开发常见错误

今天写了个方法，结果老提示错误cannot assign to self outside of a method in the init family,想了半天都不知道怎么破

-(id)initwithUrlStr:(NSString *)urlStr setBlock:(void (^)(HttpDownLoadBlock *, BOOL))a

{

    if (self=[super init]) {

        self.data=[NSMutableData dataWithCapacity:0];

        self.httpRequestBlock=a;

    }

    return self;

}