ios - 呈现 UIAlertController (以操作表样式)导致神秘的自动布局警告

Question

可以通过一个简单的项目重现该问题，如下所示:

当用户单击导航栏上的 + 按钮时，代码会实例化并以操作表样式呈现 UIAlertController。

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }

    @IBAction func buttonTapped(_ sender: Any) {
        let actionSheet = UIAlertController(title: nil,
                                            message: nil,
                                            preferredStyle: .actionSheet)
        actionSheet.addAction(UIAlertAction(title: "Create Income",
                                            style: .default) { _ in
                                                print("Income!") })
        actionSheet.addAction(UIAlertAction(title: "Create Outcome",
                                            style: .default) { _ in
                                                print("Outcome!")})
        actionSheet.addAction(UIAlertAction(title: "Cancel", style: .cancel) { _ in })
        self.present(actionSheet, animated: true, completion: nil)
    }
}

用户界面正确显示。问题是当操作表出现时 Xcode 会发出以下自动布局警告:

2020-03-12 11:24:57.002113+0800 alertcontroler_test[12757:936231] [LayoutConstraints] 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:0x600002d11b30 UIView:0x7f851961c560.width == - 16   (active)>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x600002d11b30 UIView:0x7f851961c560.width == - 16   (active)>

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

当操作表显示时，我检查 Xcode 中的 UI 层次结构，发现一些 subview 后面带有感叹号:

我用谷歌搜索了这个问题，但没有发现有人报告类似的问题(即，当 UIAlertController 显示时出现自动布局警告)。我找到了一些有关如何在 UIViewAlertForUnsatisfiableConstraints() 上设置断点以及如何调试汇编代码的信息。但我认为这是 UIAlertController 的内部问题，并且我创建和呈现 UIAlertController 的方式是常见且典型的。有什么建议我应该做什么吗？

这是在最新版本的 Xcode (11.3.1) 和 iOS (13.3) 上找到的。如果我将样式更改为警报，问题就会消失。任何帮助将不胜感激。

Answer 1

这是一个 Apple 团队尚未修复的错误，该错误与动画相关的警报和操作表有关。您可以点击以下链接来验证这一点:-

UIAlertController's actionSheet gives constraint error on iOS 12.2 / 12.3

Swift default AlertViewController breaking constraints

解决方案是将动画值传递为 false，例如:-

controller.present(alertController, animated: false, completion: nil)

我展示了没有动画的 UIAlertController，并且警告消失了。

或者您可以尝试 stackoverflow 的链接之一中提到的解决方案:-

@IBAction func buttonTapped(_ sender: UIButton) {
 ...
self.present(alertController,animated: true,completion: nil)



alertController.view.subviews.flatMap({$0.constraints}).filter{ (one: NSLayoutConstraint)-> (Bool)  in
  return (one.constant < 0) && (one.secondItem == nil) &&  (one.firstAttribute == .width)


}.first?.isActive = false

  }