address comment

src/objective-c/ProtoRPC/ProtoRPC.m

@@ -39,9 +39,9 @@
   if ((self = [super init])) {
     _responseHandler = handler;
     if (dispatchQueue == nil) {
-      _responseDispatchQueue = dispatchQueue;
-    } else {
       _responseDispatchQueue = dispatch_get_main_queue();
+    } else {
+      _responseDispatchQueue = dispatchQueue;
     }
   }
   return self;

src/objective-c/tests/InteropTests/InteropTests.m

@@ -567,7 +567,8 @@ static dispatch_once_t initGlobalInterceptorFactory;
   XCTAssertNotNil([[self class] host]);
   // The test does not work on a remote server since it does not echo a trailer
   if ([[self class] isRemoteTest]) return;
-  __weak XCTestExpectation *expectComplete = [self expectationWithDescription:@"received complete"];
+  XCTestExpectation *expectComplete = [self expectationWithDescription:@"call complete"];
+  XCTestExpectation *expectCompleteMainQueue = [self expectationWithDescription:@"main queue call complete"];
 
   RMTSimpleRequest *request = [RMTSimpleRequest message];
   request.responseType = RMTPayloadType_Compressable;
@@ -600,7 +601,22 @@ static dispatch_once_t initGlobalInterceptorFactory;
         [expectComplete fulfill];
       }
         responseDispatchQueue:dispatch_queue_create(NULL, DISPATCH_QUEUE_SERIAL)];
+  __block GRPCUnaryResponseHandler *handlerMainQueue = [[GRPCUnaryResponseHandler alloc]
+      initWithResponseHandler:^(GPBMessage *response, NSError *error) {
+        XCTAssertNil(error, @"Unexpected error: %@", error);
+        RMTSimpleResponse *expectedResponse = [RMTSimpleResponse message];
+        expectedResponse.payload.type = RMTPayloadType_Compressable;
+        expectedResponse.payload.body = [NSMutableData dataWithLength:314159];
+        XCTAssertEqualObjects(response, expectedResponse);
+        XCTAssertEqualObjects(handlerMainQueue.responseHeaders[@"x-grpc-test-echo-initial"], @"test-header");
+        XCTAssertEqualObjects(handlerMainQueue.responseTrailers[@"x-grpc-test-echo-trailing-bin"],
+                              trailer_data);
+        [expectCompleteMainQueue fulfill];
+      }
+        responseDispatchQueue:nil];
+
   [[_service unaryCallWithMessage:request responseHandler:handler callOptions:options] start];
+  [[_service unaryCallWithMessage:request responseHandler:handlerMainQueue callOptions:options] start];
   [self waitForExpectationsWithTimeout:TEST_TIMEOUT handler:nil];
 }