Let set NSDictionary as headers, and init the property (not nil)

src/objective-c/GRPCClient/GRPCCall.h

@@ -40,34 +40,42 @@
 
 // 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.
+// 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).
+// 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.
+// Each RPC uses a different HTTP2 stream, and thus multiple simultaneous RPCs can be multiplexed
+// transparently on the same TCP connection.
 @interface GRPCCall : NSObject<GRXWriter>
 
-// These HTTP2 headers will be passed to the server as part of this call. Each
-// HTTP2 header is a name-value pair with string names and either string or binary values.
+// 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 ...",
-//     @"SomeBinaryHeader": someData
-// };
+// call.requestMetadata = @{@"Authorization": @"Bearer ..."};
+//
+// call.requestMetadata[@"SomeBinaryHeader"] = someData;
 //
 // After the call is started, modifying this won't have any effect.
-@property(nonatomic, readwrite) NSMutableDictionary *requestMetadata;
+//
+// 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 isn't populated until the first event is delivered to the handler.
+// 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.
+//
+// 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

src/objective-c/GRPCClient/GRPCCall.m

@@ -82,6 +82,8 @@
   // correct ordering.
   GRPCDelegateWrapper *_responseWriteable;
   id<GRXWriter> _requestWriter;
+
+  NSMutableDictionary *_requestMetadata;
 }
 
 @synthesize state = _state;
@@ -116,10 +118,22 @@
     _callQueue = dispatch_queue_create("org.grpc.call", NULL);
 
     _requestWriter = requestWriter;
+
+    _requestMetadata = [NSMutableDictionary dictionary];
   }
   return self;
 }
 
+#pragma mark Metadata
+
+- (NSMutableDictionary *)requestMetadata {
+  return _requestMetadata;
+}
+
+- (void)setRequestMetadata:(NSDictionary *)requestMetadata {
+  _requestMetadata = [NSMutableDictionary dictionaryWithDictionary:requestMetadata];
+}
+
 #pragma mark Finish
 
 - (void)finishWithError:(NSError *)errorOrNil {

src/objective-c/tests/GRPCClientTests.m

@@ -156,8 +156,7 @@ static GRPCMethodName *kUnaryCallMethod;
                                            method:kUnaryCallMethod
                                    requestsWriter:requestsWriter];
 
-  call.requestMetadata = [NSMutableDictionary dictionaryWithDictionary:
-      @{@"Authorization": @"Bearer bogusToken"}];
+  call.requestMetadata[@"Authorization"] = @"Bearer bogusToken";
 
   id<GRXWriteable> responsesWriteable = [[GRXWriteable alloc] initWithValueHandler:^(NSData *value) {
     XCTFail(@"Received unexpected response: %@", value);