NSLayoutConstraint 的 Unsatisfiable Constraint 以编程方式更改 - swift

Question

我在 viewDidLoad 中设置了一个约束，然后在键盘出现时更改它的常量。 这是初始设置

bottomConstraint = NSLayoutConstraint(item: bottomBar, attribute: .bottom, relatedBy: .equal, toItem: view, attribute: .bottom, multiplier: 1, constant: 0)
    view.addConstraint(bottomConstraint)

然后我在收到键盘通知时更改常量：

@objc func handleKeyboardNotification(notification: NSNotification){
    
    if let keyboardFrame: NSValue = notification.userInfo?[UIResponder.keyboardFrameEndUserInfoKey] as? NSValue {
        let keyboardRectangle = keyboardFrame.cgRectValue
        let keyboardHeight = keyboardRectangle.height
        
        let isKeyboardShowing = notification.name == UIResponder.keyboardWillShowNotification
        
        bottomConstraint?.constant = isKeyboardShowing ? -keyboardHeight : 0
        
        UIView.animate(withDuration:0.1, delay: 0 , options: .curveEaseOut , animations: {
            self.view.layoutIfNeeded()
        } , completion: {(completed) in
        })
    }
}

这很有趣，因为我只是更改常量而不添加另一个约束。尽管如此，我还是在控制台中收到了这个警告：

Unable to simultaneously satisfy constraints.
Probably at least one of the constraints in the following list is one you don't want. 
Try this: 
    (1) look at each constraint and try to figure out which you don't expect; 
    (2) find the code that added the unwanted constraint or constraints and fix it. 
(
"<NSLayoutConstraint:0x282ae4ff0 UIView:0x109f11110.bottom == UIView:0x109f13240.bottom   (active)>",
"<NSLayoutConstraint:0x282ae8050 UIView:0x109f11110.bottom == UIView:0x109f13240.bottom - 291   (active)>"
 )

这基本上表明我的限制不能很好地协同工作。 我不知道我做错了什么。

编辑： 这是我用来以编程方式添加底栏的代码：

let bottomBar:UIView = {
    let v = UIView()
    v.translatesAutoresizingMaskIntoConstraints = false
    return v
}()

在 ViewdidLoad() 中

view.addSubview(bottomBar)
bottomBar.addSubview(fontView)
bottomBar.addSubview(colorPicker)
fontView.pin(to: bottomBar)
colorPicker.pin(to: bottomBar)

 func setUpConstraints(){
 NSLayoutConstraint.activate([
        bottomBar.leadingAnchor.constraint(equalTo: view.leadingAnchor),
        bottomBar.trailingAnchor.constraint(equalTo: view.trailingAnchor),
        bottomBar.heightAnchor.constraint(equalToConstant: 70),
 ])
}

Answer 1

试图让你更轻松一些......

从简单开始。

这段代码在顶部附近创建了一个文本字段，并在底部创建了一个红色的“bottomBar”视图。当键盘显示或隐藏时，bottomBar 的底部约束常量会更新（点击视图上的任意位置以关闭键盘）：

class ConstraintTestViewController: UIViewController {
    
    let bottomBar = UIView()
    
    var bottomConstraint: NSLayoutConstraint!
    
    override func viewDidLoad() {
        super.viewDidLoad()
    
        // add a text field
        let tf = UITextField()
        tf.borderStyle = .roundedRect
        tf.translatesAutoresizingMaskIntoConstraints = false
        
        bottomBar.translatesAutoresizingMaskIntoConstraints = false
        bottomBar.backgroundColor = .red
        
        view.addSubview(tf)
        view.addSubview(bottomBar)

        let g = view.safeAreaLayoutGuide
        NSLayoutConstraint.activate([
            
            // constrain text field 80-pts from Top, Width: 240, centerX
            tf.topAnchor.constraint(equalTo: g.topAnchor, constant: 80.0),
            tf.widthAnchor.constraint(equalToConstant: 240.0),
            tf.centerXAnchor.constraint(equalTo: g.centerXAnchor),
            
            // constrain bottomBar Leading and Trailing, Height: 70-pts
            bottomBar.leadingAnchor.constraint(equalTo: g.leadingAnchor),
            bottomBar.trailingAnchor.constraint(equalTo: g.trailingAnchor),
            bottomBar.heightAnchor.constraint(equalToConstant: 70.0),
            
        ])
        
        // create and add the bottom constraint
        bottomConstraint = NSLayoutConstraint(item: bottomBar, attribute: .bottom, relatedBy: .equal, toItem: view, attribute: .bottom, multiplier: 1, constant: 0)
        view.addConstraint(bottomConstraint)

        // or, use more modern syntax...
        //bottomConstraint = bottomBar.bottomAnchor.constraint(equalTo: g.bottomAnchor)
        //bottomConstraint.isActive = true
        
        // keyboard show/hide notifications
        NotificationCenter.default.addObserver(self,
                                               selector: #selector(self.handleKeyboardNotification(notification:)),
                                               name: UIResponder.keyboardWillShowNotification,
                                               object: nil)
        
        NotificationCenter.default.addObserver(self,
                                               selector: #selector(self.handleKeyboardNotification(notification:)),
                                               name: UIResponder.keyboardWillHideNotification,
                                               object: nil)

        // add a "tap on the view to dismiss the keyboard" gesture
        let t = UITapGestureRecognizer(target: self, action: #selector(self.didTap(_:)))
        view.addGestureRecognizer(t)
        
    }
    
    @objc func handleKeyboardNotification(notification: NSNotification){
        
        if let keyboardFrame: NSValue = notification.userInfo?[UIResponder.keyboardFrameEndUserInfoKey] as? NSValue {
            let keyboardRectangle = keyboardFrame.cgRectValue
            let keyboardHeight = keyboardRectangle.height
            
            let isKeyboardShowing = notification.name == UIResponder.keyboardWillShowNotification
            
            bottomConstraint.constant = isKeyboardShowing ? -keyboardHeight : 0
            
            UIView.animate(withDuration:0.1, delay: 0 , options: .curveEaseOut , animations: {
                self.view.layoutIfNeeded()
            } , completion: {(completed) in
            })
        }
    }
    
    @objc func didTap(_ g: UITapGestureRecognizer) -> Void {
        view.endEditing(true)
    }
    
}

如果运行时没有自动布局错误/警告（它会），然后开始添加您的其他 UI 元素（和支持代码）一次一个 .如果/当你再次遇到约束冲突时，你就会确切地知道你哪里出错了。

Answer 2

你没有提供足够的信息，所以有必要猜测。这是一个猜测。当你说：

fontView.pin(to: bottomBar)
colorPicker.pin(to: bottomBar)

您创建 两个 bottomBar 底部约束。当你说：

bottomConstraint = NSLayoutConstraint(item: bottomBar, attribute: .bottom, relatedBy: .equal, toItem: view, attribute: .bottom, multiplier: 1, constant: 0)
view.addConstraint(bottomConstraint)

你创建了一个第三个​​底部约束。

那么当你说

bottomConstraint?.constant = isKeyboardShowing ? -keyboardHeight : 0

您更改了一个底部约束，但没有更改其他约束。也许这就是冲突的原因。

另一种可能性是，您可能以某种未向我们展示的方式两次调用view.addConstraint。同样，这意味着您正在更改一个底部约束，而不是另一个。