关于#scala#的问题：scala编程

Question

  val cogroupRDD2 = cogroupRDD.mapValues {
    case (viewIter, cartIter, payIter) => {
   
      var viewCnt = 0
      val iter1 = viewIter.iterator
      if (iter1.hasNext) {
        viewCnt = iter1.next()
      }

      var cartCnt = 0
      val iter2 = cartIter.iterator
      if (iter2.hasNext) {
        cartCnt = iter2.next()
      }

      var payCnt = 0
      val iter3 = payIter.iterator
      if (iter3.hasNext) {
        payCnt = iter3.next()
     }
      (viewCnt, cartCnt, payCnt)
    }
  }

这里case是啥意思，求问