原理描述：

用runtime提供的函数遍历Model自身所有属性，如果属性在json中有对应的值，则将其赋值

核心方法：在NSObject的分类中添加方法

- (instancetype)initWithDict:(NSDictionary *)dict {    if (self = [self init]) {        //(1)获取类的属性及属性对应的类型        NSMutableArray * keys = [NSMutableArray array];        NSMutableArray * attributes = [NSMutableArray array];        /*         * 例子         * name = value3 attribute = T@"NSString",C,N,V_value3         * name = value4 attribute = T^i,N,V_value4         */        unsigned int outCount;        objc_property_t * properties = class_copyPropertyList([self class], &outCount);        for (int i = 0; i < outCount; i ++) {            objc_property_t property = properties[i];            //通过property_getName函数获得属性的名字            NSString * propertyName = [NSString stringWithCString:property_getName(property) encoding:NSUTF8StringEncoding];            [keys addObject:propertyName];            //通过property_getAttributes函数可以获得属性的名字和@encode编码            NSString * propertyAttribute = [NSString stringWithCString:property_getAttributes(property) encoding:NSUTF8StringEncoding];            [attributes addObject:propertyAttribute];        }        //立即释放properties指向的内存        free(properties);        //(2)根据类型给属性赋值        for (NSString * key in keys) {            if ([dict valueForKey:key] == nil) continue;            [self setValue:[dict valueForKey:key] forKey:key];        }    }    return self;}