Roll forward to cl/314747126

Signed-off-by: Alexey Polyudov <apolyudov@google.com>
Change-Id: Ie19e006429b138b3768e97dae971a43fdc5ef8bf

cpp/platform/api/settable_future_def.h

@@ -0,0 +1,45 @@
+// Copyright 2020 Google LLC
+//
+// 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
+//
+//     https://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.
+
+#ifndef PLATFORM_API_SETTABLE_FUTURE_DEF_H_
+#define PLATFORM_API_SETTABLE_FUTURE_DEF_H_
+
+#include "platform/api/listenable_future.h"
+#include "platform/exception.h"
+
+namespace location {
+namespace nearby {
+
+// A SettableFuture is a type of Future whose result can be set.
+//
+// https://google.github.io/guava/releases/20.0/api/docs/com/google/common/util/concurrent/SettableFuture.html
+//
+// Platform must implentent non-template static member functions
+//   Ptr<SettableFuture<size_t>> CreateSettableFutureSizeT()
+//   Ptr<SettableFuture<std::shared_ptr<void>>> CreateSettableFuturePtr()
+// in the location::nearby::platform::ImplementationPlatform class.
+template <typename T>
+class SettableFuture : public ListenableFuture<T> {
+ public:
+  ~SettableFuture() override = default;
+
+  virtual bool set(T value) = 0;
+
+  virtual bool setException(Exception exception) = 0;
+};
+
+}  // namespace nearby
+}  // namespace location
+
+#endif  // PLATFORM_API_SETTABLE_FUTURE_DEF_H_