extension NSObject : Equatable,Hashable {
    /// The hash value.
    ///
    /// **Axiom:** `x == y` implies `x.hashValue == y.hashValue`
    ///
    /// - Note: the hash value is not guaranteed to be stable across
    ///   different invocations of the same program.  Do not persist the
    ///   hash value across program runs.
    public var hashValue: Int { get }
}

public func ==(lhs: NSObject,rhs: NSObject) -> Bool

我找不到官方参考，但似乎是hashValue
从NSObjectProtocol调用哈希方法，并调用==
isEqual：方法(来自相同的协议)。看到更新
答案结束！

对于NSObject子类，似乎是正确的方式
要重写hash和isEqual :,这里是一个实验
表明：

覆盖hashValue和==

class ClassA : NSObject {
    let value : Int

    init(value : Int) {
        self.value = value
        super.init()
    }

    override var hashValue : Int {
        return value
    }
}

func ==(lhs: ClassA,rhs: ClassA) -> Bool {
    return lhs.value == rhs.value
}

现在创建被考虑的类的两个不同实例
“平等”，并把它们放在一起：

let a1 = ClassA(value: 13)
let a2 = ClassA(value: 13)

let nsSetA = NSSet(objects: a1,a2)
let swSetA = Set([a1,a2])

print(nsSetA.count) // 2
print(swSetA.count) // 2

您可以看到，NSSet和Set都将对象视为不同的对象。
这不是理想的结果。阵列也有意想不到的结果：

let nsArrayA = NSArray(object: a1)
let swArrayA = [a1]

print(nsArrayA.indexOfObject(a2)) // 9223372036854775807 == NSNotFound
print(swArrayA.indexOf(a2)) // nil

设置断点或添加调试输出显示被覆盖
==操作符从不调用。我不知道这是一个bug还是
意图行为。

2.覆盖哈希和isEqual：

class ClassB : NSObject {
    let value : Int

    init(value : Int) {
        self.value = value
        super.init()
    }

    override var hash : Int {
        return value
    }

    override func isEqual(object: AnyObject?) -> Bool {
        if let other = object as? ClassB {
            return self.value == other.value
        } else {
            return false
        }
    }
}

对于Swift 3，isEqual的定义：更改为

override func isEqual(_ object: Any?) -> Bool { ... }

现在所有的结果都如预期的那样

let b1 = ClassB(value: 13)
let b2 = ClassB(value: 13)

let nsSetB = NSSet(objects: b1,b2)
let swSetB = Set([b1,b2])

print(swSetB.count) // 1
print(nsSetB.count) // 1

let nsArrayB = NSArray(object: b1)
let swArrayB = [b1]

print(nsArrayB.indexOfObject(b2)) // 0
print(swArrayB.indexOf(b2)) // Optional(0)

更新：现在记录的行为
Interacting with Objective-C APIs
在“使用Swift与可可和Objective-C”参考：

The NSObject class only performs an identity comparison,so you should implement your own isEqual: method in classes that derive from the NSObject class.

As part of implementing equality for your class,be sure to implement the hash property according to the rules in Object comparison.