我试图了解猫效应是如何Cancelable
运作的。根据文档,我有以下最小应用程序
import java.util.concurrent.{Executors, ScheduledExecutorService}
import cats.effect._
import cats.implicits._
import scala.concurrent.duration._
object Main extends IOApp {
def delayedTick(d: FiniteDuration)
(implicit sc: ScheduledExecutorService): IO[Unit] = {
IO.cancelable { cb =>
val r = new Runnable {
def run() =
cb(Right(()))
}
val f = sc.schedule(r, d.length, d.unit)
// Returning the cancellation token needed to cancel
// the scheduling and release resources early
val mayInterruptIfRunning = false
IO(f.cancel(mayInterruptIfRunning)).void
}
}
override def run(args: List[String]): IO[ExitCode] = {
val scheduledExecutorService =
Executors.newSingleThreadScheduledExecutor()
for {
x <- delayedTick(1.second)(scheduledExecutorService)
_ <- IO(println(s"$x"))
} yield ExitCode.Success
}
}
当我运行这个:
❯ sbt run
[info] Loading global plugins from /Users/ethan/.sbt/1.0/plugins
[info] Loading settings for project stackoverflow-build from plugins.sbt ...
[info] Loading project definition from /Users/ethan/IdeaProjects/stackoverflow/project
[info] Loading settings for project stackoverflow from build.sbt ...
[info] Set current project to cats-effect-tutorial (in build file:/Users/ethan/IdeaProjects/stackoverflow/)
[info] Compiling 1 Scala source to /Users/ethan/IdeaProjects/stackoverflow/target/scala-2.12/classes ...
[info] running (fork) Main
[info] ()
程序此时就挂起。我有很多问题:
- 为什么程序在 1 秒后挂起而不是终止?
- 我们为什么要设置
mayInterruptIfRunning = false
?取消的全部目的不是中断正在运行的任务吗? - 这是定义的推荐方法
ScheduledExecutorService
吗?我没有在文档中看到示例。 - 该程序等待 1 秒,然后返回
()
(然后意外挂起)。如果我想退回其他东西怎么办?例如,假设我想返回一个字符串,这是一些长时间运行的计算的结果。我将如何从中提取该值IO.cancelable
?似乎困难在于IO.cancelable
返回取消操作,而不是要取消的进程的返回值。
请原谅这篇长文,但这是我的build.sbt
:
name := "cats-effect-tutorial"
version := "1.0"
fork := true
scalaVersion := "2.12.8"
libraryDependencies += "org.typelevel" %% "cats-effect" % "1.3.0" withSources() withJavadoc()
scalacOptions ++= Seq(
"-feature",
"-deprecation",
"-unchecked",
"-language:postfixOps",
"-language:higherKinds",
"-Ypartial-unification")