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87 lines
3.0 KiB
Markdown
87 lines
3.0 KiB
Markdown
# Swift 字面量本质
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## 常见的字面量默认类型
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- `public typealias IntegerLiteralType = Int`
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- `public typealias FloatLiteralType = Float`
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- `public typealias BooleanLiteralType = Bool`
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- `public typealias StringLiteralType = String`
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可以通过 typealias 修改字面量的默认类型，Demo 如下：
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```swift
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typealias IntegerLiteralType = UInt8
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var val = 8
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val
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```
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<img src="https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/SwiftChangeLiteral.png" style="zoom:25%">
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## 字面量协议
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Swift 自带的数据类型基本都可通过字面量初始化，本质原因是遵循了对应的协议
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| | | |
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| ------------- | ------------------------------------------------------ | ------------------------------------------------------------ |
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| Bool | ExpressibleByBooleanLiteral | var b:Bool = false |
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| Int | ExpressibleByIntegerLiteral | var num:Int = 2 |
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| Float、Double | ExpressibleByIntegerLiteral、ExpressibleByFloatLiteral | var height:Float = 175 <br>var height1:Float = 175.2 <br>var weight:Double = 130 <br>var weight1:Double = 130.1 |
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| Dictionary | ExpressibleByDictionaryLiteral | var dic:Dictionary = ["name": "FantasticLBP"] |
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| String | ExpressibleByStringLiteral | var name:String = "FantasticLBP" |
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| Array、Set | ExpressibleByArrayLiteral | var arr:Array = [1, 2, 3] |
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| Optional | ExpressibleByNilLiteral | var o:Optional<Int> = nil |
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## 字面量协议的使用
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Demo1
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```swift
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extension Int : ExpressibleByBooleanLiteral {
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public init(booleanLiteral value: Bool) {
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self = value ? 1 : 0
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}
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}
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var num: Int = true
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print(num) // 1
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num = false
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print(num) // 0
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```
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Demo2
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```swift
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class Student: ExpressibleByIntegerLiteral, ExpressibleByFloatLiteral, ExpressibleByStringLiteral, CustomStringConvertible {
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var name: String = ""
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var score: Double = 0
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required init(floatLiteral value: FloatLiteralType) {
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self.score = value
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}
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required init(stringLiteral value: StringLiteralType) {
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self.name = value
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}
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required init(integerLiteral value: IntegerLiteralType) {
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self.score = Double(value)
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}
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required init(extendedGraphemeClusterLiteral value: String) {
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self.name = value
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}
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required init(unicodeScalarLiteral value: String) {
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self.name = value
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}
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var description: String {
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"name is \(self.name), score is \(self.score)"
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}
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}
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var student: Student = "杭城小刘"
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print(student) // name is 杭城小刘, score is 0.0
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student = 100
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print(student) // name is , score is 100.0
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```
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