[Swift]学习笔记-可选类型/可选链

let price = p.dog?.toy?.price

p.dog?.toy?.price = 50

if let whyDog = p.dog {
    if let toy = whyDog.toy {
        toy.flying()
    }
}

// p.dog!.toy!.flying()
直接写法:
p.dog?.toy?.flying()

var opa: Optional<Int>
var opb: Int?
var nora: Int
nora = 10
 
print(nora)
print(opb)
 
输出结果:
10
nil

var opb: Int?
opb = 55
if let b = opb{
    print(opb!)
    print(b)
}
输出结果:
55
55

class Person {
    var name: String
    init(name:String){
        self.name = name
    }
    func whoami() ->String{
        print("my name is \(self.name)")
        return name
    }
}
var p0 : Person?
var p1 : Person = Person(name: "ljw")
p1.name = "zs"
p1.whoami()
输出结果:
my name is zs

    p0!.name = "ls"
   p0!.whoami()

if let p = p0{
    p.name = "ls"
    p.whoami()
}

p0 = p1
p0?.name = "ls"
p0?.whoami()

print(p0?.name)
print(p0?.whoami())
print(p1.name)
var a:String? = p0?.name
p0?.name = "ww"
var b:String = p1.name
输出结果:
nil
nil
xmg

struct Student {
    var name:String = "ljw"
    var math:Double = 99.0
    var chinese:Double = 99.0
    var english:Double = 99.0

subscript(course:String) ->Double?{
        get{
            switch course{
            case "math":
                return math
            case "chinese":
                return chinese
            case "english":
                return english
            default:
                return nil
            }
        }
        set{
            switch course{
            case "math":
                因为返回的是可选类型
                math = newValue!
            case "chinese":
                chinese = newValue!
            case "english":
                english = newValue!
            default:
                print("not found")
            }
           
        }
    }
}
var stu:Student? = Student()
可选链调用下标索引不需要., 直接在问号后面写上[]即可
print(stu?["math"])
输出结果:Optional(99.0)

stu?.name = "ww"
print(stu?.name)
输出结果:Optional("ww")

stu?["math"] = 88
print(stu?["math"])
输出结果:Optional(88.0)

let res: Void? = stu?.name = "ljw"
print(res)
输出结果:Optional(()) 代表成功.

stu = nil
let res: Void? = stu?.name = "zl"
print(res)
输出结果:nil 代表失败

class A {
    var name:String = "ljw"
}
class B{
    var a1:A?
}
class C{
    var b1:B = B()
}
class D{
    var c1: C?
}

var a1 = A()
var b1 = B()
var c1 = C()
var d1 = D()

d1.c1 = c1
通过d直接给b赋值
由于D中的C是可选值, 所以需要在C后面加上?
d1.c1?.b1.a1 = a1
通过d直接获取a中的name
其实只需要在可选值后面加上问号即可, 如果可选值不存在, 那么后面的链失效
print(d1.c1?.b1.a1?.name)
输出结果:Optional("ljw")