Some nit fixes

src/objective-c/GRPCClient/GRPCCall.m

@@ -108,6 +108,7 @@ const char *kCFStreamVarName = "grpc_cfstream";
   /** Flags whether call has been canceled. */
   BOOL _canceled;
   /** Flags whether call has been finished. */
+  BOOL _finished;
 }
 
 - (instancetype)initWithRequestOptions:(GRPCRequestOptions *)requestOptions
@@ -144,6 +145,7 @@ const char *kCFStreamVarName = "grpc_cfstream";
     dispatch_set_target_queue(responseHandler.dispatchQueue, _dispatchQueue);
     _started = NO;
     _canceled = NO;
+    _finished = YES;
   }
 
   return self;
@@ -252,7 +254,7 @@ const char *kCFStreamVarName = "grpc_cfstream";
 
 - (void)finish {
   dispatch_async(_dispatchQueue, ^{
-    NSAssert(self->started, @"Call not started.");
+    NSAssert(self->_started, @"Call not started.");
     NSAssert(!self->_canceled, @"Call arleady canceled.");
     NSAssert(!self->_finished, @"Call already half-closed.");
     if (self->_call) {

src/objective-c/GRPCClient/private/GRPCChannel.m

@@ -53,7 +53,7 @@ static GRPCChannelPool *gChannelPool;
 /** Reduce call ref count to the channel and maybe set the timer. */
 - (void)unrefChannel;
 
-/** Disconnect the channel immediately. */
+/** Disconnect the channel. Any further ref/unref are discarded. */
 - (void)disconnect;
 
 @end
@@ -67,9 +67,8 @@ static GRPCChannelPool *gChannelPool;
   dispatch_queue_t _dispatchQueue;
 
   /**
-   * Date and time when last timer is scheduled. When a timer is fired, if
-   * _lastDispatch + _destroyDelay < now, it can be determined that another timer is scheduled after
-   * schedule of the current timer, hence the current one should be discarded.
+   * Date and time when last timer is scheduled. If a firing timer's scheduled date is different
+   * from this, it is discarded.
    */
   NSDate *_lastDispatch;
 }
@@ -113,7 +112,7 @@ static GRPCChannelPool *gChannelPool;
         dispatch_time_t delay =
             dispatch_time(DISPATCH_TIME_NOW, (int64_t)self->_destroyDelay * NSEC_PER_SEC);
         dispatch_after(delay, self->_dispatchQueue, ^{
-          [self timerFireWithScheduleDate:now];
+          [self timedDisconnectWithScheduleDate:now];
         });
       }
     }
@@ -131,7 +130,7 @@ static GRPCChannelPool *gChannelPool;
   });
 }
 
-- (void)timerFireWithScheduleDate:(NSDate *)scheduleDate {
+- (void)timedDisconnectWithScheduleDate:(NSDate *)scheduleDate {
   dispatch_async(_dispatchQueue, ^{
     if (self->_disconnected || self->_lastDispatch != scheduleDate) {
       return;
@@ -183,6 +182,8 @@ static GRPCChannelPool *gChannelPool;
         grpc_slice_unref(host_slice);
       }
       grpc_slice_unref(path_slice);
+    } else {
+      NSAssert(self->_unmanagedChannel != nil, @"Invalid channeg.");
     }
   });
   return call;
@@ -255,10 +256,9 @@ static GRPCChannelPool *gChannelPool;
 }
 
 + (nullable instancetype)createChannelWithConfiguration:(GRPCChannelConfiguration *)config {
+  NSAssert(config != nil, @"configuration cannot be empty");
   NSString *host = config.host;
-  if (host.length == 0) {
-    return nil;
-  }
+  NSAssert(host.length != 0, @"host cannot be nil");
 
   NSDictionary *channelArgs;
   if (config.callOptions.additionalChannelArgs.count != 0) {
@@ -287,6 +287,9 @@ static GRPCChannelPool *gChannelPool;
 
   GRPCChannelConfiguration *channelConfig =
       [[GRPCChannelConfiguration alloc] initWithHost:host callOptions:callOptions];
+  if (channelConfig == nil) {
+    return nil;
+  }
 
   return [gChannelPool channelWithConfiguration:channelConfig];
 }

src/objective-c/GRPCClient/private/GRPCChannelPool.h

@@ -57,8 +57,6 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @interface GRPCChannelPool : NSObject
 
-- (instancetype)init;
-
 /**
  * Return a channel with a particular configuration. If the channel does not exist, execute \a
  * createChannel then add it in the pool. If the channel exists, increase its reference count.

src/objective-c/GRPCClient/private/GRPCChannelPool.m

@@ -192,6 +192,7 @@ extern const char *kCFStreamVarName;
 }
 
 - (GRPCChannel *)channelWithConfiguration:(GRPCChannelConfiguration *)configuration {
+  NSAssert(configuration != nil, @"Must has a configuration");
   GRPCChannel *channel;
   @synchronized(self) {
     if ([_channelPool objectForKey:configuration]) {