计算字符串的字符数

在开发时，我们经常遇到限制一个字符串的长度，有时候会要求不超过N个字符，简单封装了下。

1. 计算字符串的字符数

extension String {
    /// 字符数
    var charactersCount: Int {
        guard !isEmpty else { return 0 }
        var bytesCount = count
        let array = Array(self)
        for i in 0..<count {
            let charValue = array[i]
            if let asciiValue = charValue.utf8.first {
                if asciiValue < 0 || asciiValue > 127 {
                    bytesCount += 1
                }
            }
        }
        return bytesCount
    }
}

2. 截取前N个字符

extension String {
    /// 获取前maxCharacterCount个字符对应的字符串
    /// - Parameter maxCharacterCount: 最大字符数
    /// - Returns: 截取的字符串
    func prefixText(with maxCharacterCount: Int) -> Self {
        guard !isEmpty else { return "" }

        var maxLength = maxCharacterCount
        var subLength = 0
        let array = Array(self)
        for i in 0..<count {
            let charValue = array[i]
            maxLength -= 1

            if let asciiValue = charValue.utf8.first {
                if asciiValue < 0 || asciiValue > 127 {
                    maxLength -= 1
                }
            }

            if maxLength < 0 {
                break
            } else {
                subLength += 1
            }
        }
        return Self(prefix(subLength))
    }
}

3. 监听UITextField内容变化，若字符数超出N个，则截取前N个字符数

extension ViewController: UITextViewDelegate {
    func textViewDidChange(_ textView: UITextView) {
        guard let text = textView.text.qjn_stringByTrim() else {
            // do something
            return
        }

        if let selectedRange = textView.markedTextRange, let pos = textView.position(from: selectedRange.start, offset: 0) {
            return
        }

        DispatchQueue.global().async {
            if text.charactersCount > N {
                DispatchQueue.main.asyncAfter(deadline: .now() + 0.01) {
                    textView.text = text.prefixText(with: N)
                    // do something
                }
            } else {
                // do something
            }
        }
    }
}