下面是一个简单的实现(Swift 5.4+)，使用resultBuilder来清理语法!

@resultBuilder
public struct StringBuilder {
    public static func buildBlock(_ components: String...) -> String {
        return components.reduce("", +)
    }
}

public extension String {
    init(@StringBuilder _ builder: () -> String) {
        self.init(builder())
    }
}

用法:

String {
    "Hello "
    "world!"
} 
// "Hello world!"

加上@Connor的回答，也需要有\n。以下是修改后的代码:

var text:String = "This is some text \n" +
                  "over multiple lines"

迅速:

@connor是正确答案，但如果你想在打印语句中添加行，你要寻找的是\n和/或\r，这些被称为转义序列或转义字符，这是一个关于该主题的Apple文档链接。

例子:

print("First line\nSecond Line\rThirdLine...")

一种方法是将标签文本设置为attributedText，并更新字符串变量以包含换行符的HTML (<br />)。

例如:

var text:String = "This is some text<br />over multiple lines"
label.attributedText = text

输出:

This is some text
over multiple lines

希望这能有所帮助!

下面是一个代码片段，用n个字符分隔字符串:

//: A UIKit based Playground for presenting user interface

import UIKit
import PlaygroundSupport

class MyViewController : UIViewController {
    override func loadView() {

        let str = String(charsPerLine: 5, "Hello World!")
        print(str) // "Hello\n Worl\nd!\n"

    }
}

extension String {

    init(charsPerLine:Int, _ str:String){

        self = ""
        var idx = 0
        for char in str {
            self += "\(char)"
            idx = idx + 1
            if idx == charsPerLine {
                self += "\n"
                idx = 0
            }
        }

    }
}

Swift 4支持多行字符串字面值。除了换行符，它们还可以包含未转义的引号。

var text = """
    This is some text
    over multiple lines
    """

旧版本的Swift不允许你在多行上有一个字面值，但你可以在多行上添加字面值:

var text = "This is some text\n"
         + "over multiple lines\n"