如何使用 Swift 以编程方式添加约束

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【中文标题】如何使用 Swift 以编程方式添加约束【英文标题】：How to add constraints programmatically using Swift 【发布时间】：2014-10-03 14:25:22 【问题描述】：

自上周以来，我一直在尝试解决这个问题，但没有更进一步。好的，所以我需要使用以下代码将 Swift 中的一些 约束 以编程方式 应用到 UIView： 

var new_view:UIView! = UIView(frame: CGRectMake(0, 0, 100, 100));
new_view.backgroundColor = UIColor.redColor();
view.addSubview(new_view);

var constX:NSLayoutConstraint = NSLayoutConstraint(item: new_view, attribute: NSLayoutAttribute.CenterX, relatedBy: NSLayoutRelation.Equal, toItem: self.view, attribute: NSLayoutAttribute.CenterX, multiplier: 1, constant: 0);
self.view.addConstraint(constX);

var constY:NSLayoutConstraint = NSLayoutConstraint(item: new_view, attribute: NSLayoutAttribute.CenterY, relatedBy: NSLayoutRelation.Equal, toItem: self.view, attribute: NSLayoutAttribute.CenterY, multiplier: 1, constant: 0);
self.view.addConstraint(constY);

var constW:NSLayoutConstraint = NSLayoutConstraint(item: new_view, attribute: NSLayoutAttribute.Width, relatedBy: NSLayoutRelation.Equal, toItem: new_view, attribute: NSLayoutAttribute.Width, multiplier: 1, constant: 0);
self.view.addConstraint(constW);

var constH:NSLayoutConstraint = NSLayoutConstraint(item: new_view, attribute: NSLayoutAttribute.Height, relatedBy: NSLayoutRelation.Equal, toItem: new_view, attribute: NSLayoutAttribute.Height, multiplier: 1, constant: 0);
self.view.addConstraint(constH);

但是 Xcode 返回这个奇怪的输出：

2014-10-03 09:48:12.657 Test[35088:2454916] 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:0x7fa4ea446830 UIView:0x7fa4ea429290.centerX == UIView:0x7fa4ea4470f0.centerX>",
"<NSAutoresizingMaskLayoutConstraint:0x7fa4ea4516c0 h=--& v=--& UIView:0x7fa4ea429290.midX == + 50>",
"<NSLayoutConstraint:0x7fa4ea452830 'UIView-Encapsulated-Layout-Width' H:[UIView:0x7fa4ea4470f0(375)]>",
"<NSAutoresizingMaskLayoutConstraint:0x7fa4ea446db0 h=-&- v=-&- 'UIView-Encapsulated-Layout-Left' H:|-(0)-[UIView:0x7fa4ea4470f0]   (Names: '|':UIWindow:0x7fa4ea444b20 )>"
)

Will attempt to recover by breaking constraint <NSLayoutConstraint:0x7fa4ea446830 UIView:0x7fa4ea429290.centerX == UIView:0x7fa4ea4470f0.centerX>

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.

2014-10-03 09:48:12.658 Test[35088:2454916] 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:0x7fa4ea44d160 UIView:0x7fa4ea429290.centerY == UIView:0x7fa4ea4470f0.centerY>",
"<NSAutoresizingMaskLayoutConstraint:0x7fa4ea451b30 h=--& v=--& UIView:0x7fa4ea429290.midY == + 50>",
"<NSLayoutConstraint:0x7fa4ea44cf00 'UIView-Encapsulated-Layout-Height' V:[UIView:0x7fa4ea4470f0(667)]>",
"<NSAutoresizingMaskLayoutConstraint:0x7fa4ea452700 h=-&- v=-&- 'UIView-Encapsulated-Layout-Top' V:|-(0)-[UIView:0x7fa4ea4470f0]  (Names: '|':UIWindow:0x7fa4ea444b20 )>"
)

Will attempt to recover by breaking constraint <NSLayoutConstraint:0x7fa4ea44d160 UIView:0x7fa4ea429290.centerY == UIView:0x7fa4ea4470f0.centerY>

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.

你能帮帮我吗？ 非常感谢

【问题讨论】：

请将错误消息粘贴为代码块，而不是引用文本。这意味着您需要在每行的开头放置四个空格，而不是&gt;。这次我给你修好了。 您缺少“translateAutoresizingMaskIntoConstraints = false” 您可能希望将这个很棒的库用于动态和简单的约束。 github.com/SnapKit/SnapKit 确保您看到On ios, what are the differences between margins, edge insets, content insets, alignment rects, layout margins, anchors。它将改善您在边距、锚点、布局指南之间的决策...... 这里将解释如何通过代码添加约束：slicode.com/… 【参考方案1】：

您是否计划在UIViewController 的UIView 中居中放置一个宽度：100 和高度：100 的平方UIView？如果是这样，您可以尝试以下 6 种自动布局样式之一（Swift 5 / iOS 12.2）：

1。使用NSLayoutConstraint 初始化器

override func viewDidLoad() 
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let horizontalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.centerX, relatedBy: NSLayoutConstraint.Relation.equal, toItem: view, attribute: NSLayoutConstraint.Attribute.centerX, multiplier: 1, constant: 0)
    let verticalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.centerY, relatedBy: NSLayoutConstraint.Relation.equal, toItem: view, attribute: NSLayoutConstraint.Attribute.centerY, multiplier: 1, constant: 0)
    let widthConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.width, relatedBy: NSLayoutConstraint.Relation.equal, toItem: nil, attribute: NSLayoutConstraint.Attribute.notAnAttribute, multiplier: 1, constant: 100)
    let heightConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.height, relatedBy: NSLayoutConstraint.Relation.equal, toItem: nil, attribute: NSLayoutConstraint.Attribute.notAnAttribute, multiplier: 1, constant: 100)
    view.addConstraints([horizontalConstraint, verticalConstraint, widthConstraint, heightConstraint])


override func viewDidLoad() 
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let horizontalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.centerX, relatedBy: NSLayoutConstraint.Relation.equal, toItem: view, attribute: NSLayoutConstraint.Attribute.centerX, multiplier: 1, constant: 0)
    let verticalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.centerY, relatedBy: NSLayoutConstraint.Relation.equal, toItem: view, attribute: NSLayoutConstraint.Attribute.centerY, multiplier: 1, constant: 0)
    let widthConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.width, relatedBy: NSLayoutConstraint.Relation.equal, toItem: nil, attribute: NSLayoutConstraint.Attribute.notAnAttribute, multiplier: 1, constant: 100)
    let heightConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.height, relatedBy: NSLayoutConstraint.Relation.equal, toItem: nil, attribute: NSLayoutConstraint.Attribute.notAnAttribute, multiplier: 1, constant: 100)
    NSLayoutConstraint.activate([horizontalConstraint, verticalConstraint, widthConstraint, heightConstraint])


override func viewDidLoad() 
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.centerX, relatedBy: NSLayoutConstraint.Relation.equal, toItem: view, attribute: NSLayoutConstraint.Attribute.centerX, multiplier: 1, constant: 0).isActive = true
    NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.centerY, relatedBy: NSLayoutConstraint.Relation.equal, toItem: view, attribute: NSLayoutConstraint.Attribute.centerY, multiplier: 1, constant: 0).isActive = true
    NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.width, relatedBy: NSLayoutConstraint.Relation.equal, toItem: nil, attribute: NSLayoutConstraint.Attribute.notAnAttribute, multiplier: 1, constant: 100).isActive = true
    NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.height, relatedBy: NSLayoutConstraint.Relation.equal, toItem: nil, attribute: NSLayoutConstraint.Attribute.notAnAttribute, multiplier: 1, constant: 100).isActive = true

2。使用视觉格式语言

override func viewDidLoad() 
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let views = ["view": view!, "newView": newView]
    let horizontalConstraints = NSLayoutConstraint.constraints(withVisualFormat: "H:[view]-(<=0)-[newView(100)]", options: NSLayoutConstraint.FormatOptions.alignAllCenterY, metrics: nil, views: views)
    let verticalConstraints = NSLayoutConstraint.constraints(withVisualFormat: "V:[view]-(<=0)-[newView(100)]", options: NSLayoutConstraint.FormatOptions.alignAllCenterX, metrics: nil, views: views)
    view.addConstraints(horizontalConstraints)
    view.addConstraints(verticalConstraints)


override func viewDidLoad() 
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let views = ["view": view!, "newView": newView]
    let horizontalConstraints = NSLayoutConstraint.constraints(withVisualFormat: "H:[view]-(<=0)-[newView(100)]", options: NSLayoutConstraint.FormatOptions.alignAllCenterY, metrics: nil, views: views)
    let verticalConstraints = NSLayoutConstraint.constraints(withVisualFormat: "V:[view]-(<=0)-[newView(100)]", options: NSLayoutConstraint.FormatOptions.alignAllCenterX, metrics: nil, views: views)
    NSLayoutConstraint.activate(horizontalConstraints)
    NSLayoutConstraint.activate(verticalConstraints)

3。混合使用 NSLayoutConstraint 初始化程序和可视格式语言

override func viewDidLoad() 
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let views = ["newView": newView]
    let widthConstraints = NSLayoutConstraint.constraints(withVisualFormat: "H:[newView(100)]", options: NSLayoutConstraint.FormatOptions(rawValue: 0), metrics: nil, views: views)
    let heightConstraints = NSLayoutConstraint.constraints(withVisualFormat: "V:[newView(100)]", options: NSLayoutConstraint.FormatOptions(rawValue: 0), metrics: nil, views: views)
    let horizontalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.centerX, relatedBy: NSLayoutConstraint.Relation.equal, toItem: view, attribute: NSLayoutConstraint.Attribute.centerX, multiplier: 1, constant: 0)
    let verticalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.centerY, relatedBy: NSLayoutConstraint.Relation.equal, toItem: view, attribute: NSLayoutConstraint.Attribute.centerY, multiplier: 1, constant: 0)
    view.addConstraints(widthConstraints)
    view.addConstraints(heightConstraints)
    view.addConstraints([horizontalConstraint, verticalConstraint])


override func viewDidLoad() 
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let views = ["newView": newView]
    let widthConstraints = NSLayoutConstraint.constraints(withVisualFormat: "H:[newView(100)]", options: NSLayoutConstraint.FormatOptions(rawValue: 0), metrics: nil, views: views)
    let heightConstraints = NSLayoutConstraint.constraints(withVisualFormat: "V:[newView(100)]", options: NSLayoutConstraint.FormatOptions(rawValue: 0), metrics: nil, views: views)
    let horizontalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.centerX, relatedBy: NSLayoutConstraint.Relation.equal, toItem: view, attribute: NSLayoutConstraint.Attribute.centerX, multiplier: 1, constant: 0)
    let verticalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.centerY, relatedBy: NSLayoutConstraint.Relation.equal, toItem: view, attribute: NSLayoutConstraint.Attribute.centerY, multiplier: 1, constant: 0)
    NSLayoutConstraint.activate(widthConstraints)
    NSLayoutConstraint.activate(heightConstraints)
    NSLayoutConstraint.activate([horizontalConstraint, verticalConstraint])


override func viewDidLoad() 
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let views = ["newView": newView]
    let widthConstraints = NSLayoutConstraint.constraints(withVisualFormat: "H:[newView(100)]", options: NSLayoutConstraint.FormatOptions(rawValue: 0), metrics: nil, views: views)
    let heightConstraints = NSLayoutConstraint.constraints(withVisualFormat: "V:[newView(100)]", options: NSLayoutConstraint.FormatOptions(rawValue: 0), metrics: nil, views: views)
    NSLayoutConstraint.activate(widthConstraints)
    NSLayoutConstraint.activate(heightConstraints)
    NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.centerX, relatedBy: NSLayoutConstraint.Relation.equal, toItem: view, attribute: NSLayoutConstraint.Attribute.centerX, multiplier: 1, constant: 0).isActive = true
    NSLayoutConstraint(item: newView, attribute: NSLayoutConstraint.Attribute.centerY, relatedBy: NSLayoutConstraint.Relation.equal, toItem: view, attribute: NSLayoutConstraint.Attribute.centerY, multiplier: 1, constant: 0).isActive = true

4。使用UIView.AutoresizingMask

注意：Springs 和 Struts 会在运行时翻译成相应的自动布局约束。

override func viewDidLoad() 
    let newView = UIView(frame: CGRect(x: 0, y: 0, width: 100, height: 100))
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = true
    newView.center = CGPoint(x: view.bounds.midX, y: view.bounds.midY)
    newView.autoresizingMask = [UIView.AutoresizingMask.flexibleLeftMargin, UIView.AutoresizingMask.flexibleRightMargin, UIView.AutoresizingMask.flexibleTopMargin, UIView.AutoresizingMask.flexibleBottomMargin]

5。使用NSLayoutAnchor 

override func viewDidLoad() 
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)
    
    newView.translatesAutoresizingMaskIntoConstraints = false
    let horizontalConstraint = newView.centerXAnchor.constraint(equalTo: view.centerXAnchor)
    let verticalConstraint = newView.centerYAnchor.constraint(equalTo: view.centerYAnchor)
    let widthConstraint = newView.widthAnchor.constraint(equalToConstant: 100)
    let heightConstraint = newView.heightAnchor.constraint(equalToConstant: 100)
    view.addConstraints([horizontalConstraint, verticalConstraint, widthConstraint, heightConstraint])


override func viewDidLoad() 
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)
    
    newView.translatesAutoresizingMaskIntoConstraints = false
    let horizontalConstraint = newView.centerXAnchor.constraint(equalTo: view.centerXAnchor)
    let verticalConstraint = newView.centerYAnchor.constraint(equalTo: view.centerYAnchor)
    let widthConstraint = newView.widthAnchor.constraint(equalToConstant: 100)
    let heightConstraint = newView.heightAnchor.constraint(equalToConstant: 100)
    NSLayoutConstraint.activate([horizontalConstraint, verticalConstraint, widthConstraint, heightConstraint])


override func viewDidLoad() 
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)
    
    newView.translatesAutoresizingMaskIntoConstraints = false
    newView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
    newView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
    newView.widthAnchor.constraint(equalToConstant: 100).isActive = true
    newView.heightAnchor.constraint(equalToConstant: 100).isActive = true

6。使用intrinsicContentSize 和NSLayoutAnchor 

import UIKit

class CustomView: UIView 
    
    override var intrinsicContentSize: CGSize 
        return CGSize(width: 100, height: 100)
    
    


class ViewController: UIViewController 
    
    override func viewDidLoad() 
        let newView = CustomView()
        newView.backgroundColor = UIColor.red
        view.addSubview(newView)
        
        newView.translatesAutoresizingMaskIntoConstraints = false
        let horizontalConstraint = newView.centerXAnchor.constraint(equalTo: view.centerXAnchor)
        let verticalConstraint = newView.centerYAnchor.constraint(equalTo: view.centerYAnchor)
        NSLayoutConstraint.activate([horizontalConstraint, verticalConstraint])

结果：

【讨论】：

有没有办法为锚样式约束设置动画？ 我不知道选项 5 / 锚样式，我真的很喜欢它，因为它最适合我正在尝试做的模型。但是因为我不倾向于在添加约束后更新它们，所以我使用了一种在创建它们的同时激活它们的速记：例如newView.centerXAnchor.constraintEqualToAnchor(view.centerXAnchor).active = true 如果你想要偏移量，例如使用这个 let h = newView.centerXAnchor.constraintEqualToAnchor(view.centerXAnchor, constant: 80) 或 let w = updatesAvailablePopover.centerYAnchor.constraintEqualToAnchor(view.topAnchor, constant: 100) @JoeHuang 不，如果您想在将来更改约束，速记不起作用，因为速记正在创建新的约束。如果您希望引用以在将来更新约束，则需要使用 Imanou 在答案中使用的锚样式:) 别忘了newView.translatesAutoresizingMaskIntoConstraints = false 部分！我花了一段时间才看到。【参考方案2】：

它帮助我从视觉上学习，所以这是一个补充答案。

样板代码

override func viewDidLoad() 
    super.viewDidLoad()

    let myView = UIView()
    myView.backgroundColor = UIColor.blue
    myView.translatesAutoresizingMaskIntoConstraints = false
    view.addSubview(myView)

    // Add constraints code here
    // ...

以下每个示例都独立于其他示例。

固定左边缘

myView.leading = leadingMargin + 20

方法一：锚样式

let margins = view.layoutMarginsGuide
myView.leadingAnchor.constraint(equalTo: margins.leadingAnchor, constant: 20).isActive = true
除了leadingAnchor，还有trailingAnchor、topAnchor、bottomAnchor。

方法二：NSLayoutConstraint 样式

NSLayoutConstraint(item: myView, attribute: NSLayoutAttribute.leading, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.leadingMargin, multiplier: 1.0, constant: 20.0).isActive = true
除了.leading，还有.trailing、.top和.bottom。 除了.leadingMargin，还有.trailingMargin、.topMargin和.bottomMargin。

设置宽度和高度

width = 200height = 100

方法一：锚样式

myView.widthAnchor.constraint(equalToConstant: 200).isActive = true
myView.heightAnchor.constraint(equalToConstant: 100).isActive = true

方法二：NSLayoutConstraint 样式

NSLayoutConstraint(item: myView, attribute: NSLayoutAttribute.width, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 200).isActive = true
NSLayoutConstraint(item: myView, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 100).isActive = true

容器中心

myView.centerX = centerXmyView.centerY = centerY

方法一：锚样式

myView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
myView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true

方法二：NSLayoutConstraint 样式

NSLayoutConstraint(item: myView, attribute: NSLayoutAttribute.centerX, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerX, multiplier: 1, constant: 0).isActive = true
NSLayoutConstraint(item: myView, attribute: NSLayoutAttribute.centerY, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerY, multiplier: 1, constant: 0).isActive = true

注意事项

锚样式是优于NSLayoutConstraint 样式的首选方法，但它仅适用于iOS 9，因此如果您支持iOS 8，则仍应使用NSLayoutConstraint 样式。 上面的示例仅显示了所关注的一两个约束。但是，为了在我的测试项目中正确放置 myView，我需要有四个约束。

进一步阅读

Programmatically Creating Constraints 文档

【讨论】：

非常好！一个小提示：只应指定 leading 与 centerX 约束中的一个。 @Cyber​​Dude，是的，你是对的。我的意思是上面的所有例子都是相互独立的，而不是把所有的约束都添加到同一个视图中。不过，这并不像我希望的那样清楚。 您在view 上遗漏了必要的addConstraint() 以使布局约束注册。【参考方案3】：

如果您想填充您的超级视图，那么我建议您使用 swifty 方式：

    view.translatesAutoresizingMaskIntoConstraints = false
    let attributes: [NSLayoutAttribute] = [.top, .bottom, .right, .left]
    NSLayoutConstraint.activate(attributes.map 
        NSLayoutConstraint(item: view, attribute: $0, relatedBy: .equal, toItem: view.superview, attribute: $0, multiplier: 1, constant: 0)
    )

如果您需要非相等约束，请查看 iOS 9 中的 NSLayoutAnchor。直接使用 NSLayoutConstraint 通常更容易阅读：

    view.translatesAutoresizingMaskIntoConstraints = false
    view.topAnchor.constraint(equalTo: view.superview!.topAnchor).isActive = true
    view.bottomAnchor.constraint(equalTo: view.superview!.bottomAnchor).isActive = true
    view.leadingAnchor.constraint(equalTo: view.superview!.leadingAnchor, constant: 10).isActive = true
    view.trailingAnchor.constraint(equalTo: view.superview!.trailingAnchor, constant: 10).isActive = true

【讨论】：

基于此moment of WWDC video。逐个激活约束通常效率不高，即执行isActive = true 并不总是一个好主意。将相关约束组合成一个并使用NSLayoutConstraint.activate 一次性激活它们会更好，性能更好。【参考方案4】：

游乐场中多个视图的约束。

swift 3+

  var yellowView: UIView!
    var redView: UIView!

    override func loadView() 

        // UI

        let view = UIView()
        view.backgroundColor = .white

        yellowView = UIView()
        yellowView.backgroundColor = .yellow
        view.addSubview(yellowView)

        redView = UIView()
        redView.backgroundColor = .red
        view.addSubview(redView)

        // Layout
        redView.translatesAutoresizingMaskIntoConstraints = false
        yellowView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            yellowView.topAnchor.constraint(equalTo: view.topAnchor, constant: 20),
            yellowView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 20),
            yellowView.widthAnchor.constraint(equalToConstant: 80),
            yellowView.heightAnchor.constraint(equalToConstant: 80),

            redView.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: -20),
            redView.trailingAnchor.constraint(equalTo: view.trailingAnchor,constant: -20),
            redView.widthAnchor.constraint(equalToConstant: 80),
            redView.heightAnchor.constraint(equalToConstant: 80)
            ])

        self.view = view

在我看来，xcode 游乐场是学习添加的最佳场所 以编程方式约束。

【讨论】：

【参考方案5】：

我们可以在 swift 5.1 中轻松做到这一点

设置 1

子视图与视图中心对齐

使用浮动设置子视图宽度高度

view.addSubview(myView1)
myView1.translatesAutoresizingMaskIntoConstraints = false
NSLayoutConstraint.activate([
    myView1.centerXAnchor.constraint(equalTo: view.centerXAnchor),
    myView1.centerYAnchor.constraint(equalTo: view.centerYAnchor),
    myView1.widthAnchor.constraint(equalToConstant: 100),
    myView1.heightAnchor.constraint(equalToConstant: 100),
])

设置 2

子视图与视图前导和顶部锚点对齐

使用视图宽度高度设置子视图宽度

    view.addSubview(myView2)
    myView2.translatesAutoresizingMaskIntoConstraints = false

    NSLayoutConstraint.activate([
        myView2.leadingAnchor.constraint(equalTo: view.leadingAnchor,constant: 16),
        myView2.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor,constant: 16),
        myView2.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 0.3),
        myView2.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 0.3)
    ])

【讨论】：

【参考方案6】：

基本上它涉及 3 个步骤

fileprivate func setupName()  

    lblName.text = "Hello world"

    // Step 1
    lblName.translatesAutoresizingMaskIntoConstraints = false

    //Step 2
    self.view.addSubview(lblName)

    //Step 3
    NSLayoutConstraint.activate([
        lblName.centerXAnchor.constraint(equalTo: self.view.centerXAnchor),
        lblName.centerYAnchor.constraint(equalTo: self.view.centerYAnchor)
    ])

这会将标签“hello world”放在屏幕中心。

请参考链接Autolayout constraints programmatically

【讨论】：

【参考方案7】：

正如错误消息所暗示的，问题在于您的 NSAutoresizingMaskLayoutConstraints 类型的约束与您的显式约束冲突，因为 new_view.translatesAutoresizingMaskIntoConstraints 设置为 true。

这是您在代码中创建的视图的默认设置。你可以这样关闭它：

var new_view:UIView! = UIView(frame: CGRectMake(0, 0, 100, 100))
new_view.translatesAutoresizingMaskIntoConstraints = false

另外，您的宽度和高度限制很奇怪。如果您希望视图具有恒定的宽度，这是正确的方法：

new_view.addConstraint(NSLayoutConstraint(
    item:new_view, attribute:NSLayoutAttribute.Width,
    relatedBy:NSLayoutRelation.Equal,
    toItem:nil, attribute:NSLayoutAttribute.NotAnAttribute,
    multiplier:0, constant:100))

（将 100 替换为您想要的宽度。）

如果您的部署目标是 iOS 9.0 或更高版本，您可以使用以下更短的代码：

new_view.widthAnchor.constraintEqualToConstant(100).active = true

无论如何，对于这样的布局（固定大小并在父视图中居中），使用自动调整掩码并让系统将掩码转换为约束会更简单：

var new_view:UIView! = UIView(frame: CGRectMake(0, 0, 100, 100))
new_view.backgroundColor = UIColor.redColor();
view.addSubview(new_view);

// This is the default setting but be explicit anyway...
new_view.translatesAutoresizingMaskIntoConstraints = true

new_view.autoresizingMask = [ .FlexibleTopMargin, .FlexibleBottomMargin,
    .FlexibleLeftMargin, .FlexibleRightMargin ]

new_view.center = CGPointMake(view.bounds.midX, view.bounds.midY)

请注意，即使您还使用自动布局，使用自动调整大小也是完全合法的。 （UIKit 在内部的很多地方仍然使用自动调整大小。）问题是很难对使用自动调整大小的视图应用额外的约束。

【讨论】：

如果 UIKit 在内部使用自动调整大小可以解释锚定在中心的约束的不当行为吗？当我将 UIButtons 集合组织为 UIview 的子类时，锚约束似乎起作用，但当我将它们组织为视图控制器中的函数时，它们不起作用。如果您有空，可以看看这篇文章 [***.com/questions/40650951/…‌ ler-we-cant-actually-add-any-new-fences-due] 不，这并不能解释代码的不当行为。【参考方案8】：

为 Swift 3 更新

import UIKit

class ViewController: UIViewController 

let redView: UIView = 

    let view = UIView()
    view.translatesAutoresizingMaskIntoConstraints = false
    view.backgroundColor = .red
    return view
()

override func viewDidLoad() 
    super.viewDidLoad()

    setupViews()
    setupAutoLayout()


func setupViews() 

    view.backgroundColor = .white
    view.addSubview(redView)


func setupAutoLayout() 

    // Available from iOS 9 commonly known as Anchoring System for AutoLayout...
    redView.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 20).isActive = true
    redView.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -20).isActive = true

    redView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
    redView.heightAnchor.constraint(equalToConstant: 300).isActive = true

    // You can also modified above last two lines as follows by commenting above & uncommenting below lines...
    // redView.topAnchor.constraint(equalTo: view.topAnchor, constant: 20).isActive = true
    // redView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true

约束类型

 /*
// regular use
1.leftAnchor
2.rightAnchor
3.topAnchor
// intermediate use
4.widthAnchor
5.heightAnchor
6.bottomAnchor
7.centerXAnchor
8.centerYAnchor
// rare use
9.leadingAnchor
10.trailingAnchor
etc. (note: very project to project)
*/

【讨论】：

别忘了加，youViewName.translatesAutoresizingMaskIntoConstraints = false【参考方案9】：

自动布局是通过对图像应用约束来实现的。使用 NSLayoutConstraint。可以在所有设备上实现理想而美观的设计。请尝试以下代码。

import UIKit

class ViewController: UIViewController 

override func viewDidLoad() 
super.viewDidLoad()

let myImageView:UIImageView = UIImageView()
myImageView.backgroundColor = UIColor.red
myImageView.image = UIImage(named:"sample_dog")!
myImageView.translatesAutoresizingMaskIntoConstraints = false
myImageView.layer.borderColor = UIColor.red.cgColor
myImageView.layer.borderWidth = 10
self.view.addSubview(myImageView)
        
view.removeConstraints(view.constraints)


view.addConstraint(NSLayoutConstraint(
item: myImageView,
attribute: .top,
relatedBy: .equal,
toItem: view,
attribute: .top,
multiplier: 1,
constant:100)
    
)

view.addConstraint(NSLayoutConstraint(
item: myImageView,
attribute: .centerX,
relatedBy: .equal,
toItem: view,
attribute: .centerX,
multiplier: 1,
constant:0)
)
    
view.addConstraint(NSLayoutConstraint(
item: myImageView,
attribute: .height,
relatedBy: .equal,
toItem: view,
attribute: .width,
multiplier: 0.5,
constant:40))
    
view.addConstraint(NSLayoutConstraint(
item: myImageView,
attribute: .width,
relatedBy: .equal,
toItem: view,
attribute: .width,
multiplier: 0.5,
constant:40))
    


override func didReceiveMemoryWarning() 
    super.didReceiveMemoryWarning()

【讨论】：

【参考方案10】：

想在 Imanou Petit 的回答中添加一些理论概念，以便了解自动布局的工作原理。

要了解自动布局，请将您的视图视为橡胶对象，它最初会缩小。

要在屏幕上放置一个对象，我们需要 4 个必备的东西：

对象的X坐标（水平位置）。

物体的Y坐标（垂直位置）

对象的宽度

对象的高度。

1 X 坐标：有多种方法可以为视图提供 x 坐标。

如前导约束、尾随约束、水平居中 等等

2 Y 坐标： 有多种方法可以为视图提供 y 坐标：

如顶部约束、底部约束、垂直居中等

3 对象的宽度：有两种方法可以给视图提供宽度约束：

一个。添加固定宽度约束（将此约束视为固定宽度的铁棒，并且您已将橡胶物体水平钩住，因此橡胶物体不会收缩或膨胀）

b.不要添加任何宽度约束，而是将 x 坐标约束添加到视图尾随和前导的两端，这两个约束将通过从前导和尾随两端拉/推橡胶对象来扩展/收缩橡胶对象。

4 对象的高度： 与宽度类似，也有两种方法可以对视图进行高度限制：

一个。添加固定高度约束（将此约束视为固定高度的铁棒，并且您已将橡胶物体垂直钩住，因此橡胶物体不会收缩或膨胀）

b.不要添加任何高度约束，而是在视图顶部和底部添加 x 坐标约束，这两个约束将扩大/缩小橡胶的对象从两端、顶部和底部拉/推它。

【讨论】：

【参考方案11】：

在 xcode 7.3.1 中有点不同。这就是我想出的

   // creating the view
        let newView = UIView()
        newView.backgroundColor = UIColor.redColor()
        newView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(newView)

        // creating the constraint 

        // attribute and relation cannot be set directyl you need to create a cariable of them
        let layout11 = NSLayoutAttribute.CenterX
        let layout21 = NSLayoutRelation.Equal
        let layout31 = NSLayoutAttribute.CenterY
        let layout41 = NSLayoutAttribute.Width
        let layout51 = NSLayoutAttribute.Height
        let layout61 = NSLayoutAttribute.NotAnAttribute

        // defining all the constraint
        let horizontalConstraint = NSLayoutConstraint(item: newView, attribute: layout11, relatedBy: layout21, toItem: view, attribute: layout11, multiplier: 1, constant: 0)
        let verticalConstraint = NSLayoutConstraint(item: newView, attribute: layout31, relatedBy: layout21, toItem: view, attribute: layout31, multiplier: 1, constant: 0)
        let widthConstraint = NSLayoutConstraint(item: newView, attribute: layout41, relatedBy: layout21, toItem: nil, attribute: layout61, multiplier: 1, constant: 100)
        let heightConstraint = NSLayoutConstraint(item: newView, attribute: layout51, relatedBy: layout21, toItem: nil, attribute: layout61, multiplier: 1, constant: 100)

        // adding all the constraint
        NSLayoutConstraint.activateConstraints([horizontalConstraint,verticalConstraint,widthConstraint,heightConstraint])

【讨论】：

translatesAutoresizingMaskIntoConstraints = false :))【参考方案12】：

这是以编程方式添加约束的一种方法

override func viewDidLoad() 
            super.viewDidLoad()


let myLabel = UILabel()
        myLabel.labelFrameUpdate(label: myLabel, text: "Welcome User", font: UIFont(name: "times new roman", size: 40)!, textColor: UIColor.red, textAlignment: .center, numberOfLines: 0, borderWidth: 2.0, BorderColor: UIColor.red.cgColor)
        self.view.addSubview(myLabel)


         let myLabelhorizontalConstraint = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.centerX, relatedBy: NSLayoutRelation.equal, toItem: self.view, attribute: NSLayoutAttribute.centerX, multiplier: 1, constant: 0)
        let myLabelverticalConstraint = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.centerY, relatedBy: NSLayoutRelation.equal, toItem: self.view, attribute: NSLayoutAttribute.centerY, multiplier: 1, constant: 0)
        let mylabelLeading = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.leading, relatedBy: NSLayoutRelation.equal, toItem: self.view, attribute: NSLayoutAttribute.leading, multiplier: 1, constant: 10)
        let mylabelTrailing = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.trailing, relatedBy: NSLayoutRelation.equal, toItem: self.view, attribute: NSLayoutAttribute.trailing, multiplier: 1, constant: -10)
        let myLabelheightConstraint = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 50)
        NSLayoutConstraint.activate(\[myLabelhorizontalConstraint, myLabelverticalConstraint, myLabelheightConstraint,mylabelLeading,mylabelTrailing\])


extension UILabel

    func labelFrameUpdate(label:UILabel,text:String = "This is sample Label",font:UIFont = UIFont(name: "times new roman", size: 20)!,textColor:UIColor = UIColor.red,textAlignment:NSTextAlignment = .center,numberOfLines:Int = 0,borderWidth:CGFloat = 2.0,BorderColor:CGColor = UIColor.red.cgColor)
        label.translatesAutoresizingMaskIntoConstraints = false
        label.text = text
        label.font = font
        label.textColor = textColor
        label.textAlignment = textAlignment
        label.numberOfLines = numberOfLines
        label.layer.borderWidth = borderWidth
        label.layer.borderColor = UIColor.red.cgColor

【讨论】：

【参考方案13】： 
    var xCenterConstraint : NSLayoutConstraint!
    var yCenterConstraint: NSLayoutConstraint!

 xCenterConstraint = NSLayoutConstraint(item: self.view, attribute: .CenterX, relatedBy: .Equal, toItem: (Your view NAme), attribute: .CenterX, multiplier: 1, constant: 0)
            self.view.addConstraint(xCenterConstraint)

 yCenterConstraint = NSLayoutConstraint(item: self.view, attribute: .CenterY, relatedBy: .Equal, toItem: (Your view Name), attribute: .CenterY, multiplier: 1, constant: 0)
            self.view.addConstraint(yCenterConstraint)

【讨论】：

请解释代码以及它如何回答问题。【参考方案14】：

如果你觉得上面的内容很丑。您应该考虑使用 DSL 进行约束。如SnapKit 使约束 API 更加用户友好

view.snp.makeConstraints  make in
    make.edges.equalToSuperview()

【讨论】：

【参考方案15】：

试试这个优雅的UIView 扩展来约束。你可以简单地做约束：


 - firstView.coverWholeSuperview()
 - firstView.constraints(size: CGSize(width: 44, height: 44), centerX: view.centerXAnchor, centerY: view.centerXAnchor)
 - firstView.constraints(top: view.topAnchor, 
                         leading: secondView.leadingAnchor, 
                         bottom: view.bottomAnchor, 
                         trailing: secondView.trailingAnchor, 
                         padding: UIEdgeInsets(top: 12, left: 12, bottom: 12, right: 12))

这是扩展，只需将其复制到您的项目中即可。

extension UIView 
    /// Attaches all sides of the receiver to its parent view
    func coverWholeSuperview(margin: CGFloat = 0.0) 
        let view = superview
        layoutAttachTop(to: view, margin: margin)
        layoutAttachBottom(to: view, margin: margin)
        layoutAttachLeading(to: view, margin: margin)
        layoutAttachTrailing(to: view, margin: margin)

    

    /// Attaches the top of the current view to the given view's top if it's a superview of the current view
    /// or to it's bottom if it's not (assuming this is then a sibling view).
    @discardableResult
    func layoutAttachTop(to: UIView? = nil, margin: CGFloat = 0.0) -> NSLayoutConstraint 

        let view: UIView? = to ?? superview
        let isSuperview = view == superview
        let constraint = NSLayoutConstraint(item: self, attribute: .top, relatedBy: .equal,
                                            toItem: view, attribute: isSuperview ? .top : .bottom, multiplier: 1.0,
                                            constant: margin)
        superview?.addConstraint(constraint)

        return constraint
    

    /// Attaches the bottom of the current view to the given view
    @discardableResult
    func layoutAttachBottom(to: UIView? = nil, margin: CGFloat = 0.0, priority: UILayoutPriority? = nil) -> NSLayoutConstraint 

        let view: UIView? = to ?? superview
        let isSuperview = (view == superview) || false
        let constraint = NSLayoutConstraint(item: self, attribute: .bottom, relatedBy: .equal,
                                            toItem: view, attribute: isSuperview ? .bottom : .top, multiplier: 1.0,
                                            constant: -margin)
        if let priority = priority 
            constraint.priority = priority
        
        superview?.addConstraint(constraint)

        return constraint
    

    /// Attaches the leading edge of the current view to the given view
    @discardableResult
    func layoutAttachLeading(to: UIView? = nil, margin: CGFloat = 0.0) -> NSLayoutConstraint 

        let view: UIView? = to ?? superview
        let isSuperview = (view == superview) || false
        let constraint = NSLayoutConstraint(item: self, attribute: .leading, relatedBy: .equal,
                                            toItem: view, attribute: isSuperview ? .leading : .trailing, multiplier: 1.0,
                                            constant: margin)
        superview?.addConstraint(constraint)

        return constraint
    

    /// Attaches the trailing edge of the current view to the given view
    @discardableResult
    func layoutAttachTrailing(to: UIView? = nil, margin: CGFloat = 0.0, priority: UILayoutPriority? = nil) -> NSLayoutConstraint 

        let view: UIView? = to ?? superview
        let isSuperview = (view == superview) || false
        let constraint = NSLayoutConstraint(item: self, attribute: .trailing, relatedBy: .equal,
                                            toItem: view, attribute: isSuperview ? .trailing : .leading, multiplier: 1.0,
                                            constant: -margin)
        if let priority = priority 
            constraint.priority = priority
        
        superview?.addConstraint(constraint)

        return constraint
    

    // For anchoring View
    struct AnchoredConstraints 
        var top, leading, bottom, trailing, width, height, centerX, centerY: NSLayoutConstraint?
    

    @discardableResult
    func constraints(top: NSLayoutYAxisAnchor? = nil, leading: NSLayoutXAxisAnchor? = nil, bottom: NSLayoutYAxisAnchor? = nil,
                trailing: NSLayoutXAxisAnchor? = nil, padding: UIEdgeInsets = .zero, size: CGSize = .zero,
                centerX: NSLayoutXAxisAnchor? = nil, centerY: NSLayoutYAxisAnchor? = nil,
                centerXOffset: CGFloat = 0, centerYOffset: CGFloat = 0) -> AnchoredConstraints 

        translatesAutoresizingMaskIntoConstraints = false
        var anchoredConstraints = AnchoredConstraints()

        if let top = top 
            anchoredConstraints.top = topAnchor.constraint(equalTo: top, constant: padding.top)
        

        if let leading = leading 
            anchoredConstraints.leading = leadingAnchor.constraint(equalTo: leading, constant: padding.left)
        

        if let bottom = bottom 
            anchoredConstraints.bottom = bottomAnchor.constraint(equalTo: bottom, constant: -padding.bottom)
        

        if let trailing = trailing 
            anchoredConstraints.trailing = trailingAnchor.constraint(equalTo: trailing, constant: -padding.right)
        

        if size.width != 0 
            anchoredConstraints.width = widthAnchor.constraint(equalToConstant: size.width)
        

        if size.height != 0 
            anchoredConstraints.height = heightAnchor.constraint(equalToConstant: size.height)
        

        if let centerX = centerX 
            anchoredConstraints.centerX = centerXAnchor.constraint(equalTo: centerX, constant: centerXOffset)
        

        if let centerY = centerY 
            anchoredConstraints.centerY = centerYAnchor.constraint(equalTo: centerY, constant: centerYOffset)
        

        [anchoredConstraints.top, anchoredConstraints.leading, anchoredConstraints.bottom,
         anchoredConstraints.trailing, anchoredConstraints.width,
         anchoredConstraints.height, anchoredConstraints.centerX,
         anchoredConstraints.centerY].forEach  $0?.isActive = true 

        return anchoredConstraints

【讨论】：

【参考方案16】：

以下代码在这种情况下适用于我：UIImageView 强制景观。

    imagePreview!.isUserInteractionEnabled = true
    imagePreview!.isExclusiveTouch = true
    imagePreview!.contentMode = UIView.ContentMode.scaleAspectFit
    
    // Remove all constraints
    imagePreview!.removeAllConstraints()
    
    // Add the new constraints
    let guide = view.safeAreaLayoutGuide
    imagePreview!.translatesAutoresizingMaskIntoConstraints = false
    imagePreview!.leadingAnchor.constraint(equalTo: guide.leadingAnchor).isActive = true
    imagePreview!.trailingAnchor.constraint(equalTo: guide.trailingAnchor).isActive = true
    imagePreview!.heightAnchor.constraint(equalTo: guide.heightAnchor, multiplier: 1.0).isActive = true

removeAllConstraints 是一个扩展

extension UIView 
    
    func removeAllConstraints() 
        var _superview = self.superview
        
        func removeAllConstraintsFromView(view: UIView)  for c in view.constraints  view.removeConstraint(c)  
        
        while let superview = _superview 
            for constraint in superview.constraints 
                
                if let first = constraint.firstItem as? UIView, first == self 
                    superview.removeConstraint(constraint)
                
                
                if let second = constraint.secondItem as? UIView, second == self 
                    superview.removeConstraint(constraint)
                
            
            
            _superview = superview.superview
        
        
        self.removeConstraints(self.constraints)
        self.translatesAutoresizingMaskIntoConstraints = true

【讨论】：

一些建议：我永远不会强行打开一些东西。在访问可选项之前始终保持警惕，否则您会遇到一些崩溃。此外，您不需要删除所有约束。这是对处理能力的浪费，并可能导致问题。只需在 viewDidLoad 或 init 函数中添加视图加载时所需的约束即可。【参考方案17】：

您将所有已定义的约束添加到 self.view，这是错误的，因为应将宽度和高度约束添加到您的 newView。

另外，据我了解，您希望将宽度和高度设置为 100:100。 在这种情况下，您应该将代码更改为：

var constW = NSLayoutConstraint(item: newView, 
    attribute: .Width, 
    relatedBy: .Equal, 
    toItem: nil, 
    attribute: .NotAnAttribute, 
    multiplier: 1, 
    constant: 100)
newView.addConstraint(constW)

var constH = NSLayoutConstraint(item: newView, 
    attribute: .Height, 
    relatedBy: .Equal, 
    toItem: nil, 
    attribute: .NotAnAttribute, 
    multiplier: 1, 
    constant: 100)
newView.addConstraint(constH)

【讨论】：

【参考方案18】：

您可以使用Snapkit 以编程方式设置约束。

class ViewController: UIViewController 
    
    let rectView: UIView = UIView(frame: CGRect(x: 0, y: 0, width: 100, height: 100))
    
    override func viewDidLoad() 
        super.viewDidLoad()
        setupViews()
    
    
    private func setupViews() 
        rectView.backgroundColor = .red
        view.addSubview(rectView)

        rectView.snp.makeConstraints 
          $0.center.equalToSuperview()

【讨论】：

【参考方案19】：

该错误是由自动调整掩码自动创建的约束引起的，它们是因为 UIView 属性 translatesAutoresizingMaskIntoConstraints 默认为 true 而创建的。

考虑使用BoxView 来摆脱所有手动创建约束的样板，并使您的代码简洁易读。使用 BoxView 制作有问题的布局非常简单：

boxView.items = [
   new_view.boxed.centerX().centerY().relativeWidth(1.0).relativeHeight(1.0)
]

【讨论】：

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