code: /*
   * Copyright (c) 2016-present, RxJava Contributors.
   *
   * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
   * compliance with the License. You may obtain a copy of the License at
   *
   * http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software distributed under the License is
   * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
   * the License for the specific language governing permissions and limitations under the License.
   */
  
  package io.reactivex.rxjava3.core;
  
  import io.reactivex.rxjava3.annotations.NonNull;
  
  /**
   * Interface to map/wrap a downstream {@link Observer} to an upstream {@code Observer}.
   *
   * @param <Downstream> the value type of the downstream
   * @param <Upstream> the value type of the upstream
   */
  @FunctionalInterface
  public interface ObservableOperator<@NonNull Downstream, @NonNull Upstream> {
      /**
       * Applies a function to the child {@link Observer} and returns a new parent {@code Observer}.
       * @param observer the child {@code Observer} instance
       * @return the parent {@code Observer} instance
       * @throws Throwable on failure
       */
      @NonNull
      Observer<? super Upstream> apply(@NonNull Observer<? super Downstream> observer) throws Throwable;
  }
  