平常,我們判斷兩個對象是否相等可以用==
示例:

class AA:NSObject {
    let id:String
    init(id:String) {
        self.id = id
        super.init();
    }
}

let a1 = AA(id: "2")
let a2 = AA(id: "2")

結(jié)果:

a1 == a2 
//結(jié)果:false,因?yàn)楸緛砭褪莾蓚€對象

這里重寫

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

class AA:NSObject {
    let id:String
    init(id:String) {
        self.id = id
        super.init();
    }
    
    override func isEqual(_ object: Any?) -> Bool {
        guard let other = object as? AA else {
            return false
        }
        return id == other.id
    }
}

結(jié)果

a1 == a2 
//結(jié)果:true,因?yàn)槲覀冏宨d相等成為其相等的條件

結(jié)論:

我們可以自由的定制兩個對象相等的條件,只要重寫override func isEqual(_ object: Any?) -> Bool

重點(diǎn)來啦

class AA:NSObject {
    let id:String
    init(id:String) {
        self.id = id
        super.init();
    }
    
    override func isEqual(_ object: Any?) -> Bool {
        guard let other = object as? AA else {
            return false
        }
        return id == other.id
    }
}

let a0 = AA(id: "1")
let a1 = AA(id: "2")
let a2 = AA(id: "2")
let a3 = AA(id: "4")

var a:Set<AA> = [a0,a1]
var b:Set<AA> = [a2,a3]

let c = a.subtracting(b)

結(jié)果為:

c:[a0,a1]

為什么,a1不能移除,因?yàn)閍1等于a2阿.
原因是,Set是用哈希表存的,哈希表里要找相同的對象,首先就要有相同的哈希值,再然后是isEqual.所以在AA里加override var hashValue: Int

class AA:NSObject {
    let id:String
    init(id:String) {
        self.id = id
        super.init();
    }
    
    override func isEqual(_ object: Any?) -> Bool {
        guard let other = object as? AA else {
            return false
        }
        return id == other.id
    }

    override var hashValue: Int{
        return Int(id)!
    }
}

let c = a.subtracting(b)

結(jié)果為:

c:[a0]