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Action composition

This chapter introduces several ways of defining generic action functionality.

Basic action composition

Let’s start with the simple example of a logging decorator: we want to log each call to this action.

The first way is not to define our own Action, but just to provide a helper method building a standard Action:

def LoggingAction(f: Request[AnyContent] => Result): Action[AnyContent] = {
  Action { request =>
    Logger.info("Calling action")
    f(request)
  }
}

That you can use as:

def index = LoggingAction { request =>
  Ok("Hello World")
}

This is simple but it works only with the default parse.anyContent body parser as we don’t have a way to specify our own body parser. We can of course define an additional helper method:

def LoggingAction[A](bp: BodyParser[A])(f: Request[A] => Result): Action[A] = {
  Action(bp) { request =>
    Logger.info("Calling action")
    f(request)
  }
}

And then:

def index = LoggingAction(parse.text) { request =>
  Ok("Hello World")
}

Wrapping existing actions

Another way is to define our own LoggingAction that would be a wrapper over another Action:

case class Logging[A](action: Action[A]) extends Action[A] {
  
  def apply(request: Request[A]): Result = {
    Logger.info("Calling action")
    action(request)
  }
  
  lazy val parser = action.parser
}

Now you can use it to wrap any other action value:

def index = Logging { 
  Action { 
    Ok("Hello World")
  }
}

Note that it will just re-use the wrapped action body parser as is, so you can of course write:

def index = Logging { 
  Action(parse.text) { 
    Ok("Hello World")
  }
}

Another way to write the same thing but without defining the Logging class, would be:

def Logging[A](action: Action[A]): Action[A] = {
  Action(action.parser) { request =>
    Logger.info("Calling action")
    action(request)
  }
} 

The following example is wrapping an existing action to add session variable:

def addSessionVar[A](action: Action[A]) = Action(action.parser) { request =>
  action(request).withSession("foo" -> "bar")
}

A more complicated example

Let’s look at the more complicated but common example of an authenticated action. The main problem is that we need to pass the authenticated user to the wrapped action.

def Authenticated(action: User => EssentialAction): EssentialAction = {
  
  // Let's define a helper function to retrieve a User
  def getUser(request: RequestHeader): Option[User] = {
    request.session.get("user").flatMap(u => User.find(u))
  }
  
  // Now let's define the new Action
  EssentialAction { request =>
    getUser(request).map(u => action(u)(request)).getOrElse {
      Done(Unauthorized)
    }
  }
  
}

You can use it like this:

def index = Authenticated { user =>
  Action { request =>
    Ok("Hello " + user.name)
  }
}

Note: There is already an Authenticated action in play.api.mvc.Security.Authenticated with a more generic implementation than this example.

In the previous section we said that an Action[A] was a Request[A] => Result function but this is not entirely true. Actually the Action[A] trait is defined as follows:

trait EssentialAction extends (RequestHeader => Iteratee[Array[Byte], Result])

trait Action[A] extends EssentialAction {
  def parser: BodyParser[A]
  def apply(request: Request[A]): Result
  def apply(headers: RequestHeader): Iteratee[Array[Byte], Result] = …
}

An EssentialAction is a function that takes the request headers and gives an Iteratee that will eventually parse the request body and produce a HTTP result. Action[A] implements EssentialAction as follow: it parses the request body using its body parser, gives the built Request[A] object to the action code and returns the action code’s result. An Action[A] can still be thought of as a Request[A] => Result function because it has an apply(request: Request[A]): Result method.

The EssentialAction trait is useful to compose actions with code that requires to read some information from the request headers before parsing the request body.

Our Authenticated implementation above tries to find a user id in the request session and calls the wrapped action with this user if found, otherwise it returns a 401 UNAUTHORIZED status without even parsing the request body.

Another way to create the Authenticated action

Let’s see how to write the previous example without wrapping the whole action:

def Authenticated(f: (User, Request[AnyContent]) => Result) = {
  Action { request =>
    val result = for {
      id <- request.session.get("user")
      user <- User.find(id)
    } yield f(user, request)
    result getOrElse Unauthorized
  }
}

To use this:

def index = Authenticated { (user, request) =>
   Ok("Hello " + user.name)
}

A problem here is that you can’t mark the request parameter as implicit anymore. You can solve that using currying:

def Authenticated(f: User => Request[AnyContent] => Result) = {
  Action { request =>
    val result = for {
      id <- request.session.get("user")
      user <- User.find(id)
    } yield f(user)(request)
    result getOrElse Unauthorized
  }
}

Then you can do this:

def index = Authenticated { user => implicit request =>
   Ok("Hello " + user.name)
}

Another (probably simpler) way is to define our own subclass of Request as AuthenticatedRequest (so we are merging both parameters into a single parameter):

case class AuthenticatedRequest(
  user: User, private val request: Request[AnyContent]
) extends WrappedRequest(request)

def Authenticated(f: AuthenticatedRequest => Result) = {
  Action { request =>
    val result = for {
      id <- request.session.get("user")
      user <- User.find(id)
    } yield f(AuthenticatedRequest(user, request))
    result getOrElse Unauthorized
  }
}

And then:

def index = Authenticated { implicit request =>
   Ok("Hello " + request.user.name)
}

We can of course extend this last example and make it more generic by making it possible to specify a body parser:

case class AuthenticatedRequest[A](
  user: User, private val request: Request[A]
) extends WrappedRequest(request)

def Authenticated[A](p: BodyParser[A])(f: AuthenticatedRequest[A] => Result) = {
  Action(p) { request =>
    val result = for {
      id <- request.session.get("user")
      user <- User.find(id)
    } yield f(Authenticated(user, request))
    result getOrElse Unauthorized
  }
}

// Overloaded method to use the default body parser
import play.api.mvc.BodyParsers._
def Authenticated(f: AuthenticatedRequest[AnyContent] => Result): Action[AnyContent]  = {
  Authenticated(parse.anyContent)(f)
}

Next: Content negotiation