Swift中Range的使用

相比OC中的NSRange，在Swift中使用Range是一个比较麻烦的事情，犹记得第一个使用，感觉写法很复杂，这里简单介绍下它的用法。

Close Ragne: a...b

这种操作创建了一个包括a和b的区间，有两种不同的闭区间，CloseRange和CountableClosedRange

• CloseRange

在Swift中所有Ranges中的元素都是可比的，遵循Comparable协议，这让我们可以获取集合中任意区间的元素

let myRange: ClosedRange = 1...3

let myArray = ["a", "b", "c", "d", "e"]
myArray[myRange] // ["b", "c", "d"]

• CountableClosedRange

它同CloseRange的区别就是它可以遍历，遵循了Sequence协议

let myRange: CountableClosedRange = 1...3

let myArray = ["a", "b", "c", "d", "e"]
myArray[myRange] // ["b", "c", "d"]

for index in myRange {
    print(myArray[index])
}

Half-Open Ranges: a..<b

这是个左闭右开的区间，包括a但是不包括b。同样也有两种不同的类型：Range和CountableRange

• Range

使用同ClosedRange类似

let myRange: Range = 1..<3

let myArray = ["a", "b", "c", "d", "e"]
myArray[myRange] // ["b", "c"]

• CountableRange

let myRange: CountableRange = 1..<3

let myArray = ["a", "b", "c", "d", "e"]
myArray[myRange] // ["b", "c"]

for index in myRange {
    print(myArray[index])
}

NSRange

在swift中同样也是需要使用NSRange，因为通过不同的含义来创建的。

let myNSRange = NSRange(location: 3, length: 2)

let myRange: Range = 3..<5

它是通过起始位置和长度来确定区间范围，不同于Range通过起始和结束的位置。

Ranges with Strings

//Range
var myString = "abcde"
let start = myString.index(myString.startIndex, offsetBy: 1)
let end = myString.index(myString.startIndex, offsetBy: 4)
let myRange = start..<end
myString.substring(with: myRange) // "bcd"

//NSRange
let myNSRange = NSRange(location: 1, length: 3)

let myNSString: NSString = "abcde"
myNSString.substring(with: myNSRange) // "bcd"

可以看到NSRange比Range要简洁多了，既然这样，为什么苹果还要想出个Range类型呢？

//Range
var myString = "a😀cde"
let start2 = myString.index(myString.startIndex, offsetBy: 1)
let end2 = myString.index(myString.startIndex, offsetBy: 4)
let myRange2 = start2..<end2
myString.substring(with: myRange2) // "😀cd"

//NSRange
let myNSString2: NSString = "a😀cde"
myNSString2.substring(with: myNSRange) // "😀c"    Where is the "d"!?

因为emoji笑脸占用了两个UTF-16单元去存储，所以和我们预期的就有所不同了。

Extension

可是要使用String时，Range写法好复杂，该怎么办？这里我们可以拓展String对象。

extension String {
    subscript(r: ClosedRange<Int>) -> String {
        let start = index(startIndex, offsetBy: r.lowerBound)
        let end = index(startIndex, offsetBy: r.upperBound)
        return self[start...end]
    }
    
    subscript(r: Range<Int>) -> String {
        get {
            let start = index(startIndex, offsetBy: r.lowerBound)
            let end = index(startIndex, offsetBy: r.upperBound)
            return self[start..<end]
        }
    }
}

//usage
"abcde"[1...3] = "bcd"
"abcde"[1..<3] = "bc"