可选项(Optional)
- 可选项,一般也叫可选类型,它允许将值设置为
nil - 在类型名称后面加个
?来定义一个可选项
var name: String? = "Jack"
name = nil
var age: Int?//默认就是nil
age = 10
age = nil
var arr = [1,27,19,28]
func get (_ index: Int) -> Int? {
if index < 0 || index > arr.count {
return nil
}
return arr[index]
}
print(get(1))//Optional(27)
print(get(-1))//nil
print(get(12))//nil
强制解包(Forced Unwrapping)
可选项是对其他类型的一层包装,可以将他理解为一个盒子
如果为
nil,那么他是个空盒子-
如果不为
nil,那么盒子里装的是:被包装类型的数据.
image.png 如果要从可选项中取出被包装的数据(将盒子里装的东西取出来),需要使用
感叹号!进行强制解包.如果对值为
nil的可选项(空盒子)进行轻质解包,将会产生运行时错误.
判断可选项是否包含值
var age: Int?//默认就是nil
age = 10
age = nil
if age != nil {
print(age!)
}else{
print("age 为 nil")
}
可选项绑定(Optional Binding)
- 可以使用
可选绑定来判断可选项是否包含值 - 如果包含就自动解包,把值赋给一个临时的常量或变量,并返回
true.否则返回false
if let number = Int("123") {
print("字符串转换整数成功: \(number)")
}else{
print("字符串转换整数失败")
}
enum Season: String {
case spring, summer, autumn, winter
}
if let season = Season(rawValue: "3") {
switch season {
case .spring:
print(season.rawValue)
case .summer:
print(season.rawValue)
case .autumn:
print(season.rawValue)
case .winter:
print(season.rawValue)
}
}else{
print("无匹配")
}
等价写法
if let first = Int("2") {
if let second = Int("4") {
if first < second && first > 0 {
print("second > first > 0")
}
}
}
if let first = Int("2"),let second = Int("4"),first > 0 && first < second {
print("second > first > 0")
}
While循环中使用可选项绑定
//遍历数组,将所有整数相加
var arr = ["10","20","-2","22","1","c","-2",]
var index = 0
var sum = 0
while let num = Int(arr[index]),num > 0 {
sum += num
index += 1
}
print(sum)//30 第三个元素停止
空合运算符??(Nil-Coalescing Operator)
a ?? b
- a 是可选项.
- b 是可选项 或者 不是可选项.
- b 和 a 的存储类型必须相同.
- 如果a不为
nil,就返回a. - 如果a为 nil 就返回 b.
- 如果b是不可选项,返回a时会自动解包.
let a: Int? = 10
let b: Int? = 20
let c = a ?? b//a不为nil,b为可选项 返回a. Int? Optional(10)
let a: Int? = 10
let b: Int = 20
let c = a ?? b//a不为nil,b不是可选项 返回a! Int
let a: Int? = nil
let b: Int? = 20
let c = a ?? b//a为nil,b是可选项 返回b . Int? Optional(20)
let a: Int? = nil
let b: Int = 20
let c = a ?? b//a为nil,b不是可选项 返回b.Int
let a: Int? = nil
let b: Int? = nil
let c = a ?? b//a为nil,返回b c = nil Int?
//如果不实用??运算符
let a: Int? = nil
let b: Int = 2
let c: Int
if let tmp = a {
c = tmp
}else{
c = b
}
多个 ?? 一起使用
let a: Int? = 1
let b: Int? = 2
let c = a ?? b ?? 3 // a ?? b = a; a ?? 3 = a!;
let a: Int? = nil
let b: Int? = 2
let c = a ?? b ?? 3 // a ?? b = b; b ?? 3 = b!
let a: Int? = nil
let b: Int? = nil
let c = a ?? b ?? 3 // a ?? b = b; b ?? c = 3;
?? 和 if let 配合使用
let a: Int? = nil
let b: Int? = 2
if let c = a ?? b {
print(c)
}
//等价写法
if a != nil || b != nil {
}
if let c = a , let d = b {
print(c)
print(d)
}
//等价写法
if a != nil && b != nil {
let c = a!
let d = b!
}
guard语句
guard 条件 else {
//do somrthing
//退出当前作用域
// return break continue throw error
}
- 当
guard语句的条件为false时,就会执行大括号里面的代码 - 当
guard语句的条件为true时,就会跳过guard语句 -
guard语句特别适合用来 提前退出 - 当使用
guard语句进行可选绑定时,绑定的常量let,变量var也能在外层作用域中使用
func login(_ info: [String : String]){
guard let userName = info["userName"] else {
print("请输入用户名")
return
}
guard let passWord = info["passwd"] else {
print("请输入密码")
return
}
print(userName,passWord)
}
隐式解包
- 在某些情况下,可选项一旦被设定值之后,就会一直拥有值,在这种情况下,可以去掉检查,也不必每次访问都进行解包,因为她能确定每次访问的时候都有值
- 可以再类型后面加一个感叹号!,定义一个隐式解包的可选项
let num1: Int! = 10
let num2: Int = num1
if num1 != nil {
print(num2 + 6)
}
if let num3 = num1 {
print(num3)
}
字符串插值
-
可选项在字符串插值挥着直接打印时,编译器会发出警告
image.png
-消除警告方法
let num1: Int? = 10
print(num1!)
print(num1 ?? 0)
print(String(describing: num1))
多重可选项
- 可以使用
lldb指令frame variable -R或者fr v -R查看区别
var num1: Int? = 10
var num2: Int?? = num1
var num3: Int?? = 10
print(num1 == num2)//true
print(num1 == num3)//true
print(num2 == num3)//true
image.png
(lldb) fr v -R num1
(Swift.Optional) num1 = some {
some = {
_value = 10
}
}
(lldb) fr v -R num2
(Swift.Optional>) num2 = some {
some = some {
some = {
_value = 10
}
}
}
(lldb) fr v -R num3
(Swift.Optional>) num3 = some {
some = some {
some = {
_value = 10
}
}
}
(lldb)
image.png
(lldb) fr v -R num1
(Swift.Optional) num1 = none {
some = {
_value = 0
}
}
(lldb) fr v -R num2
(Swift.Optional>) num2 = some {
some = none {
some = {
_value = 0
}
}
}
(lldb) fr v -R num3
(Swift.Optional>) num3 = none {
some = some {
some = {
_value = 0
}
}
}
(lldb)
Snip20190628_18.png