FutureExecutor.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 <folly/functional/Invoke.h>
#include <folly/futures/Future.h>

namespace folly {

template <typename ExecutorImpl>
class FutureExecutor : public ExecutorImpl {
 public:
  template <typename... Args>
  explicit FutureExecutor(Args&&... args)
      : ExecutorImpl(std::forward<Args>(args)...) {}

  /*
   * Given a function func that returns a Future<T>, adds that function to the
   * contained Executor and returns a Future<T> which will be fulfilled with
   * func's result once it has been executed.
   *
   * For example: auto f = futureExecutor.addFuture([](){
   *                return doAsyncWorkAndReturnAFuture();
   *              });
   */
  template <typename F>
  typename std::enable_if<
      folly::isFuture<invoke_result_t<F>>::value,
      invoke_result_t<F>>::type
  addFuture(F func) {
    using T = typename invoke_result_t<F>::value_type;
    folly::Promise<T> promise;
    auto future = promise.getFuture();
    ExecutorImpl::add([promise = std::move(promise),
                       func = std::move(func)]() mutable {
      func().then([promise = std::move(promise)](folly::Try<T>&& t) mutable {
        promise.setTry(std::move(t));
      });
    });
    return future;
  }

  /*
   * Similar to addFuture above, but takes a func that returns some non-Future
   * type T.
   *
   * For example: auto f = futureExecutor.addFuture([]() {
   *                return 42;
   *              });
   */
  template <typename F>
  typename std::enable_if<
      !folly::isFuture<invoke_result_t<F>>::value,
      folly::Future<typename folly::lift_unit<invoke_result_t<F>>::type>>::type
  addFuture(F func) {
    using T = typename folly::lift_unit<invoke_result_t<F>>::type;
    folly::Promise<T> promise;
    auto future = promise.getFuture();
    ExecutorImpl::add(
        [promise = std::move(promise), func = std::move(func)]() mutable {
          promise.setWith(std::move(func));
        });
    return future;
  }
};

} // namespace folly