how to use setting defined in sbt plugin

build.sbt

// build.sbt in myplugin
sbtPlugin := true

lazy val plugin = (project in file(".")).
  settings(
    name := "myplugin",
    version := "0.1-SNAPSHOT",
    scalaVersion := "2.10.4"
  )

buildInDemo.sbt

// build.sbt in the project that using myplugin
lazy val usage = (project in file("."))
  .enablePlugins(HelloPlugin).
  settings(
    name := "sbt-test",
    version := "0.1",
    scalaVersion := "2.10.4")
  .settings(
    greeting := "Hello"
  )

HelloPlugin.scala

// source in plugin
package sbthello

import sbt._
import Keys._

object HelloPlugin extends AutoPlugin {
  object autoImport {
    val greeting = settingKey[String]("greeting")
    val obfuscate = taskKey[String]("Obfuscates files.")
  }
  import autoImport._
  lazy val baseSettings: Seq[Def.Setting[_]] = Seq(
    greeting := "Hi!",
    obfuscate := {
      println(greeting.value)
      greeting.value + " value"
    }
  )

  override val projectSettings =
    inConfig(Compile)(baseSettings)
}

plugins.sbt

addSbtPlugin("mysite" % "mysite" % "0.1-SNAPSHOT")

problem.md

此时执行的结果

> show obfuscate
Hi!
[info] Hi! value
> show greeting
[info] Hello

如果修改HelloPlugin.scala中的一句为：

from:
greeting := "Hi!",
to:
greeting in obfuscate := "Hi!",

则执行结果变为

> show greeting
[info] Hello
> show obfuscate
Hello
[info] Hello value

符合预期，但是此时如果去掉build.sbt中的greeting := "Hello"，则报错：

[error] References to undefined settings:
[error]
[error]   compile:greeting from compile:obfuscate ((sbthello.HelloPlugin) HelloPlugin.scala:40)
[error]      Did you mean compile:obfuscate::greeting ?
[error]
[error]   compile:greeting from compile:obfuscate ((sbthello.HelloPlugin) HelloPlugin.scala:40)
[error]      Did you mean compile:obfuscate::greeting ?
[error]

problemEn.md

with the files above, the execution result is:

> show obfuscate
Hi!
[info] Hi! value
> show greeting
[info] Hello

the obfuscate task cannot read the "Hello" value

if modify greeting := "Hi!" in HelloPlugin.scala to greeting in obfuscate := "Hi!",

the obfuscate task can read the "Hello" value now

> show greeting
[info] Hello
> show obfuscate
Hello
[info] Hello value

but we cannot remove greeting := "Hello" in buildInDemo.sbt now, or else it will have such errors:

[error] References to undefined settings:
[error]
[error]   compile:greeting from compile:obfuscate ((sbthello.HelloPlugin) HelloPlugin.scala:40)
[error]      Did you mean compile:obfuscate::greeting ?
[error]
[error]   compile:greeting from compile:obfuscate ((sbthello.HelloPlugin) HelloPlugin.scala:40)
[error]      Did you mean compile:obfuscate::greeting ?
[error]

找到答案了，果然很naive：
http://stackoverflow.com/questions/34784300/how-to-read-the-modified-setting-defined-in-my-own-sbt-plugin