斯威夫特的扫描仪.charactersToBeSkipped是如何工作的？

Question

有人能解释一下，为什么在默认的.charactersToBeSkipped设置为.whitespacesAndNewlines的情况下，扫描仪在“扫描1”(参见代码)中的第一个空格处停止呢？看来我不懂跳过字符的概念。

扫描2-4只是测试用例。Xcode 11 / Swift 4

游乐场：

let hexCs = CharacterSet(charactersIn: "1234567890abcdefABCDEF")
let hexWhiteCs = CharacterSet().union(hexCs).union(.whitespacesAndNewlines)

let ascii = "<19 19 19 19>"

let scanner = Scanner(string:ascii)
_ = scanner.scanString("<")
let reset = scanner.currentIndex
/// Scan 1:
var result = scanner.scanCharacters(from: hexCs) // "19"
/// Scan 2:
scanner.charactersToBeSkipped = nil
scanner.currentIndex = reset
result = scanner.scanCharacters(from: hexCs) // "19"
/// Scan 3:
scanner.currentIndex = reset
result = scanner.scanCharacters(from: hexWhiteCs) // "19 19 19 19"
/// Scan 4:
scanner.charactersToBeSkipped = .whitespacesAndNewlines
scanner.currentIndex = reset
result = scanner.scanCharacters(from: hexWhiteCs) // "19 19 19 19"

let expected = result!.filter { !CharacterSet.whitespacesAndNewlines.contains($0.unicodeScalars.first!) } // "19191919"

Answer 1

文档可以：

要跳过的字符在扫描仪检查目标之前被跳过。例如，如果扫描器忽略空格并向其发送scanInt:消息，则它跳过空格，直到找到十进制数字或其他字符。当一个元素被扫描时，没有字符被跳过。