xcode - 无法推断通用参数 'Self' (swift 4)

Question

我想为 string 做一个子字符串扩展，我尝试了以下两种方法但没有成功:

extension String
{
    func substringToFirstChar(of char: Character) -> String
    {
        let pos = self.range(of: String(char))
        let subString = self[..<pos?.lowerBound]   
        return String(subString)
    }
}

或者

extension String
{
    func substringToFirstChar(of char: Character) -> String
    {
        let pos = self.index(of: char)
        let subString = self[..<pos]
        return String(subString)
    }
}

xcode prompt error: Generic parameter 'Self' could not be inferred at the "let subString = self[.." line.

怎么做 ？

Answer 1

range(of/index(of返回一个可选的。您必须在范围表达式中解开可选项

extension String
{
    func substringToFirstChar(of char: Character) -> String?
    {
        guard let pos = self.range(of: String(char))?.lowerBound else { return nil }
        // or  guard let pos = self.index(of: char) else { return nil }
        let subString = self[..<pos]
        return String(subString)
    }
}

或者 - 避免可选 - 如果没有匹配，则返回未更改的字符串

extension String
{
    func substringToFirstChar(of char: Character) -> String
    {
        guard let pos = self.range(of: String(char))?.lowerBound else { return self }
        // or  guard let pos = self.index(of: char) else { return self }
        let subString = self[..<pos]
        return String(subString)
    }
}