Scala Cats 效果 - IO 异步转换 - 它是如何工作的？

Question

这是一些使用 IO Monad 的 Scala 猫代码：

import java.util.concurrent.{ExecutorService, Executors}

import cats.effect.IO

import scala.concurrent.{ExecutionContext, ExecutionContextExecutor}
import scala.util.control.NonFatal

object Program extends App {

  type CallbackType = (Either[Throwable, Unit]) => Unit

  // IO.async[Unit] is like a Future that returns Unit on completion.
  // Unlike a regular Future, it doesn't start to run until unsafeRunSync is called.
  def forkAsync(toRun: () => Unit)(executor: ExecutorService): IO[Unit] = IO.async[Unit] { callback: CallbackType =>
    // "callback" is a function that either takes a throwable (Left) or whatever toRun returns (Right).
    println("LalalaAsync: " + Thread.currentThread().getName)
    executor.execute(new Runnable {
      def run(): Unit = {
        val nothing: Unit = toRun() // Note: This line executes the body and returns nothing, which is of type Unit.
        try {
          callback(Right(nothing)) // On success, the callback returns nothing
        } catch {
          case NonFatal(t) => callback(Left(t)) // On failure, it returns an exception
        }
      }
    })
  }

  def forkSync(toRun: () => Unit)(executor: ExecutorService): IO[Unit] = IO.apply {
    println("LalalaSync: " + Thread.currentThread().getName)
    executor.execute(new Runnable {
      def run(): Unit = {
        toRun()
      }
    })
  }

  val treadPool: ExecutorService = Executors.newSingleThreadExecutor()
  val mainThread: Thread = Thread.currentThread()

  val Global: ExecutionContextExecutor = ExecutionContext.global

  /*
  Output:
    1 Hello World printed synchronously from Main.main
    LalalaSync: scala-execution-context-global-12
    Hello World printed synchronously from thread pool.pool-1-thread-1
    LalalaAsync: scala-execution-context-global-12
    Hello World printed asynchronously from thread pool.pool-1-thread-1
    2 Hello World printed synchronously from Global .scala-execution-context-global-12
   */
  val program = for {
    _ <- IO {
      println("1 Hello World printed synchronously from Main." + Thread.currentThread().getName) // "main" thread
    }
    _ <- IO.shift(Global) // Shift to Global Execution Context
    _ <- forkSync { () =>
      println("Hello World printed synchronously from thread pool." + Thread.currentThread().getName) // "pool-1-thread-1" thread
    }(treadPool)
    _ <- forkAsync { () =>
      println("Hello World printed asynchronously from thread pool." + Thread.currentThread().getName) // "pool-1-thread-1" thread
    }(treadPool)
    _ <- IO.shift(Global) // Shift to Global Execution Context
    _ <- IO {
      println("2 Hello World printed synchronously from Global ." + Thread.currentThread().getName) // "scala-execution-context-global-13" thread
    }
  } yield ()

  program.unsafeRunSync()
}

要运行它，您需要添加：

libraryDependencies ++= Seq(
  "org.typelevel" %% "cats" % "0.9.0",
  "org.typelevel" %% "cats-effect" % "0.3"
),

到你的 build.sbt 文件。

注意输出：

  /*
  Output:
    1 Hello World printed synchronously from Main.main
    LalalaSync: scala-execution-context-global-12
    Hello World printed synchronously from thread pool.pool-1-thread-1
    LalalaAsync: scala-execution-context-global-12
    Hello World printed asynchronously from thread pool.pool-1-thread-1
    2 Hello World printed synchronously from Global .scala-execution-context-global-12
 */

基本上，我不明白 IO.shift(Global) 或 IO.async 是如何工作的。

比如为什么我调用了“forkAsync”之后，如果我不调用“IO.shift(Global)”，后面的同步IO对象都运行在“pool-1-thread-1”中。另外，这个例子中的 forkAsync 和 forkSync 有什么区别？它们都从 ExecutionContext.global 开始，然后在“pool.pool-1-thread-1”中执行一个 Runnable。

就像 forkAsync 和 forkSync 做完全相同的事情还是 forkAsync 做不同的事情？如果他们在做同样的事情，那么在 IO.async 中包装代码有什么意义？如果他们不做同样的事情，他们有什么不同？

Answer 1

比如为什么我调用了“forkAsync”之后，如果我不调用“IO.shift(Global)”，后面的同步IO对象都运行在“pool-1-thread-1”中。

更重要的问题是你为什么期望它在全局上评估“后续同步 IO 对象”？

IO 内部没有线程池的概念，它不知道global，因此它无法切换回您的默认线程池，因此您确实需要触发手动切换。

升级到最新版本1.0.0 并且您在ContextShift 中也有evalOn，它将在指定的线程池上执行IO 操作，然后切换回您的“全局”，我想这是你想要什么。

另外，这个例子中的 forkAsync 和 forkSync 有什么区别？

您的forkSync 触发Runnable 的执行，但不等待其完成。这是一场火灾，忘记了。这意味着后续的连锁动作不会产生反压力。

一些建议：

1. 升级到最新版本 (1.0.0)
2. 阅读文档：https://typelevel.org/cats-effect/datatypes/io.html