Add task to Build.scala - sbt

Question

The document http://www.scala-sbt.org/0.13.0/docs/Detailed-Topics/Tasks.html explains how to add a task to build.sbt, but how do you add one to build.scala? Thanks

Answer 1

The part where you declare the TaskKey is the same in either format: val myTask = taskKey....
The part where you write out your Initialize[Task[T]] is the same: myTask := ....
The only difference is the context in which the latter thing appears.
In the .sbt format, it appears by itself, separated from other things by blank lines.
In the .scala format, you have to add the setting to the project. That's documented at http://www.scala-sbt.org/release/docs/Getting-Started/Full-Def.html and is the same regardless of whether we're talking about a task or a regular setting.
Here's a complete working example:
import sbt._
object MyBuild extends Build {
val myTask = taskKey[Unit]("...")
lazy val root =
Project(id = "MyProject", base = file("."))
.settings(
myTask := { println("hello") }
)
}

Answer 2

When defining a task in one project of a multi project build and using a "root" project to aggregate other projects, the aggregation means that any tasks in subprojects can be run from the root project as well, in fact this will run the tasks in all subprojects - see Multi-project builds. This is generally useful, for example to compile all subprojects when the compile task is run from the root project. However in this case it is a little confusing.
So the task is not accessible in all projects, but is accessible in both the subproject where you define the task, and the aggregating (root) project. The task is still running in the project where it is defined, it can just be called when in the root project.
To demonstrate this, we can have the same "hello" task defined in multiple sub projects, which are aggregated in a root project:
import sbt._
import Keys._
object Build extends Build {
val hwsettings = Defaults.defaultSettings ++ Seq(
organization := "organization",
version := "0.1",
scalaVersion := "2.10.4"
)
val hello = TaskKey[Unit]("hello", "Prints hello.")
lazy val projectA = Project(
"a",
file("a"),
settings = hwsettings ++ Seq(
hello := {println("Hello from project A!")}
))
lazy val projectB = Project(
"b",
file("b"),
settings = hwsettings ++ Seq(
hello := {println("Hello from project B!")}
))
lazy val projectC = Project(
"c",
file("c"),
settings = hwsettings
)
lazy val root = Project (
"root",
file ("."),
settings = hwsettings
) aggregate (projectA, projectB, projectC)
}
Note that projects a and b have "hello" tasks, and c does not. When we use "hello" from the root project, the aggregation causes the task to run in projects a AND b:
> project root
[info] Set current project to root (in build file:/Users/trepidacious/temp/multiProjectTasks/)
> hello
Hello from project A!
Hello from project B!
[success] Total time: 0 s, completed 24-Dec-2014 23:00:23
We can also switch to project a or b, and running hello will only run the task in the project we are in:
> project a
[info] Set current project to a (in build file:/Users/trepidacious/temp/multiProjectTasks/)
> hello
Hello from project A!
[success] Total time: 0 s, completed 24-Dec-2014 23:00:27
> project b
[info] Set current project to b (in build file:/Users/trepidacious/temp/multiProjectTasks/)
> hello
Hello from project B!
[success] Total time: 0 s, completed 24-Dec-2014 23:00:30
Finally, if we switch to project c, hello is not defined:
> project c
[info] Set current project to c (in build file:/Users/trepidacious/temp/multiProjectTasks/)
> hello
[error] Not a valid command: hello (similar: shell, help, reload)
[error] No such setting/task
[error] hello
[error] ^
>
This aggregation can be disabled, as described here.