如何通过其优先级更改内在内容大小

Question

我希望第二个标签 (subview2) 保持不变，并让第一个标签 (subview1) 随着容器视图大小的减小而缩小，但无论我如何调整第二个标签仍然会缩小优先级：

import UIKit
import PlaygroundSupport

let containerView = UIView(frame: CGRect(origin: .zero, size: .init(width: 200, height: 300)))
let subview1 = UILabel()
subview1.text = "Hello"
subview1.backgroundColor = .red
containerView.addSubview(subview1)
subview1.translatesAutoresizingMaskIntoConstraints = false
let subview2 = UILabel()
subview2.text = "Hello"
subview2.backgroundColor = .cyan
containerView.addSubview(subview2)
subview2.translatesAutoresizingMaskIntoConstraints = false

let views: [String: Any] = ["subview1": subview1, "subview2": subview2]
let con1 = NSLayoutConstraint.constraints(withVisualFormat: "V:|-(20)-[subview1]", metrics: nil, views: views)
let con2 = NSLayoutConstraint.constraints(withVisualFormat: "V:|-(20)-[subview2]", metrics: nil, views: views)
let con3 = NSLayoutConstraint.constraints(withVisualFormat: "H:|-(30)-[subview1]", metrics: nil, views: views)
let con4 = NSLayoutConstraint.constraints(withVisualFormat: "H:[subview2]-(30)-|", metrics: nil, views: views)
let con5 = NSLayoutConstraint.constraints(withVisualFormat: "H:[subview1(>=100)]-(>=30)-[subview2(>=100)]", metrics: nil, views: views)
NSLayoutConstraint.activate(con1 + con2 + con3 + con4 + con5)

subview1.setContentCompressionResistancePriority(UILayoutPriority.defaultLow, for: .horizontal)
subview2.setContentCompressionResistancePriority(UILayoutPriority.defaultHigh, for: .horizontal)

PlaygroundPage.current.needsIndefiniteExecution = true
PlaygroundPage.current.liveView = containerView

我试过手动增加subview2的优先级，但还是不行：

let priority1 = subview2.contentCompressionResistancePriority(for: .horizontal)
print(priority1) // 750
let priority2 = subview2.contentCompressionResistancePriority(for: .horizontal)
print(priority2) // 750
subview2.setContentCompressionResistancePriority(priority2 + 100, for: .horizontal)

Answer 1

VFL（视觉格式语言）非常过时且有限。我强烈建议您改用更现代（更灵活）的语法。

也就是说，问题出在这一行：

let con5 = NSLayoutConstraint.constraints(withVisualFormat: 
            "H:[subview1(>=100)]-(>=30)-[subview2(>=100)]", metrics: nil, views: views)

你的代码说：

• subview1 宽度必须大于或等于100
• subview2 宽度必须大于或等于100

所以，setContentCompressionResistancePriority 行什么也不做。您已经说过“这些是必需的最小宽度。”

如果您将该行更改为：

let con5 = NSLayoutConstraint.constraints(withVisualFormat:
            "H:[subview1]-(>=30)-[subview2]", metrics: nil, views: views)

你应该得到你想要的结果。

不过，您的代码使用了更现代的约束语法——您可能会发现它更合乎逻辑且更易于理解：

NSLayoutConstraint.activate([

    // both labels 20-pts from containerView Top
    subview1.topAnchor.constraint(equalTo: containerView.topAnchor, constant: 20.0),
    subview2.topAnchor.constraint(equalTo: containerView.topAnchor, constant: 20.0),

    // left label Leading 30-pts from containerView Leading
    subview1.leadingAnchor.constraint(equalTo: containerView.leadingAnchor, constant: 30.0),

    // right label Trailing 30-pts from containerView Trailing
    subview2.trailingAnchor.constraint(equalTo: containerView.trailingAnchor, constant: -30.0),

    // right label Leading at least 30-pts from left label Trailing
    subview2.leadingAnchor.constraint(greaterThanOrEqualTo: subview1.trailingAnchor, constant: 30.0),

])

// don't do any of this...
//let views: [String: Any] = ["subview1": subview1, "subview2": subview2]
//let con1 = NSLayoutConstraint.constraints(withVisualFormat: "V:|-(20)-[subview1]", metrics: nil, views: views)
//let con2 = NSLayoutConstraint.constraints(withVisualFormat: "V:|-(20)-[subview2]", metrics: nil, views: views)
//let con3 = NSLayoutConstraint.constraints(withVisualFormat: "H:|-(30)-[subview1]", metrics: nil, views: views)
//let con4 = NSLayoutConstraint.constraints(withVisualFormat: "H:[subview2]-(30)-|", metrics: nil, views: views)
//let con5 = NSLayoutConstraint.constraints(withVisualFormat: "H:[subview1]-(>=30)-[subview2]", metrics: nil, views: views)
//NSLayoutConstraint.activate(con1 + con2 + con3 + con4 + con5)

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编辑 -回答评论...

如果您希望每个标签的宽度为 >= 100，但允许它们压缩（左标签首先压缩），您需要添加优先级小于 .required 的宽度约束：

// both labels should have a width of >= 100
let leftWidth = subview1.widthAnchor.constraint(greaterThanOrEqualToConstant: 100.0)
let rightWidth = subview2.widthAnchor.constraint(greaterThanOrEqualToConstant: 100.0)

// set the Priority on the width constraints to less than required
//  to allow auto-layout to break that constraint if needed
leftWidth.priority = .defaultHigh
rightWidth.priority = .defaultHigh

// activate the width constraints
leftWidth.isActive = true
rightWidth.isActive = true

// tell leftLabel to compress before rightLabel
subview1.setContentCompressionResistancePriority(UILayoutPriority.defaultLow, for: .horizontal)
subview2.setContentCompressionResistancePriority(UILayoutPriority.defaultHigh, for: .horizontal)