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60 lines
1.9 KiB
60 lines
1.9 KiB
// Copyright 2021 gRPC authors. |
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// |
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// Licensed under the Apache License, Version 2.0 (the "License"); |
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// you may not use this file except in compliance with the License. |
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// You may obtain a copy of the License at |
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// |
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// http://www.apache.org/licenses/LICENSE-2.0 |
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// |
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// Unless required by applicable law or agreed to in writing, software |
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// distributed under the License is distributed on an "AS IS" BASIS, |
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
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// See the License for the specific language governing permissions and |
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// limitations under the License. |
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#ifndef GRPC_CORE_LIB_PROMISE_POLL_H |
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#define GRPC_CORE_LIB_PROMISE_POLL_H |
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#include <grpc/support/port_platform.h> |
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#include <stddef.h> |
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#include "absl/types/variant.h" |
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namespace grpc_core { |
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// A type that signals a Promise is still pending and not yet completed. |
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// Allows writing 'return Pending{}' and with automatic conversions gets |
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// upgraded to a Poll<> object. |
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struct Pending { |
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constexpr bool operator==(Pending) const { return true; } |
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}; |
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// The result of polling a Promise once. |
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// |
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// Can be either pending - the Promise has not yet completed, or ready - |
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// indicating that the Promise has completed AND should not be polled again. |
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template <typename T> |
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using Poll = absl::variant<Pending, T>; |
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// Variant of Poll that serves as a ready value |
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static constexpr size_t kPollReadyIdx = 1; |
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// PollTraits tells us whether a type is Poll<> or some other type, and is |
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// leveraged in the PromiseLike/PromiseFactory machinery to select the |
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// appropriate implementation of those concepts based upon the return type of a |
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// lambda, for example (via enable_if). |
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template <typename T> |
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struct PollTraits { |
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static constexpr bool is_poll() { return false; } |
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}; |
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template <typename T> |
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struct PollTraits<Poll<T>> { |
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using Type = T; |
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static constexpr bool is_poll() { return true; } |
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}; |
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} // namespace grpc_core |
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#endif // GRPC_CORE_LIB_PROMISE_POLL_H
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