Set user's dispatch queue's handler to internal serial queue

src/objective-c/GRPCClient/GRPCCall.h

@@ -174,9 +174,7 @@ extern id const kGRPCTrailersKey;
 
 /**
  * All the responses must be issued to a user-provided dispatch queue. This property specifies the
- * dispatch queue to be used for issuing the notifications. A serial queue should be provided if
- * the order of responses (initial metadata, message, message, ..., message, trailing metadata)
- * needs to be maintained.
+ * dispatch queue to be used for issuing the notifications.
  */
 @property(atomic, readonly) dispatch_queue_t dispatchQueue;
 

src/objective-c/GRPCClient/GRPCCall.m

@@ -101,6 +101,12 @@ const char *kCFStreamVarName = "grpc_cfstream";
   if (requestOptions.host.length == 0 || requestOptions.path.length == 0) {
     [NSException raise:NSInvalidArgumentException format:@"Neither host nor path can be nil."];
   }
+  if (requestOptions.safety > GRPCCallSafetyCacheableRequest) {
+    [NSException raise:NSInvalidArgumentException format:@"Invalid call safety value."];
+  }
+  if (responseHandler == nil) {
+    [NSException raise:NSInvalidArgumentException format:@"Response handler required."];
+  }
 
   if ((self = [super init])) {
     _requestOptions = [requestOptions copy];
@@ -114,6 +120,7 @@ const char *kCFStreamVarName = "grpc_cfstream";
       // Fallback on earlier versions
       _dispatchQueue = dispatch_queue_create(NULL, DISPATCH_QUEUE_SERIAL);
     }
+    dispatch_set_target_queue(responseHandler.dispatchQueue, _dispatchQueue);
     _started = NO;
   }
 

src/objective-c/ProtoRPC/ProtoRPC.h

@@ -48,9 +48,7 @@
 
 /**
  * All the responses must be issued to a user-provided dispatch queue. This property specifies the
- * dispatch queue to be used for issuing the notifications. A serial queue should be provided if
- * the order of responses (initial metadata, message, message, ..., message, trailing metadata)
- * needs to be maintained.
+ * dispatch queue to be used for issuing the notifications.
  */
 @property(atomic, readonly) dispatch_queue_t dispatchQueue;
 

src/objective-c/ProtoRPC/ProtoRPC.m

@@ -72,6 +72,16 @@
                        responseHandler:(id<GRPCProtoResponseHandler>)handler
                            callOptions:(GRPCCallOptions *)callOptions
                          responseClass:(Class)responseClass {
+  if (requestOptions.host.length == 0 || requestOptions.path.length == 0) {
+    [NSException raise:NSInvalidArgumentException format:@"Neither host nor path can be nil."];
+  }
+  if (requestOptions.safety > GRPCCallSafetyCacheableRequest) {
+    [NSException raise:NSInvalidArgumentException format:@"Invalid call safety value."];
+  }
+  if (handler == nil) {
+    [NSException raise:NSInvalidArgumentException format:@"Response handler required."];
+  }
+
   if ((self = [super init])) {
     _requestOptions = [requestOptions copy];
     _handler = handler;
@@ -82,6 +92,7 @@
     } else {
       _dispatchQueue = dispatch_queue_create(nil, DISPATCH_QUEUE_SERIAL);
     }
+    dispatch_set_target_queue(handler.dispatchQueue, _dispatchQueue);
 
     [self start];
   }