Swift 条件字符串插值

Question

当我需要根据某些条件（在本例中为 isFavorite）构建字符串时，我经常陷入这样的代码编写过程中：

let me = Contact(name: "Stefan", isFavorite: true)

var message = "Contact \(me.name)"
if me.isFavorite {
    message.append(" is a favorite contact")
}

对于这样一个简单的任务，这些是 4 行或者复杂的三元运算符（if ? then : else）。对此我一直心存愧疚...

有没有办法用 Swift 更优雅地做到这一点？

Answer 1

Swift 5

确实有 - 我在 UIKonf 2019 上找到了答案，我从 Erica Sadun 那里听说有一种方法可以通过使用 Swift 5 String Interpolation 在一行中实现这一点。您只需要这个可重复使用的扩展：

extension String.StringInterpolation {
    mutating func appendInterpolation(if condition: @autoclosure () -> Bool, _ literal: StringLiteralType) {
        guard condition() else { return }
        appendLiteral(literal)
    }
}

它使您能够将 4 行转换为一行：

"Contact: \(me.name)\(if: me.isFavorite, " is a favorite contact")"

您可以在 Erica 的文章中阅读 Swift 的字符串插值的不同可能性：https://ericasadun.com/2018/12/12/the-beauty-of-swift-5-string-interpolation/

游乐场

开始动手做这个：

import Foundation

// *****************************************************************************
// Many thanks to Erica Sadun for describing the new possibilities of string
// interpolation in Swift 5 
// *****************************************************************************


// *****************************************************************************
// Conditional Interpolation

struct Contact {
    let name: String
    let isFavorite: Bool
}
let me = Contact(name: "Stefan", isFavorite: true)

// ***************************************************************** Swift 4

var message = "Contact \(me.name)"
if me.isFavorite {
    message.append(" is favorite")
}
message // we need 4 lines to construct this message!!!
"Contact: \(me.name)\(me.isFavorite ? " is favorite" : "")" // or a complex ternary operator

// ***************************************************************** Swift 5

extension String.StringInterpolation {

    mutating func appendInterpolation(if condition: @autoclosure () -> Bool, _ literal: StringLiteralType) {
        guard condition() else { return }
        appendLiteral(literal)
    }
}
"Contact: \(me.name)\(if: me.isFavorite, " is favorite")" // simple and clean - no extras

// *****************************************************************************
// Optional Interpolation

let optionalMe: Contact? = Contact(name: "Stefan", isFavorite: true)

// ***************************************************************** Swift 4

"Contact: \(optionalMe?.name)" // shows warning

// ***************************************************************** Swift 5

// Interpolate nil value
extension String.StringInterpolation {
    /// Provides `Optional` string interpolation without forcing the
    /// use of `String(describing:)`.
    public mutating func appendInterpolation<T>(_ value: T?, default defaultValue: String) {
        if let value = value {
            appendInterpolation(value)
        } else {
            appendLiteral(defaultValue)
        }
    }
}
let nilContact: Contact? = nil
"Contact: \(nilContact?.name, default: "nil")"

// Strip `Optional`
extension String.StringInterpolation {
    /// Interpolates an optional using "stripped" interpolation, omitting
    /// the word "Optional" from both `.some` and `.none` cases
    public mutating func appendInterpolation<T>(describing value: T?) {
        if let value = value {
            appendInterpolation(value)
        } else {
            appendLiteral("nil")
        }
    }

}
"Contact: \(describing: optionalMe?.name)"

Answer 2

您可以使用Ternary operator

let msg = "Contact \(me.name)" + (me.isFavorite ? " is a favorite contact" : "")

Answer 3

我倾向于创建第二个 String 变量，它可能为空也可能不为空，然后无条件地附加它...

let me = Contact(name: "Stefan", isFavorite: true)
let favorite = me.isFavorite ? " is a favorite contact" : ""

var message = "Contact \(me.name)\(favorite)"

Swift 条件字符串插值

Swift Conditional String Interpolation

string

string-interpolation

swift

Swift 5

游乐场