如何正确编写约束代码

Question

我想在 UITableViewCell 中设置一个 UITextView。在 TableView 函数的一些方法中，我写了下面的代码，但是它会导致以下错误：

Terminating app due to uncaught exception 'NSGenericException', reason: 'Unable to activate constraint with anchors and because they have no common ancestor. Does the constraint or its anchors reference items in different view hierarchies? That's illegal.'

这是我在 tableView 中的代码：

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = newTableView.dequeueReusableCell(withIdentifier: "NewCell", for: indexPath) as! NewCellTableViewCell
    let tableTextView = UITextView()
    tableTextView.translatesAutoresizingMaskIntoConstraints = false
    tableTextView.leadingAnchor.constraint(equalTo: cell.leadingAnchor).isActive = true
    tableTextView.topAnchor.constraint(equalTo: cell.topAnchor).isActive = true
    tableTextView.widthAnchor.constraint(equalTo: cell.widthAnchor).isActive = true
    tableTextView.heightAnchor.constraint(equalTo: cell.heightAnchor).isActive = true
    cell.addSubview(tableTextView)
    return cell
}

Answer 1

首先你应该打电话给：

cell.addSubview(tableTextView)

然后附加约束条件，此外：

All constraints must involve only views that are within scope of the receiving view. Specifically, any views involved must be either the receiving view itself, or a subview of the receiving view.

所以您的代码可能是：

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = newTableView.dequeueReusableCell(withIdentifier: "NewCell", for: indexPath) as! NewCellTableViewCell
    let tableTextView = UITextView()
    cell.addSubView(tableTextView)
    tableTextView.translatesAutoresizingMaskIntoConstraints = false

    tableTextView.leadingAnchor.constraint(equalTo: cell.leadingAnchor).isActive = true
    tableTextView.topAnchor.constraint(equalTo: cell.topAnchor).isActive = true
    tableTextView.widthAnchor.constraint(equalTo: cell.widthAnchor).isActive = true
    tableTextView.heightAnchor.constraint(equalTo: cell.heightAnchor).isActive = true
    return cell
}

Answer 2

必须在应用约束之前将子视图添加到父视图：

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = newTableView.dequeueReusableCell(withIdentifier: "NewCell", for: indexPath) as! NewCellTableViewCell
    let tableTextView = UITextView()
    tableTextView.translatesAutoresizingMaskIntoConstraints = false
    cell.addSubView(tableTextView)
    tableTextView.leadingAnchor.constraint(equalTo: cell.leadingAnchor).isActive = true
    tableTextView.topAnchor.constraint(equalTo: cell.topAnchor).isActive = true
    tableTextView.widthAnchor.constraint(equalTo: cell.widthAnchor).isActive = true
    tableTextView.heightAnchor.constraint(equalTo: cell.heightAnchor).isActive = true

    return cell
}

如何正确编写约束代码

How to properly write the constraint code

xcode

swift

swift4

xcode9