添加后更改约束会导致约束错误 (swift)

Question

我正在努力将错误消息添加到我的登录屏幕。

虽然代码运行没问题，但可以按我的要求执行。它会导致执行约束错误。

以下是受影响的约束：

self.view.addConstraints(
    NSLayoutConstraint.constraintsWithVisualFormat(
        "V:|[topBar]-32-[emailInputField]-32-[passwordInputField]-32-[signInButton]-16-[resetPasswordButton]-[signUpButton]", options: nil, metrics: nil, views: viewsDictionary))
self.view.addConstraints(
    NSLayoutConstraint.constraintsWithVisualFormat(
        "V:|[topBar]-32-[emailIconBox]-32-[passwordIconBox]", options: nil, metrics: nil, views: viewsDictionary))

这里是导致 errorView 出现的函数。

// sign in button function
func signIn(sender: UIButton) {
self.view.endEditing(true)

if emailInputField.text.isEmpty {
    println("NO EMAIL")

    errorLabel.attributedText = UILabel.ErrorMessage("Form fields cannot be empty!")


    var viewsDictionary = [ "topBar":topBar,
        "errorView":errorView,
        "errorLabel":errorLabel,
        "emailInputField":emailInputField,
        "emailIconBox":emailIconBox,
        "passwordInputField":passwordInputField,
        "passwordIconBox":passwordIconBox,
        "signInButton":signInButton,
        "resetPasswordButton":resetPasswordButton,
        "signUpButton":signUpButton]

    self.view.addConstraints(
        NSLayoutConstraint.constraintsWithVisualFormat(
            "V:|[topBar]-16-[errorView]-16-[emailInputField]-32-[passwordInputField]-32-[signInButton]-16-[resetPasswordButton]-[signUpButton]", options: nil, metrics: nil, views: viewsDictionary))
    self.view.addConstraints(
        NSLayoutConstraint.constraintsWithVisualFormat(
            "V:|[topBar]-16-[errorView]-16-[emailIconBox]-32-[passwordIconBox]", options: nil, metrics: nil, views: viewsDictionary))
    self.view.addConstraints(
        NSLayoutConstraint.constraintsWithVisualFormat(
            "V:[errorView(50)]", options: NSLayoutFormatOptions(0), metrics: nil, views: viewsDictionary))
}
}

如何在不破坏约束的情况下更改约束？

我尝试了 self.view.updateConstraints() - 但没有任何作用。我也尝试在添加约束之前删除约束，但仍然存在错误。

如有任何帮助，我们将不胜感激！

编辑：

我找到了一个 objective-c 解决方案，用一个常量约束一个变量，然后改变常量。 - 我尝试将其翻译成 swift。当我运行它时没有错误，但是我的 errorView 没有显示（高度没有改变）

这是我尝试过的：

我在ViewControllerclass（viewDidLoad之前）声明了变量

var errorViewHeight = NSLayoutConstraint()
在我函数的开头，我指定了它。

errorViewHeight = NSLayoutConstraint(item:errorView, attribute:NSLayoutAttribute.Height, relatedBy:NSLayoutRelation.Equal, toItem:nil, attribute:NSLayoutAttribute.NotAnAttribute, multiplier:1.0 , constant:0)
最后，在 if 语句 returns 为真之后，我尝试为其设置动画。

UIView.animateWithDuration(0.2, 动画: { () -> Void in self.errorViewHeight.constant = 50 self.view.layoutIfNeeded() })

编辑：

错误信息如下：

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. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSLayoutConstraint:0x170089e20 V:[UIView:0x174191780(0)]>",
    "<NSLayoutConstraint:0x170089fb0 V:[UIView:0x174191780(50)]>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x170089fb0 V:[UIView:0x174191780(50)]>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.

Answer 1

我想出了如何使用常量更改约束。

在viewControllerclass中我定义了约束：

var errorViewHeight:NSLayoutConstraint!

并且在 viewDidLoad 中，我指定并将其添加到我的 UIView

errorViewHeight = NSLayoutConstraint(item:errorView, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier:1.0, constant:0)
errorView.addConstraint(errorViewHeight)

从那时起，我可以使用以下代码行正确更改高度

errorViewHeight.constant = 50

希望对寻找相同问题的人有所帮助。

Answer 2

您可以通过添加以下扩展来更改任何约束，如果您要部署到多个设备，那将是极好的：

extension UIView
{
    func updateHieghtForView()
    {
        var constant = CGFloat()
        // change the constant by switching device type by screen size 
        let allConstraints = self.constraints
        for heightConstraint in allConstraints
        {
            if heightConstraint.firstAttribute == .Height 
            {
                self.removeConstraint(heightConstraint)
            }
        }
        let layOutConstaint = NSLayoutConstraint(item: self,
                                                 attribute: .Height,
                                                 relatedBy: .Equal,
                                                 toItem: nil,
                                                 attribute:.Height,
                                                 multiplier: 1,
                                                 constant: constant)
        self.addConstraint(layOutConstaint)
        self.updateConstraints()
    }
}

Answer 3

我的回答我想补充的是，有些人有在viewDidLayout方法中设置约束的习惯，所以当你改变或编辑任何约束时，它会给你这个错误因为 viewDidLayout 不像 viewDidLoad 或其他函数那样工作。所以我的建议是，如果你正在做这样的事情，请使用 viewDidAppear 而不是 viewDidLayout，最好使用 viewDidLoad.

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