ios - 以编程方式将现有约束复制到新对象

Question

我允许用户在按下按钮时创建一个新的文本字段。我想以编程方式从现有文本字段复制前导和尾随约束。我的代码:

 @IBAction func addAnotherTextField(sender: AnyObject) {



    let newTextField = UITextField.init(frame: CGRectMake(20, positionY, self.view.frame.size.width-40, 30))
    newTextField.delegate = self
    newTextField.tag = fieldCount
    newTextField.placeholder = "You created this!"
    newTextField.borderStyle = UITextBorderStyle.RoundedRect

    newTextField.translatesAutoresizingMaskIntoConstraints = false

    let leadingConstraint = NSLayoutConstraint(item: newTextField, attribute: NSLayoutAttribute.Leading, relatedBy: NSLayoutRelation.Equal, toItem: nameTextField, attribute: NSLayoutAttribute.Leading, multiplier: 1, constant: 0)

    let trailingConstraint = NSLayoutConstraint(item: newTextField, attribute: NSLayoutAttribute.Trailing, relatedBy: NSLayoutRelation.Equal, toItem: nameTextField, attribute: NSLayoutAttribute.Leading, multiplier: 1, constant: 0)

    newTextField.addConstraint(leadingConstraint)
    newTextField.addConstraint(trailingConstraint)

    view.addSubview(newTextField)


    fieldCount++
    positionY = positionY + 15 + newTextField.frame.size.height



}

}

不幸的是，上面的代码在运行时崩溃了。

Answer 1

您需要首先添加SubView，然后才添加约束。 您无法在不相关的 UIView 之间连接约束...

来自文档:

Discussion: The constraint 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. Constraints that are added to a view are said to be held by that view. The coordinate system used when evaluating the constraint is the coordinate system of the view that holds the constraint.