ios - 未计算 UITableViewCell 中的约束

Question

我有以下单元格以编程方式设置约束:

class RadioButtonCell: UITableViewCell {

static let identifier = "RadioButtonCell"

let radioButton = RadioButton()
let labelTitle = UILabel()

private var didUpdateConstraints = false

override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
    super.init(style: style, reuseIdentifier: reuseIdentifier)
    setupSubViews()
}

required init?(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
}

private func setupSubViews() {
    radioButton.translatesAutoresizingMaskIntoConstraints = false
    labelTitle.translatesAutoresizingMaskIntoConstraints = false
    contentView.addSubview(radioButton)
    contentView.addSubview(labelTitle)
}

override func updateConstraints() {
    super.updateConstraints()
    if !didUpdateConstraints {
        radioButton.anchor(leading: contentView.leadingAnchor, padding: UIEdgeInsets(top: 0, left: Constants.UI.defaultMarginX2, bottom: 0, right: 0))
        radioButton.centerYAnchor.constraint(equalTo: contentView.centerYAnchor).isActive = true
        labelTitle.anchor(leading: radioButton.trailingAnchor, padding: UIEdgeInsets(top: 0, left: Constants.UI.defaultMarginX2, bottom: 0, right: 0))
        labelTitle.centerYAnchor.constraint(equalTo: contentView.centerYAnchor).isActive = true
        didUpdateConstraints = true
    }
  }
}

anchor 方法只是一个添加约束的辅助方法。约束设置正确(Autolayout 没有问题)。

然后在 cellForRowAtIndexPath 方法中，我像这样创建单元格:

guard let cell = tableView.dequeueReusableCell(withIdentifier: RadioButtonCell.identifier, for: indexPath) as? RadioButtonCell else { return UITableViewCell() }
radtioButtonController.addButton(cell.radioButton)
cell.labelTitle.text = "test"
return cell

这在 tableView 中创建了以下布局(这显然是错误的):

如果我将约束的设置移动到 setupSubViews() 方法，则布局是正确的:

class RadioButtonCell: UITableViewCell {

static let identifier = "RadioButtonCell"

let radioButton = RadioButton()
let labelTitle = UILabel()

private var didUpdateConstraints = false

override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
    super.init(style: style, reuseIdentifier: reuseIdentifier)
    setupSubViews()
}

required init?(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
}

private func setupSubViews() {
    radioButton.translatesAutoresizingMaskIntoConstraints = false
    labelTitle.translatesAutoresizingMaskIntoConstraints = false
    contentView.addSubview(radioButton)
    contentView.addSubview(labelTitle)
    radioButton.anchor(leading: contentView.leadingAnchor, padding: UIEdgeInsets(top: 0, left: Constants.UI.defaultMarginX2, bottom: 0, right: 0))
    radioButton.centerYAnchor.constraint(equalTo: contentView.centerYAnchor).isActive = true
    labelTitle.anchor(leading: radioButton.trailingAnchor, padding: UIEdgeInsets(top: 0, left: Constants.UI.defaultMarginX2, bottom: 0, right: 0))
    labelTitle.centerYAnchor.constraint(equalTo: contentView.centerYAnchor).isActive = true
 }
}

为什么会这样？我认为我们应该在 updateConstraints 方法中设置约束...

谢谢你的回答:)

编辑 我发现，当我调用 cell.updateConstraintsIfNeeded() 或 cell.setNeedsUpdateConstraints( ) 在 cellForRowAtIndexPath 中。为什么我们需要告诉单元格重新计算约束条件？当使用 IB 添加约束时，我们不需要这样做......

Answer 1

好吧，在您的第一个示例中，您不仅在 init 中将约束添加到您的 UI 元素。您只需设置您的 subview 。

override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
    super.init(style: style, reuseIdentifier: reuseIdentifier)
    setupSubViews()
    // No explicit constraint setup happened
}

为了设置您的约束，您已经覆盖了 override func updateConstraints() { ... } 方法。我们来看看official documentation from Apple .

In short, your override will be in effect when you notify the system that you need constraints' update. As a result you need to explicitly inform the system by invoking setNeedsUpdateConstraints() or updateConstraintsIfNeeded().

---

---

让我们看看你的第二个例子。您在 private func setupSubViews(){ ... } 中嵌入了约束设置。因此，在调用此函数时，您的约束已准备好应用。无需系统调用。