Play 2.4.x how use Messages and not MessagesApi I18N

Question

I've done a little bit different form the internationalization guide, at the play I18N guide

I forced the result to the language in the query string, it works, but it needs to be done in the "correct way". It's a good form to keep the work and look for better approach later:

NOTE: I USED THE "MessagesApi" to make it happen.

please see the code below:

package controllers
import play.api._
import play.api.mvc._
import play.api.i18n.I18nSupport
import play.api.i18n.Messages.Implicits._
import play.api.i18n.MessagesApi
import javax.inject.Inject
import play.api.i18n.Lang
import play.api.i18n._
class Application  @Inject() ( val messagesApi: MessagesApi) extends   Controller with I18nSupport {

def index = Action {  implicit request =>
            request.getQueryString("lang") match{
        case Some(lang) =>  messagesApi.setLang(Ok(views.html.index()(messagesApi,Lang(lang))),Lang(lang))
        case None => messagesApi.setLang(Ok(views.html.index()(messagesApi,Lang("en"))),Lang("en"))
    }
}}

index.scala.html

 @()(implicit message: MessagesApi ,l: Lang)
 <li><a href="./?lang=en"><img src="@routes.Assets.versioned("images/BR.png")" /></a></li>
 <li><a href="./?lang=en"><img src="@routes.Assets.versioned("images/US.gif")" /></a></li>
 <header>
    <h1>@message("intro")</h1>
 </header>
    <p>@Html(message("description"))</p>

conf/application.conf

 play.i18n.langs = [ "en", "pt","fr" ]

Answer 1

If you mix in the I18nSupport trait into your controller then you have an implicit conversion in scope which translates a RequestHeader into a Messages instance. If you look into the request2Messages method then you can see that it calls the MessagesApi.preferred(request: RequestHeader) method.

So in your case you must create a subclass of DefaultMessagesApi and override the MessagesApi.preferred method to retrieve the Lang from query string as currently implemented in your controller. Then you can bind your instance to the MessagesApi trait, so that it gets automatically injected.

To bind your instance you should create your own I18nModule similar to the default one provided by Play.

Note: For Guice Injection only because it's the default method used by Play. For compile time DI you must follow another approach.

package modules

import play.api.i18n._
import play.api.{Configuration, Environment}
import play.api.inject.Module

class I18nModule extends Module {
  def bindings(environment: Environment, configuration: Configuration) = {
    Seq(
      bind[Langs].to[DefaultLangs],
      bind[MessagesApi].to[YourMessagesApi]
    )
  }
}

Then you must disable the default Play I18nModule module and enable yours.

play.modules.disabled += "play.api.i18n.I18nModule"
play.modules.enabled += "modules.I18nModule"

Now in your template you must only pass the implicit Messages instance.

 @()(implicit messages: Messages)