从Swift2开始，String不再遵守CollectionType协议，即不再是集合类型，但是String的属性characters遵守CollectionType协议。

String的characters属性的类型是CharacterView(结构体)

获取字符串String中单独的字符Character：

for character in "Dog".characters {
    print(character)
}

使用Character类型的数组构造字符串String：

let catCharacters: [Character] = ["C","a","t","!"]
let catString = String(catCharacters)

将字符串String转化为字符数组[Character]：

let string: String = "abcdefg"
var characters = [Character]()
for character in string.characters {
    characters.append(character)
}