Ios 自动布局在显示1个标签的情况下不起作用，但在显示2个标签的情况下起作用

当标签是容器视图层次结构中唯一的标签时，标签会在其容器外部展开。如果有两个标签，它可以正常工作，并且两个标签都位于容器视图中

我的实际用例更复杂，但我试图将其简化为下面的代码

Xcode 10.2游乐场代码（Swift 5）：

结果如下图所示：

如果我尝试使用2个标签，它将生成正确的结果，这样标签就不会扩展其容器的宽度：

import UIKit
import Foundation
import PlaygroundSupport

let viewController = UIViewController()
viewController.view.backgroundColor = UIColor.green

let containerView = UIView()
containerView.backgroundColor = .gray
containerView.translatesAutoresizingMaskIntoConstraints = false
viewController.view.addSubview(containerView)

containerView.widthAnchor.constraint(equalToConstant: 200).isActive = true
containerView.heightAnchor.constraint(equalToConstant: 200).isActive = true
containerView.centerXAnchor.constraint(equalTo: viewController.view.centerXAnchor).isActive = true
containerView.centerYAnchor.constraint(equalTo: viewController.view.centerYAnchor).isActive = true

let topologyView = UIView()
topologyView.backgroundColor = .blue
topologyView.translatesAutoresizingMaskIntoConstraints = false
containerView.addSubview(topologyView)

let leadingConstraint = topologyView.leadingAnchor.constraint(greaterThanOrEqualTo: containerView.leadingAnchor)
print(leadingConstraint.priority)
leadingConstraint.priority = .defaultLow
leadingConstraint.isActive = true

let trailingConstraint = topologyView.trailingAnchor.constraint(greaterThanOrEqualTo: containerView.trailingAnchor)
trailingConstraint.priority = .defaultLow
trailingConstraint.isActive = true

let topConstraint = topologyView.topAnchor.constraint(greaterThanOrEqualTo: containerView.topAnchor)
topConstraint.priority = .defaultLow
topConstraint.isActive = true

let bottomConstraint = topologyView.bottomAnchor.constraint(greaterThanOrEqualTo: containerView.bottomAnchor)
bottomConstraint.priority = .defaultLow
bottomConstraint.isActive = true

topologyView.centerXAnchor.constraint(equalTo: containerView.centerXAnchor).isActive = true
topologyView.centerYAnchor.constraint(equalTo: containerView.centerYAnchor).isActive = true

let label1Title = "1234"
let label1 = UILabel()
label1.translatesAutoresizingMaskIntoConstraints = false
label1.text = Array(repeating: label1Title, count: 10).joined()
label1.setContentHuggingPriority(.required, for: .horizontal)
label1.setContentCompressionResistancePriority(.required, for: .horizontal)
topologyView.addSubview(label1)

label1.leadingAnchor.constraint(equalTo: topologyView.leadingAnchor).isActive = true
label1.trailingAnchor.constraint(equalTo: topologyView.trailingAnchor).isActive = true
label1.topAnchor.constraint(equalTo: topologyView.topAnchor).isActive = true

let label2 = UILabel()
label2.translatesAutoresizingMaskIntoConstraints = false
label2.text = "123124128"
label2.setContentHuggingPriority(.required, for: .horizontal)
label2.setContentCompressionResistancePriority(.required, for: .horizontal)
topologyView.addSubview(label2)

label2.leadingAnchor.constraint(equalTo: topologyView.leadingAnchor).isActive = true
label2.trailingAnchor.constraint(equalTo: topologyView.trailingAnchor).isActive = true
label2.bottomAnchor.constraint(equalTo: topologyView.bottomAnchor).isActive = true
label2.topAnchor.constraint(equalTo: label1.bottomAnchor).isActive = true

let window = UIWindow(frame: CGRect(x: 0, y: 0, width: 300, height: 350))
window.rootViewController = viewController

PlaygroundPage.current.liveView = window
PlaygroundPage.current.needsIndefiniteExecution = true

window.makeKeyAndVisible()

print(topologyView.contentHuggingPriority(for: .horizontal))
print(topologyView.contentCompressionResistancePriority(for: .horizontal))

print(containerView.contentHuggingPriority(for: .horizontal))
print(containerView.contentCompressionResistancePriority(for: .horizontal))

结果如下图所示：

我希望当我只显示一个标签时，标签不会扩展到其容器之外

---

我想知道为什么前导约束和尾随约束在第一种情况下（使用1个标签）不能正常工作，而在第二种情况下（使用2个标签）却能正常工作？

我发现您正在设置

label1.setContentHuggingPriority（.required，for:.horizontal）

。请尝试将优先级降低到

high

而不是

require

您遇到了一些问题

如果将

label2

的文本更改为：

label2.text = "01234567890 ABCDEFGHIJ"

您将获得以下信息：

正如你所看到的，有两个标签并不能“解决问题”。。。看起来是这样的，因为第二个标签使用了非常短的字符串

发生的情况是：当自动布局处理约束并发现它不能完全满足所有约束时，它使用优先级来确定它可以在不抛出错误的情况下打破哪些约束。在您的情况下，您正在设置各种约束和优先级，这些约束和优先级错误地授予自动布局更改布局的权限

因此，首先，我不知道您是从哪里想到将

topologyView

前导、尾随、顶部和底部约束的优先级设置为

.defaultLow

。它的作用是显式地告诉自动布局忽略那些约束（如果需要）。然后添加一个比topologyView宽的标签，自动布局将遵循您的说明并打破限制

在您的“双标签”示例中，自动布局优先于

label2

——这恰好使它看起来正确（直到您将标签加宽）

接下来，您错误地使用了

大于或等于
。通过设置：

topologyView.trailingAnchor.constraint(greaterThanOrEqualTo: containerView.trailingAnchor)

你说的是“让
地形视图的后缘
超越
集装箱视图的后缘
。你真正想要的是小于：

topologyView.trailingAnchor.constraint(lessThanOrEqualTo: containerView.trailingAnchor)

这将使
地形视图的后缘小于
集装箱视图的后缘

现在，还不清楚您是否希望标签与
containerView
的全宽相匹配，或者如果标签都很短，您是否希望标签居中

如果是全宽，则使用
equalTo
…如果居中，则使用
greathanorequalto
前导和
lesshanorequalto
尾随

同样的情况也适用于底部约束

因此，您的“单一标签”示例变成：

import UIKit
import Foundation
import PlaygroundSupport

let viewController = UIViewController()
viewController.view.backgroundColor = UIColor.green

let containerView = UIView()
containerView.backgroundColor = .gray
containerView.translatesAutoresizingMaskIntoConstraints = false
viewController.view.addSubview(containerView)

containerView.widthAnchor.constraint(equalToConstant: 200).isActive = true
containerView.heightAnchor.constraint(equalToConstant: 200).isActive = true
containerView.centerXAnchor.constraint(equalTo: viewController.view.centerXAnchor).isActive = true
containerView.centerYAnchor.constraint(equalTo: viewController.view.centerYAnchor).isActive = true

let topologyView = UIView()
topologyView.backgroundColor = .blue
topologyView.translatesAutoresizingMaskIntoConstraints = false
containerView.addSubview(topologyView)

let leadingConstraint = topologyView.leadingAnchor.constraint(greaterThanOrEqualTo: containerView.leadingAnchor)
// use default priority
//leadingConstraint.priority = .defaultLow
leadingConstraint.isActive = true

// use lessThanOrEqualTo
let trailingConstraint = topologyView.trailingAnchor.constraint(lessThanOrEqualTo: containerView.trailingAnchor)
// use default priority
//trailingConstraint.priority = .defaultLow
trailingConstraint.isActive = true

let topConstraint = topologyView.topAnchor.constraint(greaterThanOrEqualTo: containerView.topAnchor)
// use default priority
//topConstraint.priority = .defaultLow
topConstraint.isActive = true

// use lessThanOrEqualTo
let bottomConstraint = topologyView.bottomAnchor.constraint(lessThanOrEqualTo: containerView.bottomAnchor)
// use default priority
//bottomConstraint.priority = .defaultLow
bottomConstraint.isActive = true

topologyView.centerXAnchor.constraint(equalTo: containerView.centerXAnchor).isActive = true
topologyView.centerYAnchor.constraint(equalTo: containerView.centerYAnchor).isActive = true

let label1Title = "1234"
let label1 = UILabel()
label1.backgroundColor = .yellow
label1.translatesAutoresizingMaskIntoConstraints = false
label1.text = Array(repeating: label1Title, count: 10).joined()
// we can leave Hugging and Compression at default Priority
//label1.setContentHuggingPriority(.required, for: .horizontal)
//label1.setContentCompressionResistancePriority(.required, for: .horizontal)
topologyView.addSubview(label1)

label1.leadingAnchor.constraint(equalTo: topologyView.leadingAnchor).isActive = true
label1.trailingAnchor.constraint(equalTo: topologyView.trailingAnchor).isActive = true
label1.bottomAnchor.constraint(equalTo: topologyView.bottomAnchor).isActive = true
label1.topAnchor.constraint(equalTo: topologyView.topAnchor).isActive = true

let window = UIWindow(frame: CGRect(x: 0, y: 0, width: 300, height: 350))
window.rootViewController = viewController

PlaygroundPage.current.liveView = window
PlaygroundPage.current.needsIndefiniteExecution = true

window.makeKeyAndVisible()

导致（短标签）：


或（长标签）：


您的“两个标签”示例是：

import UIKit
import Foundation
import PlaygroundSupport

let viewController = UIViewController()
viewController.view.backgroundColor = UIColor.green

let containerView = UIView()
containerView.backgroundColor = .gray
containerView.translatesAutoresizingMaskIntoConstraints = false
viewController.view.addSubview(containerView)

containerView.widthAnchor.constraint(equalToConstant: 200).isActive = true
containerView.heightAnchor.constraint(equalToConstant: 200).isActive = true
containerView.centerXAnchor.constraint(equalTo: viewController.view.centerXAnchor).isActive = true
containerView.centerYAnchor.constraint(equalTo: viewController.view.centerYAnchor).isActive = true

let topologyView = UIView()
topologyView.backgroundColor = .blue
topologyView.translatesAutoresizingMaskIntoConstraints = false
containerView.addSubview(topologyView)

let leadingConstraint = topologyView.leadingAnchor.constraint(greaterThanOrEqualTo: containerView.leadingAnchor)
// use default priority
//leadingConstraint.priority = .defaultLow
leadingConstraint.isActive = true

// use lessThanOrEqualTo
let trailingConstraint = topologyView.trailingAnchor.constraint(lessThanOrEqualTo: containerView.trailingAnchor)
// use default priority
//trailingConstraint.priority = .defaultLow
trailingConstraint.isActive = true

let topConstraint = topologyView.topAnchor.constraint(greaterThanOrEqualTo: containerView.topAnchor)
// use default priority
//topConstraint.priority = .defaultLow
topConstraint.isActive = true

// use lessThanOrEqualTo
let bottomConstraint = topologyView.bottomAnchor.constraint(lessThanOrEqualTo: containerView.bottomAnchor)
// use default priority
//bottomConstraint.priority = .defaultLow
bottomConstraint.isActive = true

topologyView.centerXAnchor.constraint(equalTo: containerView.centerXAnchor).isActive = true
topologyView.centerYAnchor.constraint(equalTo: containerView.centerYAnchor).isActive = true

let label1Title = "1234"
let label1 = UILabel()
label1.backgroundColor = .yellow
label1.translatesAutoresizingMaskIntoConstraints = false
label1.text = Array(repeating: label1Title, count: 10).joined()
// we can leave Hugging and Compression at default Priority
//label1.setContentHuggingPriority(.required, for: .horizontal)
//label1.setContentCompressionResistancePriority(.required, for: .horizontal)
topologyView.addSubview(label1)

label1.leadingAnchor.constraint(equalTo: topologyView.leadingAnchor).isActive = true
label1.trailingAnchor.constraint(equalTo: topologyView.trailingAnchor).isActive = true
label1.topAnchor.constraint(equalTo: topologyView.topAnchor).isActive = true

let label2 = UILabel()
label2.backgroundColor = .orange
label2.translatesAutoresizingMaskIntoConstraints = false
label2.text = "012345 ABCDE"
// we can leave Hugging and Compression at default Priority
//label2.setContentHuggingPriority(.required, for: .horizontal)
//label2.setContentCompressionResistancePriority(.required, for: .horizontal)
topologyView.addSubview(label2)

label2.leadingAnchor.constraint(equalTo: topologyView.leadingAnchor).isActive = true
label2.trailingAnchor.constraint(equalTo: topologyView.trailingAnchor).isActive = true
label2.bottomAnchor.constraint(equalTo: topologyView.bottomAnchor).isActive = true
label2.topAnchor.constraint(equalTo: label1.bottomAnchor).isActive = true

let window = UIWindow(frame: CGRect(x: 0, y: 0, width: 300, height: 350))
window.rootViewController = viewController

PlaygroundPage.current.liveView = window
PlaygroundPage.current.needsIndefiniteExecution = true

window.makeKeyAndVisible()

对于短标签和长标签结果：


您好！感谢您的回复。通过锚创建的约束不需要手动添加到父级。它们只需要被激活。请查看文档以获取参考：@LeonidKokhnovych噢，我有点困惑。所以sorry@LeonidKokhnovych很抱歉我的困惑。我只是更新我的答案。请告诉我它是否有效。我我认为您的问题是约束优先级。感谢您的回答！我已尝试将
label1.setContentHuggingPriority（.required，for:.horizontal）
更改为
label1.setContentHuggingPriority（.defaultHigh，for:.horizontal）
并且它不会改变任何东西。对我来说，它看起来更像是UIKit中的一个bug，或者我遗漏了一些重要的东西。我发现你正在设置
label1.setContentHuggingPriority（.required，For:.horizontal）
。请尝试将优先级降低到
高
，而不是
require
import UIKit
import Foundation
import PlaygroundSupport

let viewController = UIViewController()
viewController.view.backgroundColor = UIColor.green

let containerView = UIView()
containerView.backgroundColor = .gray
containerView.translatesAutoresizingMaskIntoConstraints = false
viewController.view.addSubview(containerView)

containerView.widthAnchor.constraint(equalToConstant: 200).isActive = true
containerView.heightAnchor.constraint(equalToConstant: 200).isActive = true
containerView.centerXAnchor.constraint(equalTo: viewController.view.centerXAnchor).isActive = true
containerView.centerYAnchor.constraint(equalTo: viewController.view.centerYAnchor).isActive = true

let topologyView = UIView()
topologyView.backgroundColor = .blue
topologyView.translatesAutoresizingMaskIntoConstraints = false
containerView.addSubview(topologyView)

let leadingConstraint = topologyView.leadingAnchor.constraint(greaterThanOrEqualTo: containerView.leadingAnchor)
// use default priority
//leadingConstraint.priority = .defaultLow
leadingConstraint.isActive = true

// use lessThanOrEqualTo
let trailingConstraint = topologyView.trailingAnchor.constraint(lessThanOrEqualTo: containerView.trailingAnchor)
// use default priority
//trailingConstraint.priority = .defaultLow
trailingConstraint.isActive = true

let topConstraint = topologyView.topAnchor.constraint(greaterThanOrEqualTo: containerView.topAnchor)
// use default priority
//topConstraint.priority = .defaultLow
topConstraint.isActive = true

// use lessThanOrEqualTo
let bottomConstraint = topologyView.bottomAnchor.constraint(lessThanOrEqualTo: containerView.bottomAnchor)
// use default priority
//bottomConstraint.priority = .defaultLow
bottomConstraint.isActive = true

topologyView.centerXAnchor.constraint(equalTo: containerView.centerXAnchor).isActive = true
topologyView.centerYAnchor.constraint(equalTo: containerView.centerYAnchor).isActive = true

let label1Title = "1234"
let label1 = UILabel()
label1.backgroundColor = .yellow
label1.translatesAutoresizingMaskIntoConstraints = false
label1.text = Array(repeating: label1Title, count: 10).joined()
// we can leave Hugging and Compression at default Priority
//label1.setContentHuggingPriority(.required, for: .horizontal)
//label1.setContentCompressionResistancePriority(.required, for: .horizontal)
topologyView.addSubview(label1)

label1.leadingAnchor.constraint(equalTo: topologyView.leadingAnchor).isActive = true
label1.trailingAnchor.constraint(equalTo: topologyView.trailingAnchor).isActive = true
label1.topAnchor.constraint(equalTo: topologyView.topAnchor).isActive = true

let label2 = UILabel()
label2.backgroundColor = .orange
label2.translatesAutoresizingMaskIntoConstraints = false
label2.text = "012345 ABCDE"
// we can leave Hugging and Compression at default Priority
//label2.setContentHuggingPriority(.required, for: .horizontal)
//label2.setContentCompressionResistancePriority(.required, for: .horizontal)
topologyView.addSubview(label2)

label2.leadingAnchor.constraint(equalTo: topologyView.leadingAnchor).isActive = true
label2.trailingAnchor.constraint(equalTo: topologyView.trailingAnchor).isActive = true
label2.bottomAnchor.constraint(equalTo: topologyView.bottomAnchor).isActive = true
label2.topAnchor.constraint(equalTo: label1.bottomAnchor).isActive = true

let window = UIWindow(frame: CGRect(x: 0, y: 0, width: 300, height: 350))
window.rootViewController = viewController

PlaygroundPage.current.liveView = window
PlaygroundPage.current.needsIndefiniteExecution = true

window.makeKeyAndVisible()