直接上代碼

// 創(chuàng)建textField的時候，添加輸入監(jiān)聽
textField.addTarget(self, action: #selector(textDidChanged(_ :)), for: .editingChanged)

@objc func textDidChanged(_ tf: UITextField) {
        if let _ = tf.text {
            if let positionRange = tf.markedTextRange {
                if let _  = tf.position(from: positionRange.start, offset: 0) {
                    // 正在輸入，不校驗(yàn)
                } else {
                    if let text = tf.text, text.count > self.maxCount {
                        tf.text = (text as NSString).substring(to: 8)
                    }
                }
            } else {
                // 非鍵盤輸入
                if let text = tf.text, text.count > self.maxCount {
                    tf.text = (text as NSString).substring(to: 8)
                }
            }
        }
        Log("當(dāng)前輸入的內(nèi)容:\(tf.text ?? "")")
    }