Custom Settings and Tasks

Custom Settings and Tasks

This page gets you started creating your own settings and tasks.

To understand this page, be sure you've read earlier pages in the Getting Started Guide, especially build.sbt and more about settings.

Defining a key

Keys is packed with examples illustrating how to define keys. Most of the keys are implemented in Defaults.

Keys have one of three types. SettingKey and TaskKey are described in .sbt build definition. Read about InputKey on the Input Tasks page.

Some examples from Keys:

val scalaVersion = settingKey[String]("The version of Scala used for building.")
val clean = taskKey[Unit]("Deletes files produced by the build, such as generated sources, compiled classes, and task caches.")

The key constructors have two string parameters: the name of the key ("scalaVersion") and a documentation string ("The version of scala used for building.").

Remember from .sbt build definition that the type parameter T in SettingKey[T] indicates the type of value a setting has. T in TaskKey[T] indicates the type of the task's result. Also remember from .sbt build definition that a setting has a fixed value until project reload, while a task is re-computed for every "task execution" (every time someone types a command at the sbt interactive prompt or in batch mode).

Keys may be defined in a .sbt file, .scala file, or in a plugin. Any val found in a Build object in your .scala build definition files or any val found in a Plugin object from a plugin will be imported automatically into your .sbt files.

Implementing a task

Once you've defined a key for your task, you'll need to complete it with a task definition. You could be defining your own task, or you could be planning to redefine an existing task. Either way looks the same; use := to associate some code with the task key:

val sampleStringTask = taskKey[String]("A sample string task.")

val sampleIntTask = taskKey[Int]("A sample int task.")

sampleStringTask := System.getProperty("user.home")

sampleIntTask := {
  val sum = 1 + 2
  println("sum: " + sum)

If the task has dependencies, you'd reference their value using value, as discussed in more about settings.

The hardest part about implementing tasks is often not sbt-specific; tasks are just Scala code. The hard part could be writing the "meat" of your task that does whatever you're trying to do. For example, maybe you're trying to format HTML in which case you might want to use an HTML library (you would add a library dependency to your build definition and write code based on the HTML library, perhaps).

sbt has some utility libraries and convenience functions, in particular you can often use the convenient APIs in IO to manipulate files and directories.

Use plugins!

If you find you have a lot of custom code, consider moving it to a plugin for re-use across multiple builds.

It's very easy to create a plugin, as teased earlier and discussed at more length here.