src/objective-c/GRPCClient/GRPCCall.h

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// The gRPC protocol is an RPC protocol on top of HTTP2.
//
// While the most common type of RPC receives only one request message and returns only one response
// message, the protocol also supports RPCs that return multiple individual messages in a streaming
// fashion, RPCs that accept a stream of request messages, or RPCs with both streaming requests and
// responses.
//
// Conceptually, each gRPC call consists of a bidirectional stream of binary messages, with RPCs of
// the "non-streaming type" sending only one message in the corresponding direction (the protocol
// doesn't make any distinction).
//
// Each RPC uses a different HTTP2 stream, and thus multiple simultaneous RPCs can be multiplexed
// transparently on the same TCP connection.

#import <Foundation/Foundation.h>
#import <RxLibrary/GRXWriter.h>

// Key used in |NSError|'s |userInfo| dictionary to store the response metadata sent by the server.
extern id const kGRPCStatusMetadataKey;

// Represents a single gRPC remote call.
@interface GRPCCall : GRXWriter

// These HTTP headers will be passed to the server as part of this call. Each HTTP header is a
// name-value pair with string names and either string or binary values.
//
// The passed dictionary has to use NSString keys, corresponding to the header names. The
// value associated to each can be a NSString object or a NSData object. E.g.:
//
// call.requestMetadata = @{@"Authorization": @"Bearer ..."};
//
// call.requestMetadata[@"SomeBinaryHeader"] = someData;
//
// After the call is started, modifying this won't have any effect.
//
// For convenience, the property is initialized to an empty NSMutableDictionary, and the setter
// accepts (and copies) both mutable and immutable dictionaries.
- (NSMutableDictionary *)requestMetadata; // nonatomic
- (void)setRequestMetadata:(NSDictionary *)requestMetadata; // nonatomic, copy

// This dictionary is populated with the HTTP headers received from the server. When the RPC ends,
// the HTTP trailers received are added to the dictionary too. It has the same structure as the
// request metadata dictionary.
//
// The first time this object calls |writeValue| on the writeable passed to |startWithWriteable|,
// the |responseMetadata| dictionary already contains the response headers. When it calls
// |writesFinishedWithError|, the dictionary contains both the response headers and trailers.
@property(atomic, readonly) NSDictionary *responseMetadata;

// The request writer has to write NSData objects into the provided Writeable. The server will
// receive each of those separately and in order.
// A gRPC call might not complete until the request writer finishes. On the other hand, the
// request finishing doesn't necessarily make the call to finish, as the server might continue
// sending messages to the response side of the call indefinitely (depending on the semantics of
// the specific remote method called).
// To finish a call right away, invoke cancel.
- (instancetype)initWithHost:(NSString *)host
                        path:(NSString *)path
              requestsWriter:(GRXWriter *)requestsWriter NS_DESIGNATED_INITIALIZER;

// Finishes the request side of this call, notifies the server that the RPC
// should be cancelled, and finishes the response side of the call with an error
// of code CANCELED.
- (void)cancel;

// TODO(jcanizales): Let specify a deadline. As a category of GRXWriter?
@end