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package xitrum.view

import scala.util.control.NonFatal
import xitrum.{Action, Config, DualConfig, Log}

/**
 * Template engines should extend this trait and implement its methods.
 * On startup, an instance of the configured template engine is created and used
 * for every rendering request. Thus it should be thread-safe.
 */
trait TemplateEngine {
  /**
   * Called once when Xitrum server starts. If necessary the template engine
   * should allocate its resources here.
   */
  def start(): Unit

  /**
   * Called once when Xitrum server stops. If necessary the template engine
   * should release its resources here.
   */
  def stop(): Unit

  /**
   * Renders the template at ``uri``.
   *
   * @param uri URI to the template
   *
   * @param options specific to the configured template engine
   */
  def renderTemplate(uri: String, currentAction: Action, options: Map[String, Any]): String
}

object TemplateEngine {
  /**
   * Template config in xitrum.conf can be in 2 forms:
   *
   * template = my.template.Engine
   *
   * Or if the template engine needs additional options:
   *
   * template {
   *   "my.template.Engine" {
   *     option1 = value1
   *     option2 = value2
   *   }
   * }
   */
  def loadFromConfig(): Option[TemplateEngine] = {
    if (Config.xitrum.config.hasPath("template")) {
      try {
        val engine = DualConfig.getClassInstance[TemplateEngine](Config.xitrum.config, "template")
        engine.start()
        Some(engine)
      } catch {
        case NonFatal(e) =>
          Config.exitOnStartupError("Could not load template engine, please check config/xitrum.conf", e)
          None
      }
    } else {
      Log.info("No template engine is configured")
      None
    }
  }
}