count of the latch, releasing all waiting fibers if the count reaches zero.
count: UIO[Int]	Returns the current count.

Example Usage

import zio._
import zio.concurrent.CountdownLatch

for {
  latch  <- CountdownLatch.make(100)
  count  <- Ref.make(0)
  ps     <- ZIO.collectAll(List.fill(10)(Promise.make[Nothing, Unit]))
  _      <- ZIO.forkAll(ps.map(p => latch.await *> count.update(_ + 1) *> p.succeed(())))
  _      <- latch.countDown.repeat(Schedule.recurs(99))
  _      <- ZIO.foreach_(ps)(_.await)
  result <- count.get
} yield assert(result == 10)