SF: Add promise/future utilities
promise::defer and promise::yield return a deferred future given a
lambda or value, respectively. promise::chain enables composition
of sequential futures.
Bug: 123715322
Bug: 134772048
Test: PromiseTest
Change-Id: Iff81407d60a357d76b5863c4a3a8af7760bd8c33
diff --git a/services/surfaceflinger/Promise.h b/services/surfaceflinger/Promise.h
new file mode 100644
index 0000000..a80d441
--- /dev/null
+++ b/services/surfaceflinger/Promise.h
@@ -0,0 +1,80 @@
+/*
+ * Copyright 2020 The Android Open Source Project
+ *
+ * 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 <future>
+#include <type_traits>
+#include <utility>
+
+namespace android::promise {
+namespace impl {
+
+template <typename T>
+struct FutureResult {
+ using Type = T;
+};
+
+template <typename T>
+struct FutureResult<std::future<T>> {
+ using Type = T;
+};
+
+} // namespace impl
+
+template <typename T>
+using FutureResult = typename impl::FutureResult<T>::Type;
+
+template <typename... Args>
+inline auto defer(Args... args) {
+ return std::async(std::launch::deferred, std::forward<Args>(args)...);
+}
+
+template <typename T>
+inline std::future<T> yield(T&& v) {
+ return defer([](T&& v) { return std::forward<T>(v); }, std::forward<T>(v));
+}
+
+template <typename T>
+struct Chain {
+ Chain(std::future<T>&& f) : future(std::move(f)) {}
+ operator std::future<T>&&() && { return std::move(future); }
+
+ T get() && { return future.get(); }
+
+ template <typename F, typename R = std::invoke_result_t<F, T>>
+ auto then(F&& op) && -> Chain<FutureResult<R>> {
+ return defer(
+ [](auto&& f, F&& op) {
+ R r = op(f.get());
+ if constexpr (std::is_same_v<R, FutureResult<R>>) {
+ return r;
+ } else {
+ return r.get();
+ }
+ },
+ std::move(future), std::forward<F>(op));
+ }
+
+ std::future<T> future;
+};
+
+template <typename T>
+inline Chain<T> chain(std::future<T>&& f) {
+ return std::move(f);
+}
+
+} // namespace android::promise