ios - OC，任何实例对象的哈希值都会在初始化后发生变化？

Question

1.每个对象只有哈希值，不等于其他对象？

NSObject *obj1 = [[NSObject alloc] init];
NSObject *obj2 = [[NSObject alloc] init];
obj1.hash != obj2.hash

object'hash 基于什么？

2.当我更改对象的属性时，对象的哈希值永远不会改变？？？

或

(对象哈希后的对象初始化不能更改？？？)

NSObject *obj1 = [[NSObject alloc] init];
NSObject *obj2 = [[NSObject alloc] init];
NSObject *obj3 = [[NSObject alloc] init];
NSDictionary *dic = @{@(obj1.hash):obj1,@(obj2.hash):obj2,@(obj3.hash):obj3,...};

我不知道以上两个问题，所以我不确定我写的是否正确。

其他(英语不是我的母语，语法错误请原谅我)

Answer 1

正如@Sulthan所说，hash的默认实现是object的地址。

在你的子类中，你可以重写-hash方法，你可以根据你的属性计算hash，如果是这样，当你改变属性时，hash会改变。

例如:NSMutableString，你可以查看日志查看结果，当你改变NSMutableString的内容时，它的hash改变了。

NSMutableString *s = [[NSMutableString alloc] initWithString:@"124"];
NSLog(@"%@", @([s hash]));
[s appendString:@"123"];
NSLog(@"%@", @([s hash]));

当您覆盖hash 方法并将该类的实例添加到使用对象的散列来定位对象的集合中时，您必须确保散列函数永远不会改变。

在Apple的文档中，谈到了NSObject协议(protocol)中的hash方法:

If two objects are equal (as determined by the isEqual: method), they must have the same hash value. This last point is particularly important if you define hash in a subclass and intend to put instances of that subclass into a collection. If a mutable object is added to a collection that uses hash values to determine the object’s position in the collection, the value returned by the hash method of the object must not change while the object is in the collection. Therefore, either the hash method must not rely on any of the object’s internal state information or you must make sure the object’s internal state information does not change while the object is in the collection. Thus, for example, a mutable dictionary can be put in a hash table but you must not change it while it is in there. (Note that it can be difficult to know whether or not a given object is in a collection.)

这是一个例子:

@interface Object : NSObject
@property (nonatomic) NSInteger ttt;
@end

@implementation Object

- (NSUInteger)hash {
    return _ttt;
}

@end

NSMutableSet *set = [NSMutableSet set];
Object *obj = [[Object alloc] init];
obj.ttt = 10;
[set addObject:obj];
obj.ttt = 9;
[set addObject:obj];
NSLog(@"%@", @(set.count));

即使 obj 是同一个对象，结果也是 2。原因是 NSMutableSet 使用 hash 来确定集合中是否已经存在一个对象。你可以想象有一个简单的哈希表，obj 第一次插入集合时位于 10。然后你把ttt属性改成9，它的hash变了，NSMutableSet发现9处是空的，于是又在9处插入对象。