+/*
+ * Copyright 2014 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/wangle/futures/detail/Types.h>
+
+namespace folly { namespace wangle {
+
+template <class> struct Future;
+
+/// A Timekeeper handles the details of keeping time and fulfilling delay
+/// promises. The returned Future<void> will either complete after the
+/// elapsed time, or in the event of some kind of exceptional error may hold
+/// an exception. These Futures respond to cancellation. If you use a lot of
+/// Delays and many of them ultimately are unneeded (as would be the case for
+/// Delays that are used to trigger timeouts of async operations), then you
+/// can and should cancel them to reclaim resources.
+///
+/// Users will typically get one of these via Future::sleep(Duration) or
+/// use them implicitly behind the scenes by passing a timeout to some Future
+/// operation.
+///
+/// Although we don't formally alias Delay = Future<void>,
+/// that's an appropriate term for it. People will probably also call these
+/// Timeouts, and that's ok I guess, but that term is so overloaded I thought
+/// it made sense to introduce a cleaner term.
+///
+/// Remember that Duration is a std::chrono duration (millisecond resolution
+/// at the time of writing).
+class Timekeeper {
+ public:
+ virtual ~Timekeeper() = default;
+
+ /// Returns a future that will complete after the given duration with the
+ /// elapsed time. Exceptional errors can happen but they must be
+ /// exceptional. Use the steady (monotonic) clock.
+ ///
+ /// You may cancel this Future to reclaim resources.
+ ///
+ /// This future probably completes on the timer thread. You should almost
+ /// certainly follow it with a via() call or the accuracy of other timers
+ /// will suffer.
+ virtual Future<void> after(Duration) = 0;
+
+ /// Returns a future that will complete at the requested time.
+ ///
+ /// You may cancel this Future to reclaim resources.
+ ///
+ /// NB This is sugar for `after(when - now)`, so while you are welcome to
+ /// use a std::chrono::system_clock::time_point it will not track changes to
+ /// the system clock but rather execute that many milliseconds in the future
+ /// according to the steady clock.
+ template <class Clock>
+ Future<void> at(std::chrono::time_point<Clock> when);
+};
+
+}}
+
+// now get those definitions
+#include <folly/wangle/futures/Future.h>
+
+// finally we can use Future
+namespace folly { namespace wangle {
+
+ template <class Clock>
+ Future<void> Timekeeper::at(std::chrono::time_point<Clock> when) {
+ auto now = Clock::now();
+
+ if (when <= now) {
+ return makeFuture();
+ }
+
+ return after(when - now);
+ }
+
+}}