现有答案有一些改进。

首先， UIKeyboardWillChangeFrameNotification
可能是最好的通知，因为它处理的不仅是显示/隐藏，还包括由于键盘更改（语言，使用第三方键盘等）和旋转引起的更改（但下面的注释说明指示键盘应隐藏）也可以处理以支持硬件键盘连接）。

其次，可以从通知中提取动画参数，以确保动画正确地在一起。

可能还有一些方法可以清理此代码，特别是如果您习惯于强制展开字典代码的话。

迅捷3

class MyViewController: UIViewController {

// This constraint ties an element at zero points from the bottom layout guide
@IBOutlet var keyboardHeightLayoutConstraint: NSLayoutConstraint?

override func viewDidLoad() {
    super.viewDidLoad()
    // Note that SO highlighting makes the new selector syntax (#selector()) look
    // like a comment but it isn't one
    NotificationCenter.default.addObserver(self,
        selector: #selector(self.keyboardNotification(notification:)),
        name: NSNotification.Name.UIKeyboardWillChangeFrame,
        object: nil)
}

deinit {
    NotificationCenter.default.removeObserver(self)
}

@objc func keyboardNotification(notification: NSNotification) {
    if let userInfo = notification.userInfo {
        let endFrame = (userInfo[UIKeyboardFrameEndUserInfoKey] as? NSValue)?.cgRectValue
        let endFrameY = endFrame.origin.y ?? 0
        let duration:TimeInterval = (userInfo[UIKeyboardAnimationDurationUserInfoKey] as? NSNumber)?.doubleValue ?? 0
        let animationCurveRawNSN = userInfo[UIKeyboardAnimationCurveUserInfoKey] as? NSNumber
        let animationCurveRaw = animationCurveRawNSN?.uintValue ?? UIViewAnimationOptions.curveEaseInOut.rawValue
        let animationCurve:UIViewAnimationOptions = UIViewAnimationOptions(rawValue: animationCurveRaw)
        if endFrameY >= UIScreen.main.bounds.size.height {
            self.keyboardHeightLayoutConstraint?.constant = 0.0
        } else {
            self.keyboardHeightLayoutConstraint?.constant = endFrame?.size.height ?? 0.0
        }
        UIView.animate(withDuration: duration,
                                   delay: TimeInterval(0),
                                   options: animationCurve,
                                   animations: { self.view.layoutIfNeeded() },
                                   completion: nil)
    }
}

（根据@Gabox的出色注释，编辑后考虑了键盘动画在屏幕外而不是缩小）

斯威夫特5

class MyViewController: UIViewController {

// This constraint ties an element at zero points from the bottom layout guide
@IBOutlet var keyboardHeightLayoutConstraint: NSLayoutConstraint?

override func viewDidLoad() {
    super.viewDidLoad()
    // Note that SO highlighting makes the new selector syntax (#selector()) look
    // like a comment but it isn't one
    NotificationCenter.default.addObserver(self,
        selector: #selector(self.keyboardNotification(notification:)),
        name: UIResponder.keyboardWillChangeFrameNotification,
        object: nil)
}

deinit {
    NotificationCenter.default.removeObserver(self)
}

@objc func keyboardNotification(notification: NSNotification) {
    if let userInfo = notification.userInfo {
        let endFrame = (userInfo[UIResponder.keyboardFrameEndUserInfoKey] as? NSValue)?.cgRectValue
        let endFrameY = endFrame?.origin.y ?? 0
        let duration:TimeInterval = (userInfo[UIResponder.keyboardAnimationDurationUserInfoKey] as? NSNumber)?.doubleValue ?? 0
        let animationCurveRawNSN = userInfo[UIResponder.keyboardAnimationCurveUserInfoKey] as? NSNumber
        let animationCurveRaw = animationCurveRawNSN?.uintValue ?? UIView.AnimationOptions.curveEaseInOut.rawValue
        let animationCurve:UIView.AnimationOptions = UIView.AnimationOptions(rawValue: animationCurveRaw)
        if endFrameY >= UIScreen.main.bounds.size.height {
            self.keyboardHeightLayoutConstraint?.constant = 0.0
        } else {
            self.keyboardHeightLayoutConstraint?.constant = endFrame?.size.height ?? 0.0
        }
        UIView.animate(withDuration: duration,
                                   delay: TimeInterval(0),
                                   options: animationCurve,
                                   animations: { self.view.layoutIfNeeded() },
                                   completion: nil)
    }
}