1 /*
2  * Copyright 2024 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 <mutex>
21 
22 #include <android-base/thread_annotations.h>
23 
24 namespace android::utils {
25 
26 // Allows a thread to `wait` for a future produced by a different thread. The future is returned by
27 // the first call to a function `F` that multiple threads may `callOnce`. If no `callOnce` happens,
28 // then `wait` does nothing. Otherwise, it blocks on the future, then destroys it, which resets the
29 // `OnceFuture`.
30 class OnceFuture {
31 public:
32     template <typename F>
callOnce(F f)33     void callOnce(F f) {
34         std::lock_guard lock(mMutex);
35         if (!mFuture.valid()) {
36             mFuture = f();
37         }
38     }
39 
wait()40     void wait() {
41         std::lock_guard lock(mMutex);
42         if (mFuture.valid()) {
43             mFuture.wait();
44             mFuture = {};
45         }
46     }
47 
48 private:
49     std::mutex mMutex;
50     std::future<void> mFuture GUARDED_BY(mMutex);
51 };
52 
53 } // namespace android::utils
54