问当键盘向上移动时，如何使textFields保持在位置上？斯威夫特

EN

import UIKit

class ViewController: UIViewController,UITextFieldDelegate {

//added in storyboard. removed the code
@IBOutlet weak var nextButton: UIButton!

@IBOutlet weak var scrollView: UIScrollView!
@IBOutlet weak var contentView: UIView!
    override func viewDidLoad() {
        super.viewDidLoad()
        //called whenever keyboard is shown/hidden

        registerForKeyboardNotifications()

        //when identifies single or multiple taps, call DismissKeyboard
        var tap: UITapGestureRecognizer = UITapGestureRecognizer(target: self, action: "DismissKeyboard")
        contentView.addGestureRecognizer(tap)

        //disable scroll bouncing
        scrollView.bounces = false

        //replace with your textfields
        for subs in self.contentView.subviews{
            if subs is UITextField{
                print("setting")
                (subs as! UITextField).delegate = self
            }
        }
    }


    //Call this function when the tap is recognized.
    func DismissKeyboard(){
        contentView.endEditing(true)
    }



    // Stop Editing on Return Key Tap.
    func textFieldShouldReturn(_ textField: UITextField) -> Bool {
        textField.resignFirstResponder()
        return true
    }

    //edited for next button
    weak var activeField: UITextField?
    func keyboardDidShow(_ notification: Notification) {

        //when a textfield is edited lift the button above the keyboard
        if let activeField = self.activeField,let keyboardSize =
            (notification.userInfo?[UIKeyboardFrameBeginUserInfoKey] as?
                NSValue)?.cgRectValue {

            //20 in insets and offset is just padding
            let contentInsets = UIEdgeInsets(top: 0.0, left: 0.0, bottom:
                keyboardSize.height + 20 + nextButton.bounds.height, right: 0.0)
            self.scrollView.contentInset = contentInsets

            var aRect = self.view.frame
            aRect.size.height -= keyboardSize.height


            let bottomPoint = CGPoint(x: activeField.frame.origin.x, y:activeField.frame.origin.y)

            if aRect.contains(bottomPoint){
                let scrollPoint = CGPoint(x: 0.0, y: bottomPoint.y - keyboardSize.height - 20 - nextButton.bounds.height)
                scrollView.setContentOffset(scrollPoint, animated: true)
            }

        }

    }
        func keyboardWillHide(_ notification: Notification) {

            let contentInsets = UIEdgeInsets.zero
            self.scrollView.contentInset = contentInsets
            self.scrollView.scrollIndicatorInsets = contentInsets
        }

        //Keep track of which textfield is being edited to make sure the field is visible when keyboard pops up
        func textFieldDidBeginEditing(_ textField: UITextField) {
            self.activeField = textField
        }


        func textFieldDidEndEditing(_ textField: UITextField, reason: UITextFieldDidEndEditingReason) {
            self.activeField = nil
        }

        //register for keyboard notifications
        func registerForKeyboardNotifications() {

            NotificationCenter.default.addObserver(self, selector:
                #selector(keyboardDidShow),
                                                   name: NSNotification.Name.UIKeyboardDidShow, object: nil)

            NotificationCenter.default.addObserver(self, selector:
                #selector(keyboardWillHide), name:
                NSNotification.Name.UIKeyboardWillHide, object: nil)
        }

        //remove keyBoard observers
        func deregisterFromKeyboardNotifications() {
            NotificationCenter.default.removeObserver(self, name: 
                NSNotification.Name.UIKeyboardDidShow, object: nil)

            NotificationCenter.default.removeObserver(self, name: 
                NSNotification.Name.UIKeyboardWillHide, object: nil)
        }



        deinit {
            //deregister keyboard notifications
            deregisterFromKeyboardNotifications()
        }
} //end of class

import UIKit

class AvoidingConstraint: NSLayoutConstraint {

    private var offset : CGFloat = 0
    private var keyboardVisibleHeight : CGFloat = 0

    override public func awakeFromNib() {
        super.awakeFromNib()

        offset = constant

        NotificationCenter.default.addObserver(self, selector: #selector(AvoidingConstraint.keyboardWillShowNotification(_:)), name: NSNotification.Name.UIKeyboardWillShow, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(AvoidingConstraint.keyboardWillHideNotification(_:)), name: NSNotification.Name.UIKeyboardWillHide, object: nil)
    }

    deinit {
        NotificationCenter.default.removeObserver(self)
    }

    // MARK: Notification

    func keyboardWillShowNotification(_ notification: Notification) {
        if let userInfo = notification.userInfo {
            if let frameValue = userInfo[UIKeyboardFrameEndUserInfoKey] as? NSValue {
                let frame = frameValue.cgRectValue
                keyboardVisibleHeight = frame.size.height
            }

            self.updateConstant()
            switch (userInfo[UIKeyboardAnimationDurationUserInfoKey] as? NSNumber, userInfo[UIKeyboardAnimationCurveUserInfoKey] as? NSNumber) {
            case let (.some(duration), .some(curve)):

                let options = UIViewAnimationOptions(rawValue: curve.uintValue)

                UIView.animate(
                    withDuration: TimeInterval(duration.doubleValue),
                    delay: 0,
                    options: options,
                    animations: {
                        UIApplication.shared.keyWindow?.layoutIfNeeded()
                        return
                }, completion: { finished in
                })
            default:

                break
            }

        }

    }

    func keyboardWillHideNotification(_ notification: NSNotification) {
        keyboardVisibleHeight = 0
        self.updateConstant()

        if let userInfo = notification.userInfo {

            switch (userInfo[UIKeyboardAnimationDurationUserInfoKey] as? NSNumber, userInfo[UIKeyboardAnimationCurveUserInfoKey] as? NSNumber) {
            case let (.some(duration), .some(curve)):

                let options = UIViewAnimationOptions(rawValue: curve.uintValue)

                UIView.animate(
                    withDuration: TimeInterval(duration.doubleValue),
                    delay: 0,
                    options: options,
                    animations: {
                        UIApplication.shared.keyWindow?.layoutIfNeeded()
                        return
                }, completion: { finished in
                })
            default:
                break
            }
        }
    }

    func updateConstant() {
        self.constant = offset + keyboardVisibleHeight
    }

}

let keyboardScreenEndFrame = (userInfo[UIKeyboardFrameEndUserInfoKey] as? NSValue ?? NSValue()).cgRectValue
let keyboardViewEndFrame = view.convert(keyboardScreenEndFrame, from: view.window)
UIView.animate(withDuration: 0.2) {
    if notification.name == Notification.Name.UIKeyboardWillHide {
        self.view.frame = CGRect(x: 0, y: 0, width: self.view.width, height: self.view.height)
    } else {
        let offset = (self.view.frame.size.height - self.activeField.frame.maxY) - keyboardViewEndFrame.height
        if offset < 0 {
            self.view.frame = CGRect(x: 0, y:  offset, width: self.view.width, height: self.view.height)
        } else {
            self.view.frame = CGRect(x: 0, y: 0, width: self.view.width, height: self.view.height)
        }
    }
}