Subclass GRXWriter instead of conforming to protocol

src/objective-c/GRPCClient/GRPCCall.h

@@ -52,7 +52,7 @@
 extern id const kGRPCStatusMetadataKey;
 
 // Represents a single gRPC remote call.
-@interface GRPCCall : NSObject<GRXWriter>
+@interface GRPCCall : GRXWriter
 
 // 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.

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

@@ -33,8 +33,9 @@
 
 #import <Foundation/Foundation.h>
 
+#import <RxLibrary/GRXWriter.h>
+
 @protocol GRXWriteable;
-@protocol GRXWriter;
 
 // This is a thread-safe wrapper over a GRXWriteable instance. It lets one
 // enqueue calls to a GRXWriteable instance for the main thread, guaranteeing

src/objective-c/RxLibrary/GRXBufferedPipe.h

@@ -51,7 +51,7 @@
 // pipe will keep buffering all data written to it, your application could run out of memory and
 // crash. If you want to react to flow control signals to prevent that, instead of using this class
 // you can implement an object that conforms to GRXWriter.
-@interface GRXBufferedPipe : NSObject<GRXWriteable, GRXWriter>
+@interface GRXBufferedPipe : GRXWriter<GRXWriteable>
 
 // Convenience constructor.
 + (instancetype)pipe;

src/objective-c/RxLibrary/GRXImmediateWriter.h

@@ -40,7 +40,7 @@
 //
 // Unless the writeable callback pauses them or stops them early, these writers will do all their
 // interactions with the writeable before the start method returns.
-@interface GRXImmediateWriter : NSObject<GRXWriter>
+@interface GRXImmediateWriter : GRXWriter
 
 // Returns a writer that pulls values from the passed NSEnumerator instance and pushes them to
 // its writeable. The NSEnumerator is released when it finishes.