冲突约束（不存在）（Swift 4）

Posted 2023-03-14

【中文标题】冲突约束（不存在）（Swift 4）

【英文标题】：Conflicting Constraints (that do not exist) (Swift 4)

【发布时间】：2020-10-10 18:34:54

【问题描述】：

当手机旋转时，我正在重新排列三个视图：

我设置约束如下：

    var narrowConstraints: [NSLayoutConstraint] = []
    var wideConstraints: [NSLayoutConstraint] = []

    func setConstraints1() 
        
        let cv1Height: CGFloat = 325
        let cv3Height: CGFloat = 125
        
        cv1.translatesAutoresizingMaskIntoConstraints = false
        cv2.translatesAutoresizingMaskIntoConstraints = false
        cv3.translatesAutoresizingMaskIntoConstraints = false
        
        let g = view.safeAreaLayoutGuide
        
        // wh
        narrowConstraints = [
            
            // set cv1 height
            cv1.heightAnchor.constraint(equalToConstant: cv1Height),
            // lock top, left and right to safe area
            cv1.topAnchor.constraint(equalTo: g.topAnchor),
            cv1.leadingAnchor.constraint(equalTo: g.leadingAnchor),
            cv1.trailingAnchor.constraint(equalTo: g.trailingAnchor),

            // lock left and right to safe area
            cv2.leadingAnchor.constraint(equalTo: g.leadingAnchor),
            cv2.trailingAnchor.constraint(equalTo: g.trailingAnchor),
            
            // lock top of cv2 to bottom of cv1
            cv2.topAnchor.constraint(equalTo: cv1.bottomAnchor),
            // lock bottom of cv2 to top of cv3
            cv2.bottomAnchor.constraint(equalTo: cv3.topAnchor),
            
            // set cv3 height
            cv3.heightAnchor.constraint(equalToConstant: cv3Height),
            // lock left, right and bottom to safe area
            cv3.leadingAnchor.constraint(equalTo: g.leadingAnchor),
            cv3.trailingAnchor.constraint(equalTo: g.trailingAnchor),
            cv3.bottomAnchor.constraint(equalTo: g.bottomAnchor),
        ]
        
        wideConstraints = [

            // lock top, bottom, and left to safe area
            cv1.topAnchor.constraint(equalTo: g.topAnchor),
            cv1.bottomAnchor.constraint(equalTo: g.bottomAnchor),
            cv1.leadingAnchor.constraint(equalTo: g.leadingAnchor),
            // lock right side of cv1 to left side of cv2
            cv1.trailingAnchor.constraint(equalTo: cv2.leadingAnchor),
            
            // lock right side of cv2 to safe area
            cv2.trailingAnchor.constraint(equalTo: g.trailingAnchor),
            // lock top of cv2 to safe area
            cv2.topAnchor.constraint(equalTo: g.topAnchor),
            // lock bottom of cv2 to top of cv3
            cv2.bottomAnchor.constraint(equalTo: cv3.topAnchor),

            // set cv3 height
            cv3.heightAnchor.constraint(equalToConstant: cv3Height),
            // lock bottom and right side of cv3 to safe area
            cv3.trailingAnchor.constraint(equalTo: g.trailingAnchor),
            cv3.bottomAnchor.constraint(equalTo: g.bottomAnchor),
            
            // make them all equal widths
            cv2.widthAnchor.constraint(equalTo: cv1.widthAnchor),
            cv3.widthAnchor.constraint(equalTo: cv2.widthAnchor),            
        ]
        
        if view.frame.width > view.frame.height 
            // if landscape
            NSLayoutConstraint.deactivate(narrowConstraints)
            NSLayoutConstraint.activate(wideConstraints)
        
        else  // portrait
            NSLayoutConstraint.deactivate(wideConstraints)
            NSLayoutConstraint.activate(narrowConstraints)
        
    

    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) 
        super.viewWillTransition(to: size, with: coordinator)
        coordinator.animate(alongsideTransition:  _ in
            if size.width > size.height 
                // we're transitioning to wider than tall
                NSLayoutConstraint.deactivate(self.narrowConstraints)
                NSLayoutConstraint.activate(self.wideConstraints)
             else 
                // we're transitioning to taller than wide
                NSLayoutConstraint.deactivate(self.wideConstraints)
                NSLayoutConstraint.activate(self.narrowConstraints)
            
        , completion: 
            _ in
            // if you want to do something after the transition
            //self.cv2.setNeedsDisplay()
        )
    

一切似乎都运行良好 - 除了以下错误消息：

2020-10-16 14:33:57.364778-0600 ViewPositions2[38358:6108060] [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:0x6000033a9130 UIView:0x7fb41950ad90.height == 325   (active)>",
    "<NSLayoutConstraint:0x6000033aa800 UIView:0x7fb41950ad90.top == UILayoutGuide:0x6000029bcb60'UIViewSafeAreaLayoutGuide'.top   (active)>",
    "<NSLayoutConstraint:0x6000033aa3f0 V:[UIView:0x7fb41950ad90]-(0)-[UIView:0x7fb41950dec0]   (active)>",
    "<NSLayoutConstraint:0x6000033a9ea0 UIView:0x7fb41950dec0.bottom == UIView:0x7fb41950e8f0.top   (active)>",
    "<NSLayoutConstraint:0x6000033a9c70 UIView:0x7fb41950e8f0.height == 125   (active)>",
    "<NSLayoutConstraint:0x6000033aa2b0 UIView:0x7fb41950e8f0.bottom == UILayoutGuide:0x6000029bcb60'UIViewSafeAreaLayoutGuide'.bottom   (active)>",
    "<NSLayoutConstraint:0x60000339ff20 'UIView-Encapsulated-Layout-Height' UIView:0x7fb41950f1c0.height == 375   (active)>",
    "<NSLayoutConstraint:0x60000339b840 'UIViewSafeAreaLayoutGuide-bottom' V:[UILayoutGuide:0x6000029bcb60'UIViewSafeAreaLayoutGuide']-(0)-|   (active, names: '|':UIView:0x7fb41950f1c0 )>",
    "<NSLayoutConstraint:0x60000339b7a0 'UIViewSafeAreaLayoutGuide-top' V:|-(0)-[UILayoutGuide:0x6000029bcb60'UIViewSafeAreaLayoutGuide']   (active, names: '|':UIView:0x7fb41950f1c0 )>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x6000033a9130 UIView:0x7fb41950ad90.height == 325   (active)>

经过反复试验，我发现如果cv1Height 和cv2Height 的总和超过375（横向视图高度），则会出现此错误消息。

但是，我不仅在横屏时停用了narrowConstraints，而且在横屏时cv1 永远不会在cv3 之上！

为什么会发生这种冲突？怎么预防？？？

【参考方案1】：

冲突是由于过渡期间状态栏的出现引起的。虽然您可以忽略警告（它不会影响应用的操作或外观），但可以通过降低纵向配置中 cv1 高度约束的优先级来消除警告：

let h1 = cv1.heightAnchor.constraint(equalToConstant: cv1Height)
h1.priority = .defaultHigh
            
narrowConstraints = [
                // set cv1 height
    h1,
                // lock top, left and right to safe area
    cv1.topAnchor.constraint(equalTo: g.topAnchor),
    cv1.leadingAnchor.constraint(equalTo: g.leadingAnchor),
    cv1.trailingAnchor.constraint(equalTo: g.trailingAnchor),

                // lock left and right to safe area
    cv2.leadingAnchor.constraint(equalTo: g.leadingAnchor),
    cv2.trailingAnchor.constraint(equalTo: g.trailingAnchor),
                
                // lock top of cv2 to bottom of cv1
    cv2.topAnchor.constraint(equalTo: cv1.bottomAnchor),
                // lock bottom of cv2 to top of cv3
    cv2.bottomAnchor.constraint(equalTo: cv3.topAnchor),
                
                // set cv3 height
    cv3.heightAnchor.constraint(equalToConstant: cv3Height),
                // lock left, right and bottom to safe area
    cv3.leadingAnchor.constraint(equalTo: g.leadingAnchor),
    cv3.trailingAnchor.constraint(equalTo: g.trailingAnchor),
    cv3.bottomAnchor.constraint(equalTo: g.bottomAnchor),
]

【讨论】：

令人着迷，船长。我们应该将其视为错误吗？

也许这只是 AutoLayout 的迷人特质之一？

好吧，我建议 OP 应该报告它。就像那些关于警报和工具栏的虚假冲突报告一样，令人抓狂

是的。创建minimal reproducible example 并不难，值得举报

@Paulw11 - 我刚刚发布了另一个似乎相关的问题：***.com/questions/64539800/… 这也是一个错误吗？