後端返回的資料中總會出現一些NSNull型別，當我們一處理程式就會崩潰，因此想到把返回的資料中的NSNull型別全部轉換成@”“空字串。下面是轉化方法：

1 自定義的幾個方法：放在NSDictionary類別中

#pragma mark - 私有方法
//將NSDictionary中的Null型別的專案轉化成@""

+(NSDictionary *)nullDic:(NSDictionary *)myDic
{
   NSArray *keyArr = [myDic allKeys];
   NSMutableDictionary *resDic = [[NSMutableDictionary
 
 alloc]init];
   for (int i = 0; i < keyArr.count; i ++)
   {
       id obj = [myDic objectForKey:keyArr[i]];
       obj = [self changeType:obj];
       [resDic setObject:obj forKey:keyArr[i]];
   }
   return resDic;
}

//將NSDictionary中的Null型別的專案轉化成@""

+(NSArray *)nullArr:(NSArray *)myArr

{
   NSMutableArray
 
 *resArr = [[NSMutableArray alloc] init];
   for (int i = 0; i < myArr.count; i ++)
   {
       id obj = myArr[i];
       obj = [self changeType:obj];
       [resArr addObject:obj];
   }
   return resArr;
}

//將NSString型別的原路返回

+(NSString *)stringToString:(NSString *)string

{
   return string;
}

//將Null型別的專案轉化成@""
 


+(NSString *)nullToString

{
   return @"";
}

#pragma mark - 公有方法

//型別識別:將所有的NSNull型別轉化成@""

+(id)changeType:(id)myObj

{
   if ([myObj isKindOfClass:[NSDictionary class]])
   {
       return [self nullDic:myObj];
   }
   else if([myObj isKindOfClass:[NSArray class]])
   {
       return [self nullArr:myObj];
   }
   else if([myObj isKindOfClass:[NSString class]])
   {
       return [self stringToString:myObj];
   }
   else if([myObj isKindOfClass:[NSNull class]])
   {
       return [self nullToString];
   }
   else
   {
       return myObj;
   }
}

2 使用方法

（1）原始的json串：後端返回的json串裡面包含型別NSString，NSArray，NSDictionary，NSNull型別。

{
    "status": 1,
    "service_name": null,
    "service_id": null,
    "img_url": "http://api.jgfw.me/assets/uploads/files/",
    "price": null,
    "num": 3,
    "service_info": {
        "service_type": null,
        "service_time": null,
        "service_detail": null,
        "customer_name": null,
        "customer_phone": null,
        "customer_address": "",
        "new_jishi": "",
        "old_jishi": null,
        "lat": null,
        "lon": null
    },
    "order_info": {
        "order_no": "E15031267469289848688",
        "pay_time": null,
        "order_time": null,
        "price": 0,
        "order_state": null
    }
}

（2）用SBJson庫:json串轉換成字典

NSDictionary *jsonDic = [retString JSONValue];

（3）用自定義方法轉換後的結果：將字典中所有的NSNull型別轉換成@”“

NSDictionary *dict = [NSDictionary changeType:jsonDic];

// 列印的結果
{
    "img_url" = "http://api.jgfw.me/assets/uploads/files/";
    num = 3;
    "order_info" =     {
        "order_no" = E15031267469289848688;
        "order_state" = "";
        "order_time" = "";
        "pay_time" = "";
        price = 0;
    };
    price = "";
    "service_id" = "";
    "service_info" =     {
        "customer_address" = "";
        "customer_name" = "";
        "customer_phone" = "";
        lat = "";
        lon = "";
        "new_jishi" = "";
        "old_jishi" = "";
        "service_detail" = "";
        "service_time" = "";
        "service_type" = "";
    };
    "service_name" = "";
    status = 1;
}