/*
 * Copyright 2019, gRPC Authors All rights reserved.
 *
 * 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.
 */
import Foundation
import NIO
import NIOHTTP1
import NIOHTTP2
import SwiftProtobuf

/// Base protocol for a client call to a gRPC service.
public protocol ClientCall {
  /// The type of the request message for the call.
  associatedtype RequestMessage: Message
  /// The type of the response message for the call.
  associatedtype ResponseMessage: Message

  /// HTTP/2 stream that requests and responses are sent and received on.
  var subchannel: EventLoopFuture<Channel> { get }

  /// Initial response metadata.
  var initialMetadata: EventLoopFuture<HTTPHeaders> { get }

  /// Status of this call which may be populated by the server or client.
  ///
  /// The client may populate the status if, for example, it was not possible to connect to the service.
  ///
  /// Note: despite `GRPCStatus` being an `Error`, the value will be __always__ delivered as a __success__
  /// result even if the status represents a __negative__ outcome. This future will __never__ be fulfilled
  /// with an error.
  var status: EventLoopFuture<GRPCStatus> { get }

  /// Trailing response metadata.
  ///
  /// This is the same metadata as `GRPCStatus.trailingMetadata` returned by `status`.
  var trailingMetadata: EventLoopFuture<HTTPHeaders> { get }

  /// Cancel the current call.
  ///
  /// Closes the HTTP/2 stream once it becomes available. Additional writes to the channel will be ignored.
  /// Any unfulfilled promises will be failed with a cancelled status (excepting `status` which will be
  /// succeeded, if not already succeeded).
  func cancel()
}

/// A `ClientCall` with request streaming; i.e. client-streaming and bidirectional-streaming.
public protocol StreamingRequestClientCall: ClientCall {
  /// Sends a message to the service.
  ///
  /// - Important: Callers must terminate the stream of messages by calling `sendEnd()` or `sendEnd(promise:)`.
  ///
  /// - Parameter message: The message to send.
  /// - Returns: A future which will be fullfilled when the message has been sent.
  func sendMessage(_ message: RequestMessage) -> EventLoopFuture<Void>

  /// Sends a message to the service.
  ///
  /// - Important: Callers must terminate the stream of messages by calling `sendEnd()` or `sendEnd(promise:)`.
  ///
  /// - Parameters:
  ///   - message: The message to send.
  ///   - promise: A promise to be fulfilled when the message has been sent.
  func sendMessage(_ message: RequestMessage, promise: EventLoopPromise<Void>?)

  /// Returns a future which can be used as a message queue.
  ///
  /// Callers may use this as such:
  /// ```
  /// var queue = call.newMessageQueue()
  /// for message in messagesToSend {
  ///   queue = queue.then { call.sendMessage(message) }
  /// }
  /// ```
  ///
  /// - Returns: A future which may be used as the head of a message queue.
  func newMessageQueue() -> EventLoopFuture<Void>

  /// Terminates a stream of messages sent to the service.
  ///
  /// - Important: This should only ever be called once.
  /// - Returns: A future which will be fullfilled when the end has been sent.
  func sendEnd() -> EventLoopFuture<Void>

  /// Terminates a stream of messages sent to the service.
  ///
  /// - Important: This should only ever be called once.
  /// - Parameter promise: A promise to be fulfilled when the end has been sent.
  func sendEnd(promise: EventLoopPromise<Void>?)
}

/// A `ClientCall` with a unary response; i.e. unary and client-streaming.
public protocol UnaryResponseClientCall: ClientCall {
  /// The response message returned from the service if the call is successful. This may be failed
  /// if the call encounters an error.
  ///
  /// Callers should rely on the `status` of the call for the canonical outcome.
  var response: EventLoopFuture<ResponseMessage> { get }
}