字典转字符串

 NSString *jsonString = nil;
    NSError *error;
    NSData *jsonData = [NSJSONSerialization dataWithJSONObject:parameters
                                                       options:NSJSONWritingPrettyPrinted // Pass 0 if you don't care about the readability of the generated string
                                                         error:&error];
    if (! jsonData) {
        NSLog(@"Got an error: %@", error);
    } else { 
        jsonString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
    }