Dominik Laskowski | 8e89c2a | 2020-04-27 16:08:19 -0700 | [diff] [blame^] | 1 | /* |
| 2 | * Copyright 2020 The Android Open Source Project |
| 3 | * |
| 4 | * Licensed under the Apache License, Version 2.0 (the "License"); |
| 5 | * you may not use this file except in compliance with the License. |
| 6 | * You may obtain a copy of the License at |
| 7 | * |
| 8 | * http://www.apache.org/licenses/LICENSE-2.0 |
| 9 | * |
| 10 | * Unless required by applicable law or agreed to in writing, software |
| 11 | * distributed under the License is distributed on an "AS IS" BASIS, |
| 12 | * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| 13 | * See the License for the specific language governing permissions and |
| 14 | * limitations under the License. |
| 15 | */ |
| 16 | |
| 17 | #pragma once |
| 18 | |
| 19 | #include <future> |
| 20 | #include <type_traits> |
| 21 | #include <utility> |
| 22 | |
| 23 | namespace android::promise { |
| 24 | namespace impl { |
| 25 | |
| 26 | template <typename T> |
| 27 | struct FutureResult { |
| 28 | using Type = T; |
| 29 | }; |
| 30 | |
| 31 | template <typename T> |
| 32 | struct FutureResult<std::future<T>> { |
| 33 | using Type = T; |
| 34 | }; |
| 35 | |
| 36 | } // namespace impl |
| 37 | |
| 38 | template <typename T> |
| 39 | using FutureResult = typename impl::FutureResult<T>::Type; |
| 40 | |
| 41 | template <typename... Args> |
| 42 | inline auto defer(Args... args) { |
| 43 | return std::async(std::launch::deferred, std::forward<Args>(args)...); |
| 44 | } |
| 45 | |
| 46 | template <typename T> |
| 47 | inline std::future<T> yield(T&& v) { |
| 48 | return defer([](T&& v) { return std::forward<T>(v); }, std::forward<T>(v)); |
| 49 | } |
| 50 | |
| 51 | template <typename T> |
| 52 | struct Chain { |
| 53 | Chain(std::future<T>&& f) : future(std::move(f)) {} |
| 54 | operator std::future<T>&&() && { return std::move(future); } |
| 55 | |
| 56 | T get() && { return future.get(); } |
| 57 | |
| 58 | template <typename F, typename R = std::invoke_result_t<F, T>> |
| 59 | auto then(F&& op) && -> Chain<FutureResult<R>> { |
| 60 | return defer( |
| 61 | [](auto&& f, F&& op) { |
| 62 | R r = op(f.get()); |
| 63 | if constexpr (std::is_same_v<R, FutureResult<R>>) { |
| 64 | return r; |
| 65 | } else { |
| 66 | return r.get(); |
| 67 | } |
| 68 | }, |
| 69 | std::move(future), std::forward<F>(op)); |
| 70 | } |
| 71 | |
| 72 | std::future<T> future; |
| 73 | }; |
| 74 | |
| 75 | template <typename T> |
| 76 | inline Chain<T> chain(std::future<T>&& f) { |
| 77 | return std::move(f); |
| 78 | } |
| 79 | |
| 80 | } // namespace android::promise |