在Objective-C中如何正确地覆盖isEqual: ?“陷阱”似乎是，如果两个对象相等(由isEqual:方法决定)，它们必须具有相同的散列值。
Cocoa Fundamentals Guide的Introspection部分确实有一个关于如何重写isEqual:的例子，复制如下，用于一个名为MyWidget的类:
- (BOOL)isEqual:(id)other {
if (other == self)
return YES;
if (!other || ![other isKindOfClass:[self class]])
return NO;
return [self isEqualToWidget:other];
}
- (BOOL)isEqualToWidget:(MyWidget *)aWidget {
if (self == aWidget)
return YES;
if (![(id)[self name] isEqual:[aWidget name]])
return NO;
if (![[self data] isEqualToData:[aWidget data]])
return NO;
return YES;
}
它检查指针是否相等，然后是类是否相等，最后使用isEqualToWidget:比较对象，后者只检查名称和数据属性。这个例子没有说明如何重写哈希。
让我们假设有其他属性不影响平等，比如年龄。难道不应该重写哈希方法，以便只有名称和数据影响哈希吗?如果是，你会怎么做?只是添加名称和数据的散列吗?例如:
- (NSUInteger)hash {
NSUInteger hash = 0;
hash += [[self name] hash];
hash += [[self data] hash];
return hash;
}
这足够了吗?有更好的技术吗?如果你有基本类型，比如int呢?将它们转换为NSNumber以获得它们的散列?或者像NSRect这样的结构?
(脑屁:最初把“位或”和|=写在一起。意味着添加。)
结合@tcurdt的答案和@oscar-gomez的答案来获取属性名，我们可以为isEqual和hash创建一个简单的解决方案:
NSArray *PropertyNamesFromObject(id object)
{
unsigned int propertyCount = 0;
objc_property_t * properties = class_copyPropertyList([object class], &propertyCount);
NSMutableArray *propertyNames = [NSMutableArray arrayWithCapacity:propertyCount];
for (unsigned int i = 0; i < propertyCount; ++i) {
objc_property_t property = properties[i];
const char * name = property_getName(property);
NSString *propertyName = [NSString stringWithUTF8String:name];
[propertyNames addObject:propertyName];
}
free(properties);
return propertyNames;
}
BOOL IsEqualObjects(id object1, id object2)
{
if (object1 == object2)
return YES;
if (!object1 || ![object2 isKindOfClass:[object1 class]])
return NO;
NSArray *propertyNames = PropertyNamesFromObject(object1);
for (NSString *propertyName in propertyNames) {
if (([object1 valueForKey:propertyName] != [object2 valueForKey:propertyName])
&& (![[object1 valueForKey:propertyName] isEqual:[object2 valueForKey:propertyName]])) return NO;
}
return YES;
}
NSUInteger MagicHash(id object)
{
NSUInteger prime = 31;
NSUInteger result = 1;
NSArray *propertyNames = PropertyNamesFromObject(object);
for (NSString *propertyName in propertyNames) {
id value = [object valueForKey:propertyName];
result = prime * result + [value hash];
}
return result;
}
现在，在你的自定义类中，你可以很容易地实现isEqual:和hash:
- (NSUInteger)hash
{
return MagicHash(self);
}
- (BOOL)isEqual:(id)other
{
return IsEqualObjects(self, other);
}
Sorry if I risk sounding a complete boffin here but...
...nobody bothered mentioning that to follow 'best practices' you should definitely not specify an equals method that would NOT take into account all data owned by your target object, e.g whatever data is aggregated to your object, versus an associate of it, should be taken into account when implementing equals.
If you don't want to take, say 'age' into account in a comparison, then you should write a comparator and use that to perform your comparisons instead of isEqual:.
如果您定义了一个isEqual:方法来任意执行相等比较，那么一旦您忘记了equals解释中的“扭曲”，您就会冒这个方法被其他开发人员甚至您自己误用的风险。
因此，虽然这是一个关于哈希的很好的问答，你通常不需要重新定义哈希方法，你可能应该定义一个特别的比较器。
