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01 - Swift 4 新增内容.md

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初始化一个多行字符串

可以直接在多行字符串中包含"

let quotation = """
The White Rabbit put on his spectacles.  "Where shall I begin,
please your Majesty?" he asked.
 
"Begin at the beginning," the King said gravely, "and go on
till you come to the end; then stop."
"""

可以直接在多行字符串中包含"""

let threeDoubleQuotes = """
Escaping the first quote \"""
Escaping all three quotes \"\"\"
"""

以下两个字符串是等价的：

let singleLineString = "These are the same."
let multilineString = """
These are the same.
"""

在字符串前后加空行，可以这样写：

"""
 
This string starts with a line feed.
It also ends with a line feed.
 
"""

如果一个多行字符串定义在一个方法中，实际字符串的值是不包含每一行前面的空格：

func generateQuotation() -> String {
    let quotation = """
        The White Rabbit put on his spectacles.  "Where shall I begin,
        please your Majesty?" he asked.
 
        "Begin at the beginning," the King said gravely, "and go on
        till you come to the end; then stop."
        """
    return quotation
}
print(quotation == generateQuotation())
// Prints "true"

如果把字符串改为下面的这个样子，第二行字符串前面的空格是不能忽略的：

func generateQuotation() -> String {
    let quotation = """
        The White Rabbit put on his spectacles.  "Where shall I begin,
                please your Majesty?" he asked.
 
        "Begin at the beginning," the King said gravely, "and go on
        till you come to the end; then stop."
        """
    return quotation
}
print(quotation == generateQuotation())
// Prints "false"

泛型下标 (Generic Subscripts)

下标可以是泛型的，并且可以包含where语句：

extension Container {
    subscript<Indices: Sequence>(indices: Indices) -> [Item]
        where Indices.Iterator.Element == Int {
            var result = [Item]()
            for index in indices {
                result.append(self[index])
            }
            return result
    }
}

上述代码的意思是：接受一个下标序列，并返回对应的元素组成的数组：

  • Indices必须遵循Sequence协议
  • indices必须是Indices类型
  • where语句要求Indices的元素必须是Int类型

协议组合中可以包含父类

例如下面这个例子：

class Location {
    var latitude: Double
    var longitude: Double
    init(latitude: Double, longitude: Double) {
        self.latitude = latitude
        self.longitude = longitude
    }
}
class City: Location, Named {
    var name: String
    init(name: String, latitude: Double, longitude: Double) {
        self.name = name
        super.init(latitude: latitude, longitude: longitude)
    }
}
func beginConcert(in location: Location & Named) {
    print("Hello, \(location.name)!")
}
 
let seattle = City(name: "Seattle", latitude: 47.6, longitude: -122.3)
beginConcert(in: seattle)
// Prints "Hello, Seattle!"

在beginConcert方法中，location参数必须Location类型，并且遵循Named协议。

final关键字不能用在协议扩展中

在协议扩展里面，我们不能使用final。