swift_03_TextView

知识点：

1通过TextView的textViewDidChange事件捕捉当前输入内容，从而进行限制输入字数

2.通过监听IResponder.keyboardWillChangeFrameNotification事件来监视Keyboard的弹出和收起。在对应回调中，通过note.userInfo?[UIKeyboardFrameEndUserInfoKey]来拿到键盘的endFrame，从而拿到键盘的高度，对计数器进行frame操作

3.同理，通过note.userInfo?[UIKeyboardAnimationDurationUserInfoKey]拿到键盘的动画duration，进而可以通过UIView的animation动画做到同步变化计数器的frame

mport UIKit

class ViewController: UIViewController,UITextViewDelegate {
    
    var limitedTextView:UITextView!
    var allowInputNumberLabel:UILabel!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.backgroundColor = UIColor.white
        self.initInputField()
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillChangeFrame(note:)), name:UIResponder.keyboardWillChangeFrameNotification, object: nil)

    }
    
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
    
    func initInputField() {
 
        limitedTextView = UITextView(frame: CGRect(x: 80, y: 80, width: self.view.frame.width - 80*2, height: 300))
        self.view.addSubview(limitedTextView)
        limitedTextView.delegate = self
        limitedTextView.font = UIFont.systemFont(ofSize: 20)
        limitedTextView.backgroundColor = UIColor.gray
        allowInputNumberLabel = UILabel(frame: CGRect(x: self.view.frame.width - 50, y: self.view.frame.height - 40, width: 50, height: 40))
        self.view.addSubview(allowInputNumberLabel)
        allowInputNumberLabel.text = "20"
        allowInputNumberLabel.textAlignment = .right
    }
 
    //    todo need to limit the paste action so that the length might be exceed!
    func textViewDidChange(_ textView: UITextView) {
        let currentCharactorCount = (textView.text.count)
        if  currentCharactorCount >= 20 {
            limitedTextView.resignFirstResponder()
        }
        allowInputNumberLabel.text = "\(20 - currentCharactorCount)"
    }
    
    @objc func keyboardWillChangeFrame(note: Notification) {
        let duration = note.userInfo?[UIResponder.keyboardAnimationDurationUserInfoKey] as! TimeInterval
        let endFrame = (note.userInfo?[UIResponder.keyboardFrameEndUserInfoKey] as! NSValue).cgRectValue
        let y = endFrame.origin.y
        
        //计算工具栏距离底部的间距
        let margin = UIScreen.main.bounds.height - y
        UIView.animate(withDuration: duration) {
            //            键盘弹出
            if margin > 0 {
                self.allowInputNumberLabel.frame.origin.y = self.allowInputNumberLabel.frame.origin.y - margin
            }
                //            键盘收起
            else {
                self.allowInputNumberLabel.frame.origin.y = self.view.frame.height - 40
            }
        }
    }
}