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§Action composition

This chapter introduces several ways of defining generic action functionality.

§Custom action builders

We saw previously that there are multiple ways to declare an action - with a request parameter, without a request parameter, with a body parser etc. In fact there are more than this, as we’ll see in the chapter on asynchronous programming.

These methods for building actions are actually all defined by a trait called ActionBuilder and the Action object that we use to declare our actions is just an instance of this trait. By implementing your own ActionBuilder, you can declare reusable action stacks, that can then be used to build actions.

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

The first way is to implement this functionality in the invokeBlock method, which is called for every action built by the ActionBuilder:

import play.api.mvc._

object LoggingAction extends ActionBuilder[Request] {
  def invokeBlock[A](request: Request[A], block: (Request[A]) => Future[Result]) = {
    Logger.info("Calling action")
    block(request)
  }
}

Now we can use it the same way we use Action:

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

Since ActionBuilder provides all the different methods of building actions, this also works with, for example, declaring a custom body parser:

def submit = LoggingAction(parse.text) { request =>
  Ok("Got a body " + request.body.length + " bytes long")
}

§Composing actions

In most applications, we will want to have multiple action builders, some that do different types of authentication, some that provide different types of generic functionality, etc. In which case, we won’t want to rewrite our logging action code for each type of action builder, we will want to define it in a reuseable way.

Reusable action code can be implemented by wrapping actions:

import play.api.mvc._

case class Logging[A](action: Action[A]) extends Action[A] {

  def apply(request: Request[A]): Future[Result] = {
    Logger.info("Calling action")
    action(request)
  }

  lazy val parser = action.parser
}

We can also use the Action action builder to build actions without defining our own action class:

import play.api.mvc._

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

Actions can be mixed in to action builders using the composeAction method:

object LoggingAction extends ActionBuilder[Request] {
  def invokeBlock[A](request: Request[A], block: (Request[A]) => Future[Result]) = {
    block(request)
  }
  override def composeAction[A](action: Action[A]) = new Logging(action)
}

Now the builder can be used in the same way as before:

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

We can also mix in wrapping actions without the action builder:

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

§More complicated actions

So far we’ve only shown actions that don’t impact the request at all. Of course, we can also read and modify the incoming request object:

import play.api.mvc._

def xForwardedFor[A](action: Action[A]) = Action.async(action.parser) { request =>
  val newRequest = request.headers.get("X-Forwarded-For").map { xff =>
    new WrappedRequest[A](request) {
      override def remoteAddress = xff
    }
  } getOrElse request
  action(newRequest)
}

Note: Play already has built in support for X-Forwarded-For headers.

We could block the request:

import play.api.mvc._

def onlyHttps[A](action: Action[A]) = Action.async(action.parser) { request =>
  request.headers.get("X-Forwarded-Proto").collect {
    case "https" => action(request)
  } getOrElse {
    Future.successful(Forbidden("Only HTTPS requests allowed"))
  }
}

And finally we can also modify the returned result:

import play.api.mvc._
import play.api.libs.concurrent.Execution.Implicits._

def addUaHeader[A](action: Action[A]) = Action.async(action.parser) { request =>
  action(request).map(_.withHeaders("X-UA-Compatible" -> "Chrome=1"))
}

§Different request types

While action composition allows you to perform additional processing at the HTTP request and response level, often you want to build pipelines of data transformations that add context to or perform validation on the request itself. ActionFunction can be thought of as a function on the request, parameterized over both the input request type and the output type passed on to the next layer. Each action function may represent modular processing such as authentication, database lookups for objects, permission checks, or other operations that you wish to compose and reuse across actions.

There are a few pre-defined traits implementing ActionFunction that are useful for different types of processing:

ActionTransformer can change the request, for example by adding additional information.
ActionFilter can selectively intercept requests, for example to produce errors, without changing the request value.
ActionRefiner is the general case of both of the above.
ActionBuilder is the special case of functions that take Request as input, and thus can build actions.

You can also define your own arbitrary ActionFunction by implementing the invokeBlock method. Often it is convenient to make the input and output types instances of Request (using WrappedRequest), but this is not strictly necessary.

§Authentication

One of the most common use cases for action functions is authentication. We can easily implement our own authentication action transformer that determines the user from the original request and adds it to a new UserRequest. Note that this is also an ActionBuilder because it takes a simple Request as input:

import play.api.mvc._

class UserRequest[A](val username: Option[String], request: Request[A]) extends WrappedRequest[A](request)

object UserAction extends
    ActionBuilder[UserRequest] with ActionTransformer[Request, UserRequest] {
  def transform[A](request: Request[A]) = Future.successful {
    new UserRequest(request.session.get("username"), request)
  }
}

Play also provides a built in authentication action builder. Information on this and how to use it can be found here.

Note: The built in authentication action builder is just a convenience helper to minimise the code necessary to implement authentication for simple cases, its implementation is very similar to the example above.

If you have more complex requirements than can be met by the built in authentication action, then implementing your own is not only simple, it is recommended.

§Adding information to requests

Now let’s consider a REST API that works with objects of type Item. There may be many routes under the /item/:itemId path, and each of these need to look up the item. In this case, it may be useful to put this logic into an action function.

First of all, we’ll create a request object that adds an Item to our UserRequest:

import play.api.mvc._

class ItemRequest[A](val item: Item, request: UserRequest[A]) extends WrappedRequest[A](request) {
  def username = request.username
}

Now we’ll create an action refiner that looks up that item and returns Either an error (Left) or a new ItemRequest (Right). Note that this action refiner is defined inside a method that takes the id of the item:

def ItemAction(itemId: String) = new ActionRefiner[UserRequest, ItemRequest] {
  def refine[A](input: UserRequest[A]) = Future.successful {
    ItemDao.findById(itemId)
      .map(new ItemRequest(_, input))
      .toRight(NotFound)
  }
}

§Validating requests

Finally, we may want an action function that validates whether a request should continue. For example, perhaps we want to check whether the user from UserAction has permission to access the item from ItemAction, and if not return an error:

object PermissionCheckAction extends ActionFilter[ItemRequest] {
  def filter[A](input: ItemRequest[A]) = Future.successful {
    if (!input.item.accessibleByUser(input.username))
      Some(Forbidden)
    else
      None
  }
}

§Putting it all together

Now we can chain these action functions together (starting with an ActionBuilder) using andThen to create an action:

def tagItem(itemId: String, tag: String) =
  (UserAction andThen ItemAction(itemId) andThen PermissionCheckAction) { request =>
    request.item.addTag(tag)
    Ok("User " + request.username + " tagged " + request.item.id)
  }

Play also provides a global filter API , which is useful for global cross cutting concerns.

Next: Content negotiation