How to store blocks in Objective C?

I started writing a simple JSON RPC TCP library in Objective C. I have a method that invokes a RPC Method:

- (void)invokeMethod:(NSString *)method
      withParameters:(id)parameters
           requestId:(id)requestId
             success:(void (^)(id responseObject))success
             failure:(void (^)(NSError *error))failure
{
    NSAssert(NSClassFromString(@"NSJSONSerialization"), @"NSJSONSerialization not found!");
    NSDictionary *requestObject = @{@"jsonrpc": @"2.0",
                                    @"method": method,
                                    @"params": parameters,
                                    @"id": requestId};
    NSError *error = nil;
    NSData *jsondData = [NSJSONSerialization dataWithJSONObject:requestObject options:0 error:&error];
    if (error){
        return failure(error);
    }
    [self->callbacks setObject:@{@"success": success ? [success copy] : [NSNull null],
                                 @"failure": failure ? [failure copy] : [NSNull null]}
                        forKey:requestId];
    NSString *str = [[NSString alloc] initWithData:jsondData encoding:NSUTF8StringEncoding];
    NSLog(@"Sending: %@", str);
    [self.socket writeData:jsondData withTimeout:-1 tag:1];
}

The class basically represents a TCP connection, when calling the above method, the JSON data is sent with an id over TCP to the server which either returns a success or a failure:

- (void) socket:(GCDAsyncSocket *)sender didReadData:(NSData *)data withTag:(long)tag
{
    NSError *error = nil;
    [self.socket readDataWithTimeout:-1 tag:2];
    // … rpc response parsing code here, removed for simplicity …
    // detect if error or success
    NSDictionary *cbs = [self->callbacks objectForKey:JSONRPCObjectId];
    void(^success)(id resultObject) = [cbs objectForKey:@"success"];
    success ? success(JSONRPCObjectResult) : nil;
    return;
}

Now, I am unsure how to keep track of the success and failure blocks, currently I am storing them in an NSMutableDict, using the requestId as key. Is it fine to do this or is there a better approach that I should use?

Blocks in objective-c are objects and you can treat the same way as other object, so storing them in NSDictionarys, NSArrays etc is perfectly fine. The only catch is that blocks when initially created exist in the same memory scope as local variable do and so they are no longer valid when the method that the block is defined in returns, just like all other local variables so you have to copy them first, just copy them and put the copy in the collection. There is a block copy function but you can just send them a copy message [myBlock copy];