IOS-我如何检查SBJson创建的JSON NSData

我正在尝试将一些简单的JSON发布到RESTful服务。我想形成一个json字符串，它采用以下形式：

{username:"someuser",password:"somepassword"}

我的POST在服务器上不断失败，抱怨POSTed值不存在。然而，如果我创建一个JS测试实现，那么同样的服务也可以正常工作。所以我确信问题出在我的SBJson客户端实现上。

NSDictionary *userDict = [[NSDictionary alloc] initWithObjectsAndKeys:
                          userName,@"username",
                          password, @"password",
                          nil];
SBJsonWriter *writer = [[SBJsonWriter alloc]init];
NSData *jsonData = [writer dataWithObject:userDict];    

jsonData看起来有大小，但它试图用SBJson序列化它，它失败了，我得到了nil。

NSString *test = [writer stringWithObject:jsonData]; // returns nil

我正试图在NSMutableURLRequest中发布上述内容。以下是我的实现，如果它有助于提供上下文：

-(void)registerUserName:(NSString *)userName andPassword:(NSString *)password{
responseData = nil;
responseData = [NSMutableData data];
NSDictionary *userDict = [[NSDictionary alloc] initWithObjectsAndKeys:
                          userName,@"username",
                          password, @"password",
                          nil];
SBJsonWriter *writer = [[SBJsonWriter alloc]init];
NSData *jsonData = [writer dataWithObject:userDict];
NSString *test = [writer stringWithObject:jsonData]; //always nil
NSURL *url = [[NSURL alloc] initWithString:@"http://localhost/~user/app_web/index.php/user/create"];
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url];
[request setHTTPMethod:@"POST"];
[request addValue:@"application/json" forHTTPHeaderField:@"Content-Type"];
[request addValue:@"application/json" forHTTPHeaderField:@"Accept"];
[request addValue:@"form-data" forHTTPHeaderField:@"Content-Disposition"];
[request setHTTPBody:jsonData];
[[UIApplication sharedApplication]setNetworkActivityIndicatorVisible:YES ];
currentRequestType = REGISTER_USER;
NSURLConnection *connection =[[NSURLConnection alloc]initWithRequest:request delegate:self]; 

谢谢！

更新

使用下面的建议，我现在这样做：

NSDictionary *userDict = [[NSDictionary alloc] initWithObjectsAndKeys:
                              userName,@"username",
                              password, @"password",
                              nil];

     NSString *jsonString = [userDict JSONRepresentation];
    NSData *jsonData = [jsonString dataUsingEncoding];
    [request setHTTPBody:jsonData];
        [[UIApplication sharedApplication]setNetworkActivityIndicatorVisible:YES ];
        currentRequestType = REGISTER_USER;
        NSURLConnection *connection =[[NSURLConnection alloc]initWithRequest:request delegate:self];

不幸的是，在服务器上仍然找不到我发布的值。我想我的NSMutableURLRequest一定出了问题。。

我在后台使用CodeIgniter。动作如下：

public function create(){
    if($this->input->post('username') && $this->input->post('password')){
        $username = $this->input->post('username');
        $password = $this->input->post('password');
        $this->load->model('User_model','users');
        $status = $this->users->add_user($username,$password);
        if($status){
            echo json_encode(array('success'=>true));
        } else{
            echo json_encode(array('success'=>false));
        }
    }
     echo json_encode(array('success'=>false)); // when posting I arrive here
}