如何为具有动态高度的表格单元格正确设置自动布局？

Question

我尝试创建一个UITableViewCell 并设置子视图如下：

class MyCellView: UITableViewCell {

    private let ImageView = UIImageView()
    private let titleView = UILabel()

    override init(style: UITableViewCellStyle, reuseIdentifier: String?) {

        super.init(style: style, reuseIdentifier: reuseIdentifier)

        contentView.addSubview(ImageView)
        contentView.addSubview(titleView)

        ImageView.translatesAutoresizingMaskIntoConstraints = false
        ImageView.topAnchor.constraint(equalTo: contentView.topAnchor).isActive = true
        ImageView.leftAnchor.constraint(equalTo: contentView.leftAnchor).isActive = true
        ImageView.rightAnchor.constraint(equalTo: contentView.rightAnchor).isActive = true
        ImageView.heightAnchor.constraint(equalTo: contentView.widthAnchor, multiplier: 0.6).isActive = true

        titleView.translatesAutoresizingMaskIntoConstraints = false
        titleView.topAnchor.constraint(equalTo: ImageView.bottomAnchor).isActive = true
        titleView.leftAnchor.constraint(equalTo: contentView.leftAnchor).isActive = true
        titleView.rightAnchor.constraint(equalTo: contentView.rightAnchor).isActive = true
        titleView.bottomAnchor.constraint(equalTo: contentView.bottomAnchor).isActive = true

        titleView.numberOfLines = 0

        titleView.text = " "   // <-- *** please note this line

    }
}

在表格视图中，我已将其设置为使用动态高度：

tableView.estimatedRowHeight = 80
tableView.rowHeight = UITableViewAutomaticDimension

我的问题是，如果我没有在单元格中设置titleView 的文本，例如我在单元格视图中备注最后一行：

// titleView.text = " "

我会从 XCode 中得到约束错误。但如果我设置文本，一切正常。

文本是从 Internet 动态下载的，因此文本可能为空。另外我可以把文本设置为“”，一个空格，我想知道我在这里有什么误解吗？如果我更正某些内容，文本可以为空（“”或 nil）吗？

同样，如果我完全删除有关titleView 的代码，我只想在单元格中有一个UIImageView，我发现单元格的高度没有扩大。如何设置它以便我可以在单元格中获得动态高度的图像？

编辑：约束错误：

2017-12-22 15:35:34.715865+0800 MyProject[15774:733515] [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.
(
"<NSLayoutConstraint:0x60c000095450 V:|-(0)-[UIImageView:0x7ff40df0de90]   (active, names: '|':UITableViewCellContentView:0x7ff40df0edc0 )>",
"<NSLayoutConstraint:0x60c0000955e0 UIImageView:0x7ff40df0de90.height == 0.6*UITableViewCellContentView:0x7ff40df0edc0.width   (active)>",
"<NSLayoutConstraint:0x60c000095630 V:[UIImageView:0x7ff40df0de90]-(0)-[UILabel:0x7ff40df0e0c0]   (active)>",
"<NSLayoutConstraint:0x60c0000957c0 UILabel:0x7ff40df0e0c0.bottom == UITableViewCellContentView:0x7ff40df0edc0.bottom   (active)>",
"<NSLayoutConstraint:0x60c000096210 'UIView-Encapsulated-Layout-Height' UITableViewCellContentView:0x7ff40df0edc0.height == 248.333   (active)>",
"<NSLayoutConstraint:0x60c000095ea0 'UIView-Encapsulated-Layout-Width' UITableViewCellContentView:0x7ff40df0edc0.width == 414   (active)>"
)

Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x60c000095630 V:[UIImageView:0x7ff40df0de90]-(0)-[UILabel:0x7ff40df0e0c0]   (active)>

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.

Answer 1

• 如果你想避免警告，试试下面的代码

  class MyCellView: UITableViewCell {
  
    private let ImageView = UIImageView()
    private let titleView = UILabel()
  
    override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
  
      super.init(style: style, reuseIdentifier: reuseIdentifier)
  
      contentView.addSubview(ImageView)
      contentView.addSubview(titleView)
  
      ImageView.translatesAutoresizingMaskIntoConstraints = false
      ImageView.topAnchor.constraint(equalTo: contentView.topAnchor).isActive = true
      ImageView.leftAnchor.constraint(equalTo: contentView.leftAnchor).isActive = true
      ImageView.rightAnchor.constraint(equalTo: contentView.rightAnchor).isActive = true
      let heightConstraint = ImageView.heightAnchor.constraint(equalTo: contentView.widthAnchor, multiplier: 0.6)
      heightConstraint.priority = UILayoutPriority.defaultLow
      heightConstraint.isActive = true
  
      titleView.translatesAutoresizingMaskIntoConstraints = false
      titleView.topAnchor.constraint(equalTo: ImageView.bottomAnchor).isActive = true
      titleView.leftAnchor.constraint(equalTo: contentView.leftAnchor).isActive = true
      titleView.rightAnchor.constraint(equalTo: contentView.rightAnchor).isActive = true
      titleView.bottomAnchor.constraint(equalTo: contentView.bottomAnchor).isActive = true
  
      titleView.numberOfLines = 0
  
  //    titleView.text = " "
    }
  }
  

• 如果您完全删除有关titleView 的代码并希望在单元格中拥有一个具有动态高度的UIImageView，请为imageView 添加bottomAnchor 约束

  class MyCellView: UITableViewCell {
  
    private let ImageView = UIImageView()
    private let titleView = UILabel()
  
    override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
  
      super.init(style: style, reuseIdentifier: reuseIdentifier)
  
      contentView.addSubview(ImageView)
      contentView.addSubview(titleView)
  
      ImageView.translatesAutoresizingMaskIntoConstraints = false
      ImageView.topAnchor.constraint(equalTo: contentView.topAnchor).isActive = true
      ImageView.leftAnchor.constraint(equalTo: contentView.leftAnchor).isActive = true
      ImageView.bottomAnchor.constraint(equalTo: contentView.bottomAnchor).isActive = true
      ImageView.rightAnchor.constraint(equalTo: contentView.rightAnchor).isActive = true
      let heightConstraint = ImageView.heightAnchor.constraint(equalTo: contentView.widthAnchor, multiplier: 0.6)
      heightConstraint.priority = UILayoutPriority.defaultLow
      heightConstraint.isActive = true
    }
  }
  

Answer 2

这似乎是一个已知“错误”的结果 - tableView 设置的高度与自动布局计算的高度发生冲突。渲染单元格时，tableView 首先应用默认高度，计算自动布局高度，然后使用后者 - 至少看起来如此。见我的question。

这意味着您使用的约束是可以的（至少在我看来是这样）。只需将定义高度的约束之一设置为priority = 999（这样在停用默认高度约束之前不会导致任何冲突）。最终，无论如何都会导致使用你的约束，所以不会造成任何麻烦。

P.S.：注意 imageView 的高度限制：

ImageView.heightAnchor.constraint(equalTo: contentView.widthAnchor, multiplier: 0.6).isActive = true

如果您想省略图像（因此将其渲染为高度 = 0），那也会给您带来麻烦（同样，降低优先级可能会让您到达您想要的位置）。如果图像总是在那里，那就别管我的 P.S. :)。