Scala Cats效果 - IO异步转换 - 它是如何工作的？

Question

以下是使用IO Monad的一些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（全局）或IO.async如何工作。

例如，为什么在我调用“forkAsync”之后，如果我不调用“IO.shift（Global）”，则后续的同步IO对象将在“pool-1-thread-1”中运行。 另外，在这个例子中，forkAsync和forkSync有什么区别？ 它们都在ExecutionContext.global中启动，然后在“pool.pool-1-thread-1”中执行Runnable。

就像forkAsync和forkSync做同样的事情或者forkAsync做一些不同的事情？ 如果他们正在做同样的事情，那么在IO.async中包装代码有什么意义呢？ 如果他们没有做同样的事情，他们有什么不同？

Answer 1

例如，为什么在我调用“forkAsync”之后，如果我不调用“IO.shift（Global）”，则后续的同步IO对象将在“pool-1-thread-1”中运行。

更重要的问题是为什么你期望它评估全球的“后续同步IO对象”？

IO内部没有线程池的概念，它不了解global ，因此它无法转移回默认的线程池，因此您需要触发手动转换。

升级到最新版本1.0.0 ，你还在ContextShift中有evalOn ，它将在指定的线程池上执行IO操作，然后切换回你的“全局”，我想这就是你想要的。

另外，在这个例子中，forkAsync和forkSync有什么区别？

forkSync触发Runnable的执行，但不等待其完成。 这是一场火灾，忘记了。 这意味着后续的链式操作不会进行反压。

一些忠告：

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