iOS 11：“无法同时满足约束”

Question

总是跑到 FoodTracker 教程；遵循此步骤：“实现自定义控件”

https://developer.apple.com/library/content/referencelibrary/GettingStarted/DevelopiOSAppsSwift/ImplementingACustomControl.html#//apple_ref/doc/uid/TP40015214-CH19-SW1

当执行第一个检查点时，模拟器显示一个红色矩形而不是教程中指示的正方形；在我得到的调试窗格上：

2018-02-09 11:19:42.130595+0100 FoodTracker[7439:80369] [MC] Lazy loading NSBundle MobileCoreServices.framework  
2018-02-09 11:19:42.131628+0100 FoodTracker[7439:80369] [MC] Loaded MobileCoreServices.framework  
2018-02-09 11:19:42.165143+0100 FoodTracker[7439:80369] [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:0x60400028a3c0 UIButton:0x7f8a0bd0e330.width == 44   (active)>",  
    "<NSLayoutConstraint:0x60400028cda0 'UISV-canvas-connection' FoodTracker.RatingControl:0x7f8a0bd08890.leading == UIButton:0x7f8a0bd0e330.leading   (active)>",  
    "<NSLayoutConstraint:0x60400028ce40 'UISV-canvas-connection' H:[UIButton:0x7f8a0bd0e330]-(0)-|   (active, names: '|':FoodTracker.RatingControl:0x7f8a0bd08890 )>",  
    "<NSLayoutConstraint:0x60400028c940 'UIView-Encapsulated-Layout-Width' FoodTracker.RatingControl:0x7f8a0bd08890.width == 200   (active)>"  
)  

Will attempt to recover by breaking constraint  
<NSLayoutConstraint:0x60400028a3c0 UIButton:0x7f8a0bd0e330.width == 44   (active)>  

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.  
2018-02-09 11:19:42.165949+0100 FoodTracker[7439:80369] [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:0x60400028a370 UIButton:0x7f8a0bd0e330.height == 44   (active)>",  
    "<NSLayoutConstraint:0x60400028a320 'UISV-canvas-connection' FoodTracker.RatingControl:0x7f8a0bd08890.top == UIButton:0x7f8a0bd0e330.top   (active)>",  
    "<NSLayoutConstraint:0x60400028cf30 'UISV-canvas-connection' V:[UIButton:0x7f8a0bd0e330]-(0)-|   (active, names: '|':FoodTracker.RatingControl:0x7f8a0bd08890 )>",  
    "<NSLayoutConstraint:0x60400028c990 'UIView-Encapsulated-Layout-Height' FoodTracker.RatingControl:0x7f8a0bd08890.height == 110   (active)>"  
)  

Will attempt to recover by breaking constraint  
<NSLayoutConstraint:0x60400028a370 UIButton:0x7f8a0bd0e330.height == 44   (active)>  

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.  

我认为以编程方式创建按钮并在此处应用自定义约束时会发生一些变化：（我正在使用 Swift 和 Xcode 9）

// Add constraints  
button.translatesAutoresizingMaskIntoConstraints = false  
button.heightAnchor.constraint(equalToConstant: 44.0).isActive = true  
button.widthAnchor.constraint(equalToConstant: 44.0).isActive = true  

欢迎任何解决问题的提示。

请注意，我正在构建一个自定义控件...我在谷歌上搜索到的所有注释似乎都不适用。

Answer 1

结语

问题自己消失了。

继续练习，Xcode IDE 强调了视图设计中的一些关键点，我被邀请“单击图标”进行修复；其中一个问题与冲突的约束有关。

正如#DonMag 所建议的，RatingControl 类必须标记为@IBDesignable（这个装饰器将在后面的练习中添加）让 Xcode 立即查看自定义控件的最终结果。几乎：星星没有出现，并且控件不会自动缩放将星星添加到控件属性。直到我关闭，再次打开并重建整个项目几次。

教程（为 Swift 3.2 编写）和实际 Xcode（适用于 Swift 4 中打开的默认项目）的差异。

我的想法是，Apple 必须更新它自己的在线文档，以避免人们在试图弄清楚为什么事情没有像预期的那样工作时失去理智。

Answer 2

忽略我的前两个 cmets...

在该教程中，您将添加自定义 ratingControl 作为垂直堆栈视图的排列子视图。这就是不需要约束的原因。

关键是创建类@IBDesignable - 允许Interface Builder 和自动布局引擎使用类的固有大小 来正确调整大小/定位/显示它。

（有点奇怪，教程中没有注明。）

Answer 3

我有同样的问题。 此外，红色方块看起来比教程中显示的要大。 为了让红色方块接受大小为 44 的约束，我必须确保 RatingControl 位于堆栈视图内部，而不是下方。

我附上图片以澄清我所说的“在堆栈视图中”的意思。

error reproduced, please note red rectangle outside the stack view hierarchy

error fixed, the red rectangle is inside the stack view hierarchy on the left

Answer 4

问题是因为所有子视图的总宽度不等于 StackView 的宽度。

使用这个计算可以解决问题：

let width = (self.frame.width - self.spacing * CGFloat(starCount-1))/CGFloat(starCount)
let height = self.frame.height

............

for _ in 0..<starCount {
        let button = UIButton()
        button.translatesAutoresizingMaskIntoConstraints = false
        button.backgroundColor = UIColor.red
        button.heightAnchor.constraint(equalToConstant: height).isActive = true
        button.widthAnchor.constraint(equalToConstant: width).isActive = true
        button.addTarget(self, action: #selector(RatingControl.ratingButtonTapped(button:)), for: .touchUpInside)
        addArrangedSubview(button)
    }