将UITabBarController添加到UIView时出现约束冲突警告

我通过代码将UITabBarController添加到UIView中，尽管模拟器里所有元素的布局都符合预期，但控制台出现了无法理解且无法消除的警告。

代码示例 #

final class MainController: UIViewController {
    
    let searchArea = SearchArea()
    let tabBarContainer = UIView()
    let tabBarControl = TabBarController()
    
    override func viewDidLoad() {

        super.viewDidLoad()

        view.backgroundColor = .systemBackground
        let safeArea = view.safeAreaLayoutGuide
        
        view.addSubview(searchArea)
        searchArea.backgroundColor = .systemBackground
        searchArea.addConstraints(
            .alignTopWithTop(of: safeArea),
            .alignLeadingWithLeading(of: safeArea),
            .alignTrailingWithTrailing(of: safeArea),
            .alignHeight(with: safeArea, mult: 0.05)
        )
        searchArea.setupView()
        
        view.addSubview(tabBarContainer)
        tabBarContainer.backgroundColor = .systemBackground
        tabBarContainer.addConstraints(
            .alignTopWithBottom(of: searchArea),
            .alignLeadingWithLeading(of: safeArea),
            .alignTrailingWithTrailing(of: safeArea),
            .alignBottomWithBottom(of: safeArea)
        )
        
        tabBarControl.view.frame = tabBarContainer.bounds
        tabBarControl.tabBar.backgroundColor = .systemBackground
        tabBarContainer.addSubview(tabBarControl.view)
        addChild(tabBarControl)
        tabBarControl.didMove(toParent: self)
    }
    
}

控制台警告输出 #

2023-06-12 17:55:46.716982+0300 BFB[66775:3057149] [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. (Note: If you're seeing
NSAutoresizingMaskLayoutConstraints that you don't understand, refer
to the documentation for the UIView property
translatesAutoresizingMaskIntoConstraints) (
"<NSAutoresizingMaskLayoutConstraint:0x600003fe7340 h=-&- v=--& UINavigationBar:0x12f21ecd0.minY == 0 (active, names:
'|':UILayoutContainerView:0x12f21eb30 )>",
"<NSAutoresizingMaskLayoutConstraint:0x600003ff1e00 h=-&- v=--& UINavigationBar:0x12f21ecd0.height == 44 (active)>",
"<NSLayoutConstraint:0x600003fef110 V:[UINavigationBar:0x12f21ecd0]-(0)-[UIFocusContainerGuide:0x6000023f4870'UINavigationControllerContentFocusContainerGuide']
(active)>",
"<NSLayoutConstraint:0x600003feee40 UIFocusContainerGuide:0x6000023f4870'UINavigationControllerContentFocusContainerGuide'.bottom
== UILayoutContainerView:0x12f21eb30.bottom (active)>",
"<NSLayoutConstraint:0x600003f902d0 'UIView-Encapsulated-Layout-Height'
UILayoutContainerView:0x12f21eb30.height == 0 (active)>" )

Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x600003fef110
V:[UINavigationBar:0x12f21ecd0]-(0)-[UIFocusContainerGuide:0x6000023f4870'UINavigationControllerContentFocusContainerGuide']
(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.

我并未对UITabBarController自动创建的UINavigationBar做任何操作，警告在执行tabBarContainer.addSubview(tabBarControl.view)时出现。我试过设置translatesAutoresizingMaskIntoConstraints为false，也把自定义约束优先级设为.defaultLow，都无法解决问题。

问题出在你给tabBarControl.view设置frame的方式——仅设置初始frame无法保证布局更新时的尺寸稳定性，导致子控制器内部布局出现约束冲突。正确的做法是用Auto Layout约束让它完全填充容器视图。

修改步骤 #

1. 移除tabBarControl.view.frame = tabBarContainer.bounds这行代码
2. 禁用tabBarControl.view的自动调整掩码转换，添加约束让它填满tabBarContainer

修改后的代码片段：

// 替换原有的frame设置和子视图添加逻辑
tabBarControl.view.translatesAutoresizingMaskIntoConstraints = false
tabBarContainer.addSubview(tabBarControl.view)
addChild(tabBarControl)
tabBarControl.didMove(toParent: self)

// 添加约束固定tabBarControl.view的尺寸
NSLayoutConstraint.activate([
    tabBarControl.view.topAnchor.constraint(equalTo: tabBarContainer.topAnchor),
    tabBarControl.view.leadingAnchor.constraint(equalTo: tabBarContainer.leadingAnchor),
    tabBarControl.view.trailingAnchor.constraint(equalTo: tabBarContainer.trailingAnchor),
    tabBarControl.view.bottomAnchor.constraint(equalTo: tabBarContainer.bottomAnchor)
])

原因说明 #

警告里的UILayoutContainerView:0x12f21eb30.height == 0表明UITabBarController的根视图在布局更新时被计算为高度0——这是因为你只设置了初始frame，没有用Auto Layout固定尺寸，系统更新布局时会重置这个frame。此时内部的UINavigationBar需要44的高度，而容器视图高度为0，自然触发约束冲突。

用Auto Layout约束子控制器视图的尺寸后，系统会正确计算容器视图的高度，内部导航栏和其他元素的约束也能正常生效，警告就会消失。

内容的提问来源于stack exchange，提问作者Alexey_BH