alt+左鍵 檢視型別

常量

// 定義常量
let a = 10
let b = 20.5

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

// 轉換型別
double型轉為Int(b)

// 指定常量型別
指定變數/常量的型別寫法: 在名字後面加上 : 型別

    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[強行解包],代表告訴編譯器,!前面的變數或者常量一定有值
/// fatal error: unexpectedly found nil while unwrapping an Optional value

// 變數的可選值預設值是 nil

    var c: Int?
    print(c as Any)

// 定義可選型別

func demo3(){

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

}
 

如果要傳到其他閉包其他方法中去,需要將閉包改為逃逸型