Fix ordering issue with subscribeToAttributesWithEndpointID over XPC.

src/darwin/Framework/CHIP/MTRDeviceControllerXPCConnection.h

@@ -43,7 +43,7 @@ typedef void (^MTRGetProxyHandleHandler)(dispatch_queue_t queue, MTRDeviceContro
 @interface MTRDeviceControllerXPCConnection<MTRDeviceControllerClientProtocol> : NSObject
 
 /**
- * This method is just for test purpsoe.
+ * This method is just for test purpose.
  */
 + (MTRDeviceControllerXPCConnection *)connectionWithWorkQueue:(dispatch_queue_t)workQueue
                                                  connectBlock:(MTRXPCConnectBlock)connectBlock;
@@ -55,6 +55,7 @@ typedef void (^MTRGetProxyHandleHandler)(dispatch_queue_t queue, MTRDeviceContro
 - (void)deregisterReportHandlersWithController:(id<NSCopying>)controller
                                         nodeID:(NSNumber *)nodeID
                                     completion:(dispatch_block_t)completion;
+- (void)callSubscriptionEstablishedHandler:(dispatch_block_t)handler;
 
 @end
 

src/darwin/Framework/CHIP/MTRDeviceControllerXPCConnection.mm

@@ -203,4 +203,11 @@ - (void)handleReportWithController:(id)controller
     });
 }
 
+- (void)callSubscriptionEstablishedHandler:(dispatch_block_t)handler
+{
+    // Call the handler from our _workQueue, so that we guarantee the same
+    // number of queue hops as for handleReportWithController.
+    dispatch_async(_workQueue, handler);
+}
+
 @end

src/darwin/Framework/CHIP/MTRDeviceOverXPC.mm

@@ -272,14 +272,16 @@ - (void)subscribeToAttributesWithEndpointID:(NSNumber * _Nullable)endpointID
                                            maxInterval:params.maxInterval
                                                 params:[MTRDeviceController encodeXPCSubscribeParams:params]
                                     establishedHandler:^{
-                                        dispatch_async(queue, ^{
-                                            MTR_LOG_DEBUG("Subscription established");
-                                            subscriptionEstablishedHandler();
-                                            // The following captures the proxy handle in the closure so that the handle
-                                            // won't be released prior to block call.
-                                            __auto_type handleRetainer = handle;
-                                            (void) handleRetainer;
-                                        });
+                                        [self.xpcConnection callSubscriptionEstablishedHandler:^{
+                                            dispatch_async(queue, ^{
+                                                MTR_LOG_DEBUG("Subscription established");
+                                                subscriptionEstablishedHandler();
+                                                // The following captures the proxy handle in the closure so that the handle
+                                                // won't be released prior to block call.
+                                                __auto_type handleRetainer = handle;
+                                                (void) handleRetainer;
+                                            });
+                                        }];
                                     }];
     };