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"
