Swift字面量(literal)

在Swift中，可以通過字面量來初始化實例，比如：var a = 1, 就是通過字面量1初始化一個Int實例。那麼爲什麼能通過字面量初始化實例呢？接下來我們學習下字面量的知識。

一、字面量初始化實例

下面列舉了通過字面量來初始化的類型，後面的註釋是該類型遵守的字面量協議，一個類型可以通過字面量來初始化的原因是遵守了對應的協議。

func testLiteral() {
    
    var b: Bool = false // ExpressibleByBooleanLiteral
    var i: Int = 10 // ExpressibleByIntegerLiteral
    var f0: Float = 10 // ExpressibleByIntegerLiteral
    var f1: Float = 10.1 // ExpressibleByFloatLiteral
    var d0: Double = 10 // ExpressibleByIntegerLiteral
    var d1: Double = 10.1 // ExpressibleByFloatLiteral
    var s: String = "dandy" // ExpressibleByStringLiteral
    var arr: Array = [1, 2, 3] // ExpressibleByArrayLiteral
    var set: Set = [1, 2, 3] // ExpressibleByArrayLiteral
    var dict: Dictionary = ["dandy": 30] // ExpressibleByDictionaryLiteral
    var o: Optional<Int> = nil // ExpressibleByNilLiteral

    // 按字符串插值表示
    let message = "One cookie: $\(f1), \(f0) cookies: $\(f0 * f1)." // ExpressibleByStringInterpolation

    // 按Unicode標量文字表示
    let ñ: Unicode.Scalar = "ñ" // ExpressibleByUnicodeScalarLiteral
    let n: String = "ñ" //ExpressibleByUnicodeScalarLiteral
    let n2: Character = "ñ" //ExpressibleByUnicodeScalarLiteral
    
    // 擴展的字素簇
    let snowflake: Character = "❄︎" // ExpressibleByExtendedGraphemeClusterLiteral
    let snowflakeStr: String = "❄︎" // ExpressibleByExtendedGraphemeClusterLiteral
}

二、自定義通過字面量初始化實例的方式

如果希望自己的類型實例可以通過字面量初始化，只需要遵守相應字面量協議即可。

class ZLStudent: ExpressibleByStringLiteral, ExpressibleByIntegerLiteral {
    var name: String = ""
    var score: Double = 0
    
    required init(integerLiteral value: IntegerLiteralType) {
        self.score = Double(value)
    }
    
    required init(stringLiteral value: StringLiteralType) {
        self.name = value
    }
}