Swift基础:将闭包改为逃逸型场景

alt+左键 查看类型

常量

// 定义常量
let a = 10
let b = 20.5

// 常量运算
(X) let result = a + b
Binary operator ‘+’ cannot be applied to operands of type ‘Int’ and ‘Double

// 转换类型
double型转为Int(b)

// 指定常量类型
指定变量/常量的类型写法: 在名字后面加上 : 类型

    let a: Double = 10.5

// Cannot assign to value: ‘a’ is a ‘let’ constant

    var a: Double = 10.5
    a = 20

//Value of optional type ‘URL?’ not unwrapped; did you mean to use ‘!’ or ‘?’?

    let request = URLRequest(url: url!)

! 就代表[强行解包],代表告诉编译器,!前面的变量或者常量一定有值
/// fatal error: unexpectedly found nil while unwrapping an Optional value

// 变量的可选值默认值是 nil

    var c: Int?
    print(c as Any)

// 定义可选类型

func demo3(){

    let a:Int? = 10
    let c:Optional = 10
    let b:Optional<Int?> = 10

}

如果要传到其他闭包其他方法中去,需要将闭包改为逃逸型