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§Dependency Injection with Templates

Twirl templates can be generated as a class rather than a static object by declaring a constructor using a special @this(args) syntax at the top of the template. This means that Twirl templates can be injected into controllers directly and can manage their own dependencies, rather than the controller having to manage dependencies not only for itself, but also for the templates it has to render.

As an example, suppose a template has a dependency on a component Summarizer, which is not used by the controller:

trait Summarizer {
  /** Provide short form of string if over a certain length */
  def summarize(item: String)

Create a file app/views/IndexTemplate.scala.html using the @this syntax for the constructor:

@this(summarizer: Summarizer)
@(item: String)


And finally define the controller in Play by injecting the template in the constructor:

public MyController @Inject()(template: views.html.IndexTemplate, 
                              cc: ControllerComponents) 
  extends AbstractController(cc) {
  def index = Action { implicit request =>
    val item = "some extremely long text"

Once the template is defined with its dependencies, then the controller can have the template injected into the controller, but the controller does not see Summarizer.

If you are using Twirl outside of a Play application, you will have to manually add the @Inject annotation saying that dependency injection should be used here:

TwirlKeys.constructorAnnotations += "@javax.inject.Inject()"

Inside a play application, this is already included in the default settings.

Next: Common use cases