实现思路

把json字符串转为NSDictionary。

NSData *jsonData = [jsonStr dataUsingEncoding:NSUTF8StringEncoding];
jsonDic = [NSJSONSerialization JSONObjectWithData:jsonData options:0 error:nil];

通过runtime的方法，来获取类的所有属性值，并放入一个数组。

    u_int count;
    objc_property_t *properties  = class_copyPropertyList(class, &count);
    NSMutableArray *propertiesArray = [NSMutableArray arrayWithCapacity:count];
    for (int i = 0; i<count; i++)
    {
        const char* propertyName =property_getName(properties[i]);
        [propertiesArray addObject: [NSString stringWithUTF8String: propertyName]];
    }
    free(properties);

依然是通过runtime的方法，给model的属性赋对应的值。

Ivar ivar = class_getInstanceVariable(class, [[NSString stringWithFormat:@"_%@",prop] UTF8String]);
object_setIvar(object, ivar, arr);

利用一个递归，来解决嵌套json字符串的问题。
model中，实现propertyNameDic这个方法，可以达到key映射的功能。key为json字符串的key，value为model的属性。

@implementation JsonModel
+ (NSDictionary *)propertyNameDic {
    return @{@"Content":@"Con",@"Data":@"Da"};
}
@end

下面是具体的实现代码。

 (NSDictionary *)propertyNameDic {
    return nil;
}
+ (id)jsonToModel:(NSString *)jsonStr {
        NSDictionary *jsonDic;
        NSData *jsonData = [jsonStr dataUsingEncoding:NSUTF8StringEncoding];
        jsonDic = [NSJSONSerialization JSONObjectWithData:jsonData options:0 error:nil];
        NSData *data = [NSJSONSerialization dataWithJSONObject:jsonDic options:NSJSONWritingPrettyPrinted error:nil];
        NSLog(@"%@",[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding]);
    }
    id obj = [self finishToModel:[self class] jsonDic:jsonDic];
    return obj;

}

- (id)finishToModel:(Class)class jsonDic:(NSDictionary *)jsonDic{
    id object = [[class alloc] init];
    NSArray *propertiesArray = [self getAllProperties:class];
    for (NSString *key in jsonDic) {
        for (NSString *prop in propertiesArray) {
            if ([self propertyIsEqual:prop toKey:key withClass:class]) {
                if ([jsonDic[key] isKindOfClass:[NSDictionary class]]) {
                    NSDictionary *subDic = jsonDic[key];
                    id o = [self finishToModel:[NSClassFromString(key) class] jsonDic:subDic];
                    Ivar ivar = class_getInstanceVariable(class, [[NSString stringWithFormat:@"_%@",prop] UTF8String]);
                    object_setIvar(object, ivar, o);
                    break;
                }else if ([jsonDic[key] isKindOfClass:[NSArray class]]) {
                    NSArray *jsonArr = jsonDic[key];
                    NSMutableArray *arr = [[NSMutableArray alloc] init];
                    [jsonArr enumerateObjectsUsingBlock:^(id  _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
                    id o = [self finishToModel:[NSClassFromString(key) class] jsonDic:obj];
                        [arr addObject:o];
                    }];
                    Ivar ivar = class_getInstanceVariable(class, [[NSString stringWithFormat:@"_%@",prop] UTF8String]);
                    object_setIvar(object, ivar, arr);
                    break;
                }else {
                    Ivar ivar = class_getInstanceVariable(class, [[NSString stringWithFormat:@"_%@",prop] UTF8String]);
                    object_setIvar(object, ivar, jsonDic[key]);
                    break;
                }
            }
        }
    }
    return object;
}


//比较传进来的key是否和属性能对应上
- (BOOL)propertyIsEqual:(NSString *)prop toKey:(NSString *)key withClass:(Class)class{
    if ([prop isEqualToString:key]) {
        return YES;
    }
    
    NSDictionary *propertyNameDic = nil;
    if ([class respondsToSelector:@selector(propertyNameDic)]) {
        propertyNameDic = [class propertyNameDic];
    }
    if ([propertyNameDic[key] isEqualToString:prop]) {
        return YES;
    }else {
        return NO;
    }
}
//获取对象的所有属性，不包括属性值
- (NSArray *)getAllProperties:(Class)class
{
    u_int count;
    objc_property_t *properties  = class_copyPropertyList(class, &count);
    NSMutableArray *propertiesArray = [NSMutableArray arrayWithCapacity:count];
    for (int i = 0; i<count; i++)
    {
        const char* propertyName =property_getName(properties[i]);
        [propertiesArray addObject: [NSString stringWithUTF8String: propertyName]];
    }
    free(properties);
    return propertiesArray;

}

//获取对象的所有属性 以及属性值
- (NSDictionary *)properties_aps
{
    NSMutableDictionary *props = [NSMutableDictionary dictionary];
    unsigned int outCount, i;
    objc_property_t *properties = class_copyPropertyList([self class], &outCount);
    for (i = 0; i<outCount; i++)
    {
        objc_property_t property = properties[i];
        const char* char_f =property_getName(property);
        NSString *propertyName = [NSString stringWithUTF8String:char_f];
        id propertyValue = [self valueForKey:(NSString *)propertyName];
        if (propertyValue) [props setObject:propertyValue forKey:propertyName];
    }
    free(properties);
    return props;
}