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后端返回的数据中总会出现一些NSNull类型，当我们一处理程序就会崩溃，因此想到把返回的数据中的NSNull类型全部转换成@""空字符串。下面是转化方法：

1 自定义的几个方法：放在NSDictionary类别中

#pragma mark - 私有方法
//将NSDictionary中的Null类型的项目转化成@""

+(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;
}

//将NSDictionary中的Null类型的项目转化成@""

+(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;
}

//将NSString类型的原路返回

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

{
   return string;
}

//将Null类型的项目转化成@""

+(NSString *)nullToString

{
   return @"";
}

#pragma mark - 公有方法

//类型识别:将所有的NSNull类型转化成@""

+(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：后端返回的json串里面包含类型NSString，NSArray，NSDictionary，NSNull类型。

{
    "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库:json串转换成字典

NSDictionary *jsonDic = [retString JSONValue];

（3）用自定义方法转换后的结果：将字典中所有的NSNull类型转换成@""

NSDictionary *dict = [NSDictionary changeType:jsonDic];

// 打印的结果
{
    "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;
}