用户切换文本字段进行输入时，从UIPickerView过渡到键盘时动画丢失

Question

我有一个ViewController，其中有几个使用键盘输入的常规文本字段，还有一个UITextView，其UIPickerView实现了inputView。当这些字段中的每个字段更改为firstResponder时，显示/隐藏动画都可以正常工作……除非用户从一种类型的字段切换到另一种类型，否则就没有动画。旧的输入设备消失，并立即由新的输入设备替换。在这种情况下，我不知道如何启用动画。

下面的代码可以正常工作，除了此过渡。我考虑过延迟两个项目的出现，但是我不知道如何检测问题情况。

//MARK: - Keyboard repositioning
extension DetailViewController {


    func registerForKeyboardNotifications() {
        //Adding notifies on keyboard appearing
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardWasShown(notification:)), name: UIResponder.keyboardWillShowNotification, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillBeHidden(notification:)), name: UIResponder.keyboardWillHideNotification, object: nil)
    }

    func deregisterFromKeyboardNotifications() {
        //Removing notifies on keyboard appearing
        NotificationCenter.default.removeObserver(self, name: UIResponder.keyboardWillShowNotification, object: nil)
        NotificationCenter.default.removeObserver(self, name: UIResponder.keyboardWillHideNotification, object: nil)
    }

    @objc func keyboardWasShown(notification: NSNotification) {

        let horizontalCenter = self.view.frame.width / 2

        var info = notification.userInfo!
        let keyboardFrame: CGRect = (info[UIResponder.keyboardFrameEndUserInfoKey] as! NSValue).cgRectValue
        let keyboardHeight = keyboardFrame.size.height
        let keyboardCenterTopInPresentingVC = CGPoint(x: horizontalCenter, y: keyboardHeight)
        let keyboardCenterTopInThisVC = self.presentingViewController?.view.convert(keyboardCenterTopInPresentingVC, to: self.view) ?? keyboardCenterTopInPresentingVC

        //the distance to push the Cancel/Add buttons up by
        let adjustment = keyboardCenterTopInThisVC.y

        //use standard keyboard animation timing
        let duration = notification.userInfo![UIResponder.keyboardAnimationDurationUserInfoKey] as! Double
        UIView.animate(withDuration: duration) { () -> Void in
            self.stackViewBottomConstraint.constant = adjustment - self.stackViewBottomConstraintInitialValue!
            self.view.layoutIfNeeded()
        }
    }


    @objc func keyboardWillBeHidden(notification: NSNotification) {
        //Once keyboard disappears, restore original positions
        var info = notification.userInfo!
        let duration = info[UIResponder.keyboardAnimationDurationUserInfoKey] as! Double
        //use standard keyboard animation timing
        UIView.animate(withDuration: duration) { () -> Void in
            self.stackViewBottomConstraint.constant = self.stackViewBottomConstraintInitialValue!
            self.view.layoutIfNeeded()
        }
    }
    }

    //MARK: - UIPickerView support
    extension DetailViewController: UIPickerViewDelegate, UIPickerViewDataSource {

    func createPickerView(for thisField: UITextField) {

        let pickerView = UIPickerView()
        thisField.inputView = pickerView
        pickerView.delegate = self
        let currentFieldValueIndex = sectionHeaderItems.firstIndex{ $0.title == thisField.text}
        pickerView.selectRow(currentFieldValueIndex!, inComponent: 0, animated: true)

        let doneButton = UIBarButtonItem.init(title: "Done", style: .done, target: self, action: #selector(self.pickerDone))
        let toolBar = UIToolbar.init(frame: CGRect(x: 0, y: 0, width: view.bounds.size.width, height: 44))
        toolBar.setItems([UIBarButtonItem(barButtonSystemItem: UIBarButtonItem.SystemItem.flexibleSpace, target: nil, action: nil), doneButton],
                         animated: true)
        thisField.inputAccessoryView = toolBar
    }

    @objc
    func pickerDone() {
        priorityGroupLabel.resignFirstResponder()
    }

    func numberOfComponents(in pickerView: UIPickerView) -> Int {
        return 1
    }

    func pickerView(_ pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int {
        return sectionHeaderItems.count
    }

    func pickerView( _ pickerView: UIPickerView, titleForRow row: Int, forComponent component: Int) -> String? {
        return sectionHeaderItems[row].title
    }

    func pickerView( _ pickerView: UIPickerView, didSelectRow row: Int, inComponent component: Int) {
        let newPriorityGroup = Int16(sectionHeaderItems[row].index)
        priorityGroupLabel.text = sectionHeaderItems[row].title

        if newPriorityGroup == originalPriorityGroup {
            //reverted to no change from saved data
            priorityGroupForDisplay = originalPriorityGroup!
            priorityForDisplay = originalPriority!
            numItemsForDisplay = Int(itemsInCurrentGroup)
        } else {
            //Priority Group has been changed
            priorityGroupForDisplay = newPriorityGroup
            numItemsForDisplay = Int(itemsInCurrentGroup + 1)
            if positionSegCtlStack.isHidden == false && positionSegCtl.selectedSegmentIndex == 0 { //Top
                priorityForDisplay = 1
            } else {
                priorityForDisplay = Int16(numItemsForDisplay)
            }
        }
    } 
}