Observer.h

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/*
 * Copyright 2017-present Facebook, Inc.
 *
 * 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.
 */

#pragma once

#include <wangle/deprecated/rx/types.h> // must come first
#include <functional>
#include <memory>
#include <stdexcept>
#include <folly/Memory.h>

namespace wangle {

template <class T> struct FunctionObserver;

template <class T>
struct Observer {
  // These are what it means to be an Observer.
  virtual void onNext(const T&) = 0;
  virtual void onError(Error) = 0;
  virtual void onCompleted() = 0;

  virtual ~Observer() = default;

  template <class N, class E, class C>
  static std::unique_ptr<Observer> create(
    N&& onNextFn, E&& onErrorFn, C&& onCompletedFn)
  {
    return std::make_unique<FunctionObserver<T>>(
      std::forward<N>(onNextFn),
      std::forward<E>(onErrorFn),
      std::forward<C>(onCompletedFn));
  }

  template <class N, class E>
  static std::unique_ptr<Observer> create(N&& onNextFn, E&& onErrorFn) {
    return std::make_unique<FunctionObserver<T>>(
      std::forward<N>(onNextFn),
      std::forward<E>(onErrorFn),
      nullptr);
  }

  template <class N>
  static std::unique_ptr<Observer> create(N&& onNextFn) {
    return std::make_unique<FunctionObserver<T>>(
      std::forward<N>(onNextFn),
      nullptr,
      nullptr);
  }
};

template <class T>
struct FunctionObserver : public Observer<T> {
  typedef std::function<void(const T&)> OnNext;
  typedef std::function<void(Error)> OnError;
  typedef std::function<void()> OnCompleted;

  template <class N = OnNext, class E = OnError, class C = OnCompleted>
  FunctionObserver(N&& n, E&& e, C&& c)
    : onNext_(std::forward<N>(n)),
      onError_(std::forward<E>(e)),
      onCompleted_(std::forward<C>(c))
  {}

  void onNext(const T& val) override {
    if (onNext_) onNext_(val);
  }

  void onError(Error e) override {
    if (onError_) onError_(e);
  }

  void onCompleted() override {
    if (onCompleted_) onCompleted_();
  }

 protected:
  OnNext onNext_;
  OnError onError_;
  OnCompleted onCompleted_;
};

} // namespace wangle