主机插耳机没声音问题

如果要在Go语言中做字符串模板替换，比如配置模板实例化，你会如何做呢？

fmt.Sprintfos.Expand

先来个栗子开胃。

var config = `
app.name = ${appName}
app.ip = ${appIP}
app.port = ${appPort}
`

var dev = map[string]string{
	"appName": "my_app",
	"appIP":   "0.0.0.0",
	"appPort": "8080",
}

func main() {
	s := os.Expand(config, func(k string) string { return dev[k] })
	fmt.Println(s)
}

/**output**

app.name = my_app
app.ip = 0.0.0.0
app.port = 8080
*/

os.Expand

${xxx}${xxx}

让我们带着这两个疑问，进入源码一探究竟。

os.Expand

// Expand根据mapping函数替换字符串中的${var}或$var
// 例如, os.ExpandEnv(s)等价于os.Expand(s, os.Getenv)
func Expand(s string, mapping func(string) string) string {
	var buf []byte
	// ${} is all ASCII, so bytes are fine for this operation.
	i := 0
	for j := 0; j < len(s); j++ {
		if s[j] == '$' && j+1 < len(s) {
			if buf == nil {
				buf = make([]byte, 0, 2*len(s))
			}
			buf = append(buf, s[i:j]...)
			name, w := getShellName(s[j+1:])
			if name == "" && w > 0 {
				// Encountered invalid syntax; eat the
				// characters.
			} else if name == "" {
				// Valid syntax, but $ was not followed by a
				// name. Leave the dollar character untouched.
				buf = append(buf, s[j])
			} else {
				buf = append(buf, mapping(name)...)
			}
			j += w
			i = j + 1
		}
	}
	if buf == nil {
		return s
	}
	return string(buf) + s[i:]
}

{}

buf

i$getShellName{}

// getShellName returns the name that begins the string and the number of bytes
// consumed to extract it. If the name is enclosed in {}, it's part of a ${}
// expansion and two more bytes are needed than the length of the name.
func getShellName(s string) (string, int) {
	switch {
	case s[0] == '{':
		if len(s) > 2 && isShellSpecialVar(s[1]) && s[2] == '}' {
			return s[1:2], 3
		}
		// Scan to closing brace
		for i := 1; i < len(s); i++ {
			if s[i] == '}' {
				if i == 1 {
					return "", 2 // Bad syntax; eat "${}"
				}
				return s[1:i], i + 1
			}
		}
		return "", 1 // Bad syntax; eat "${"
	case isShellSpecialVar(s[0]):
		return s[0:1], 1
	}
	// Scan alphanumerics.
	var i int
	for i = 0; i < len(s) && isAlphaNum(s[i]); i++ {
	}
	return s[:i], i
}

// isShellSpecialVar reports whether the character identifies a special
// shell variable such as $*.
func isShellSpecialVar(c uint8) bool {
	switch c {
	case '*', '#', '$', '@', '!', '?', '-', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
		return true
	}
	return false
}

// isAlphaNum reports whether the byte is an ASCII letter, number, or underscore
func isAlphaNum(c uint8) bool {
	return c == '_' || '0' <= c && c <= '9' || 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z'
}

getShellName

${xxx}x$xx$xxxx

第一种情况是最常用的，它对名称没什么限制，适用范围广，对人友好。

$12$1

$name_$id${name}_${id}

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