当我以编程方式更改约束时，Xcode给出错误

问题描述

我有一个用viewDidLoad（）方法中的4个约束初始化的表视图。我想稍后在代码中以编程方式更改bottomAnchor，因此将其保存为变量bottomAnchor，然后为更改后的约束添加了另一个变量keyboardBottomAnchor

这些是初始约束：

tableView.translatesAutoresizingMaskIntoConstraints = false
tableView.topAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.topAnchor,constant: 50).isActive = true
tableView.leadingAnchor.constraint(equalTo: self.view.leadingAnchor).isActive = true
tableView.trailingAnchor.constraint(equalTo: self.view.trailingAnchor).isActive = true
bottomAnchor = tableViewe.bottomAnchor.constraint(equalTo: self.view.bottomAnchor,constant:  (-1) * card.cardHandleAreaHeight + -20)
bottomAnchor.isActive = true

基本上，我希望表视图在键盘打开时上升，而在键盘关闭时下降。外观如下：

@objc func keyboardAppears(notification: Notification) {
        let userInfo = notification.userInfo
        keyboardFrame = userInfo?[UIResponder.keyboardFrameEndUserInfoKey] as! CGRect
       
        //this is the original constraint
        self.bottomConstraint.isActive = false
        //Here I make a new variable to save the new constraint
        keyboardBottomConstraint = tableView.bottomAnchor.constraint(equalTo: self.view.bottomAnchor,constant: -1 * keyboardFrame.height)
        keyboardBottomConstraint.isActive = true
}

@objc func keyboardDisappears(notification: Notification) {
        NSLayoutConstraint.deactivate([keyboardBottomConstraint])
        bottomConstraint.isActive = true
}

keyboardAppears方法正在起作用（当键盘显示时，表格视图会上升），但是keyboardDisappears方法却给了我一个Unable to simultaneously satisfy constraints错误（也就是说两个{{ 1}}和bottomConstraint有效）

关于为什么会发生这种情况的任何想法？

更新：

我使用了下面的keyboardBottomConstraint（此方法有效，但仅在我第一次打开键盘时可用）

.constant

解决方法

雨燕5

请勿停用约束，只需更改其常数值即可。

我已经在下面编写了代码并对其进行了测试，可以根据需要对其进行自定义，将与您完美配合！

import UIKit

class ViewController: UIViewController
{
    // MARK: Properties

    var tableView: UITableView!
    var tableBottomConstraint: NSLayoutConstraint!

    // MARK: View Controller Life Cycle

    override func viewDidLoad()
    {
        super.viewDidLoad()
        addKeyboardObservers()
        setupTableView()
    }

    deinit
    {
        removeKeyboardObservers()
    }

    // MARK: Methods

    private func addKeyboardObservers()
    {
        NotificationCenter.default.addObserver(self,selector: #selector(keyboardWillShow(notification:)),name: UIResponder.keyboardWillShowNotification,object: nil)

        NotificationCenter.default.addObserver(self,selector: #selector(keyboardWillHide(notification:)),name: UIResponder.keyboardWillHideNotification,object: nil)
    }

    private func removeKeyboardObservers()
    {
        NotificationCenter.default.removeObserver(self,object: nil)

        NotificationCenter.default.removeObserver(self,object: nil)
    }

    private func setupTableView()
    {
        tableView = UITableView()
        tableView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(tableView)

        tableView.leadingAnchor.constraint(equalTo: view.leadingAnchor,constant: 0.0).isActive = true
        tableView.trailingAnchor.constraint(equalTo: view.trailingAnchor,constant: 0.0).isActive = true
        tableView.topAnchor.constraint(equalTo: view.topAnchor,constant: 50.0).isActive = true

        tableBottomConstraint = tableView.bottomAnchor.constraint(equalTo: view.bottomAnchor,constant: -50.0)
        tableBottomConstraint.isActive = true
    }

    // MARK: Keyboard Handling

    @objc func keyboardWillShow(notification: Notification)
    {
        if let keyboardFrame = notification.userInfo?[UIResponder.keyboardFrameEndUserInfoKey] as? CGRect
        {
            tableBottomConstraint.constant = -1 * keyboardFrame.height
            loadViewIfNeeded()
        }
    }

    @objc func keyboardWillHide(notification: Notification)
    {
        tableBottomConstraint.constant = -50.0
        loadViewIfNeeded()
    }
}

注意：不要忘记删除deinit中的键盘观察者