Custom String Interpolation

⭐️ 注意：String.StringInterpolation == DefaultStringInterpolation

讓 String interpolation 可以插入新的型別或自訂插入方式：

extend DefaultStringInterpolation
add an overload of appendInterpolation(_:)

let name = "Joe"
let age  = 17

// ⭐️ String Interpolation:
// ------------------------
// 1. String
// 2. Int
// 3. struct User
// 4. class Person

// String 本來就知道如何插入一些內建的型別
"My name is \(name), I'm \(age) years old"
//          ╰──1──╯      ╰─2──╯

struct User {
    var name: String
    var age : Int
}

let joe = User(name: "Joe", age: 17)

// 就算是一個 struct，也沒問題。
"Hi, I'm \(joe)"        // Hi, I'm User(name: "Joe", age: 17)
//       ╰─3──╯

class Person {
    var name: String = ""
    var age : Int    = 0
}

// class instance 也可以
let nobody = Person()
"Hi, I'm \(nobody)"     // Hi, I'm Person
//       ╰── 4 ──╯

// ⭐️ Custom String Interpolation
// -------------------------------
// 如果要對特定的型別做客製化的 String Interpolation，
// 就必須擴充 DefaultStringInterpolation。

// ⭐️ 注意：String.StringInterpolation == DefaultStringInterpolation
extension String.StringInterpolation {
    mutating func appendInterpolation(_ user: User) {
        appendInterpolation("My name is \(user.name) and I'm \(user.age) years old.")
    }
}

"\(joe)"        // My name is Joe and I'm 17 years old.

可控制數字的小數位數。

import Foundation     // for String(format: "%.2f", n)
import SwiftUI        // for CGFloat

extension DefaultStringInterpolation {
    // ⭐️ `\(cgfloat, format: "%.2f")`
    mutating func appendInterpolation(_ n: CGFloat, format: String) {
        let result = String(format: "%.2f", n)
        appendLiteral(result)
    }
}

let n: CGFloat = 2.3456
"\(n, format: "%.2f")"        // 2.35

import Foundation  // for NumberFormatter, DateFormatter

extension DefaultStringInterpolation {
    
    // ⭐️ "\(int, style: .spellOut)"
    mutating func appendInterpolation(_ value: Int, style: NumberFormatter.Style) {
        // create and configure NumberFormatter
        let formatter = NumberFormatter()
        formatter.numberStyle = style
        // format the value
        if let result = formatter.string(from: value as NSNumber) {
            appendLiteral(result)
        }
    }
    
    // ⭐️ "\(date, style: .short)"
    mutating func appendInterpolation(_ value: Date, style: DateFormatter.Style) {
        // create and configure DateFormatter
        let formatter = DateFormatter()
        formatter.dateStyle = .short
        // format the value
        let result = formatter.string(from: value)
        appendLiteral(result)
    }
    
    // ⭐️ "\(date, format: "yyyy.MM.dd")"
    mutating func appendInterpolation(_ value: Date, format: String) {
        // create and configure DateFormatter
        let formatter = DateFormatter()
        formatter.dateFormat = format
        // format the value
        let result = formatter.string(from: value)
        appendLiteral(result)
    }
    
    // ⭐️ "\(twitter: username)"
    mutating func appendInterpolation(twitter: String) {
        let link = "<a href=\"https://twitter.com/\(twitter)\">@\(twitter)</a>"
        appendLiteral(link)
    }
    
    // ⭐️ "\(names, empty: "default value")"
    mutating func appendInterpolation(
        _ values: [String], 
        empty defaultValue: @autoclosure () -> String
    ) {
        // ⭐️ why we need the "; return" part:
        // https://stackoverflow.com/questions/31138273/swift-error-guard-body-must-not-fall-through
        guard !values.isEmpty else { appendLiteral(defaultValue()); return }
        appendLiteral(values.joined(separator: ", "))
    }
}

// Int
let age = 38
"you are \(age)"                    // "you are 38"
"you are \(age, style: .spellOut)"  // "you are 三十八"

// Date
let now = Date()

"Today's date is \(now)"         
// "Today's date is 2020-08-16 10:17:03 +0000"

"Today's date is \(now, style: .short)"         
// "Today's date is 2020/8/16"

"Today's date is \(now, format: "yyyy.MM.dd")"  
// "Today's date is 2020.08.16"

"Twitter: \(twitter: "joe")"
// Twitter: <a href="https://twitter.com/joe">@joe</a>

// [String]
let none = [String]()
let names = ["Mike", "Jane", "Kelly"]

"Crew: \(none)"
"Crew: \(names, empty: "No one")."    // "Crew: Mike, Jane, Kelly"
"Crew: \(none, empty: "No one")"      // "Crew: No one"

Swift ⟩
- Initialization with Literals ⟩
  - DefaultStringInterpolation (struct)
  - ExpressibleByStringInterpolation
- Basic Behaviors ⟩
  - CustomStringConvertible (protocol) - for String(describing:) and print(_:)

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Last updated 2 years ago