Firebase transaction sample in Kotlin

transaction.kt

val dbReference: DatabaseReference = firebase.ref("some/db/path")
dbReference.runTransaction(object: Transaction.Handler {
      /**
       * This method will be called once, at completion, with the results of the transaction.
       *
       * @param error null if no errors occurred, otherwise it contains a description of the error
       * @param committed True if the transaction successfully completed, false if it was aborted or
       *                  an error occurred
       * @param currentData The current data at the location
       */
      override fun onComplete(error: DatabaseError?, committed: Boolean, currentData: DataSnapshot?) {
        // completion function
      }

      /**
       * This method will be called, possibly multiple times, with the current data at this location. It is
       * responsible for inspecting that data and returning a [Result] specifying either the desired new data at the
       * location or that the transaction should be aborted.
       *
       * Since this method may be called repeatedly for the same transaction, be extremely careful of any side effects
       * that may be triggered by this method. In addition, this method is called from within the Firebase Database
       * library's run loop, so care is also required when accessing data that may be in use by other threads in your
       * application.
       *
       * Best practices for this method are to rely only on the data that is passed in.
       *
       * @param target The current data at the location. Update this to the desired data at the location.
       * @return Either the new data, or an indication to abort the transaction
       */
      override fun doTransaction(target: MutableData): Transaction.Result {
        // ... transaction logic ...
      }
})