ios - 为什么约束在 UITableView 滚动时更新？

Question

我有 2 个简单的 UILabel 需要水平放置。其中之一在 View 的右侧，而第二个在左侧并获得所有可能的空间。这是我的约束条件:

    amountOfQuestions.rightAnchor.constraint(equalTo: rightAnchor, constant: -8).isActive = true
    amountOfQuestions.centerYAnchor.constraint(equalTo: centerYAnchor).isActive = true


    topicName.leftAnchor.constraint(equalTo: leftAnchor, constant: 8).isActive = true
    topicName.topAnchor.constraint(equalTo: topAnchor, constant: 8).isActive = true
    topicName.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -8).isActive = true
    topicName.rightAnchor.constraint(equalTo: amountOfQuestions.leftAnchor, constant: -8).isActive = true

看起来一切正常，但是当显示 TableView 时，它看起来像这样:

但是在我上下滚动表格几次后，它变得正常了:

为什么我的表格没有像第二张图片那样立即显示出来？

解决方案:

amountOfQuestions.rightAnchor.constraint(equalTo: rightAnchor, constant: -8).isActive = true
    amountOfQuestions.centerYAnchor.constraint(equalTo: centerYAnchor).isActive = true
    amountOfQuestions.setContentHuggingPriority(.required, for: .horizontal)
    amountOfQuestions.setContentCompressionResistancePriority(.required, for: .horizontal)

    topicName.leftAnchor.constraint(equalTo: leftAnchor, constant: 8).isActive = true
    topicName.topAnchor.constraint(equalTo: topAnchor, constant: 8).isActive = true
    topicName.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -8).isActive = true
    topicName.rightAnchor.constraint(equalTo: amountOfQuestions.leftAnchor, constant: -8).isActive = true
    topicName.setContentHuggingPriority(.required, for: .horizontal)
    topicName.setContentCompressionResistancePriority(.required, for: .vertical)


    let myBottom = topicName.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -8)
    myBottom.priority = UILayoutPriority(rawValue: UILayoutPriority.required.rawValue - 1)
    myBottom.isActive = true

Answer 1

有多个问题会影响布局并使其定义不正确:

1. 切勿将内容限制在单元格本身。始终约束到单元格的 contentView，例如:

   amountOfQuestions.rightAnchor.constraint(equalTo: contentView.rightAnchor, constant: -8).isActive = true
   

(确保将标签添加为 contentView 的 subview ，而不是单元格本身)。

1. 如果您希望标签的大小产生任何影响，您需要正确设置拥抱和压缩阻力优先级。否则布局不必尊重内容:

   // you want topic name to resize vertically to respect the content
   topicName.setHuggingPriority(.required, for: .vertical)
   topicName.setContentCompressionResistancePriority(.required, for: .vertical)
   
   // you want amount of question to resize horizontally to respect the content
   amountOfQuestions.setHuggingPriority(.required, for: .horizontal)
   amountOfQuestions.setContentCompressionResistancePriority(.required, for: .horizontal)
   

1. 为避免在第一个布局期间发生布局冲突，最好为其中一个垂直约束赋予稍低的布局优先级:

   let bottomAnchor = topicName.bottomAnchor.constraint(equalTo: contentView.bottomAnchor, constant: -8)
   bottomAnchor.priority = UILayoutPriority(rawValue: UILayoutPriority.required.rawValue - 1)
   bottomAnchor.isActive = true