FilterOnly.scala

import play.api.mvc.{ RequestHeader, EssentialAction, EssentialFilter }

/**
 * Apply underlying filter only when f returns true.
 */
class FilterOnly(underlying: EssentialFilter, f: RequestHeader => Boolean) extends EssentialFilter {
  override def apply(next: EssentialAction): EssentialAction = new EssentialAction {
    private[this] val filteredNext = underlying(next)
    override def apply(rh: RequestHeader) = if (f(rh)) filteredNext(rh) else next(rh)
  }
}

object FilterOnly {
  implicit class EssentialFilterFilterOnlyImplicit(filter: EssentialFilter) {
    def only(f: RequestHeader => Boolean) = new FilterOnly(filter, f)
  }
}