检查并比较字符串

检查字符串是否为空：

if str.isEmpty {
    // do something if the string is empty
}

// If the string is empty, replace it with a fallback:
let result = str.isEmpty ? "fallback string" : str

检查两个字符串是否相等（在 Unicode 规范等价意义上 ）：

"abc" == "def"          // false
"abc" == "ABC"          // false
"abc" == "abc"          // true

// "LATIN SMALL LETTER A WITH ACUTE" == "LATIN SMALL LETTER A" + "COMBINING ACUTE ACCENT"
"\u{e1}" == "a\u{301}"  // true

检查字符串是否以另一个字符串开头/结尾：

"fortitude".hasPrefix("fort")      // true
"Swift Language".hasSuffix("age")  // true