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;
  
  /**
   * Represents a basic, non-backpressured {@link Observable} source base interface,
   * consumable via an {@link Observer}.
   *
   * @param <T> the element type
   * @since 2.0
   */
  @FunctionalInterface
  public interface ObservableSource<@NonNull T> {
  
      /**
       * Subscribes the given {@link Observer} to this {@link ObservableSource} instance.
       * @param observer the {@code Observer}, not {@code null}
       * @throws NullPointerException if {@code observer} is {@code null}
       */
      void subscribe(@NonNull Observer<? super T> observer);
  }
  