如何以编程方式将约束从标签添加到 UICollectionViewCell？

Question

I have a question regarding constraints to a UICollectionViewCell.

An image is included to help visualize what I mean.

I have a UICollectionViewController. Inside this controller I have added a UICollectionViewCell, which only takes up a part of the screen.

My question is: How do I add constraints from a label (see image) to that UICollectionViewCell, such that I know that the label is always the same distance from the UICollectionViewCell. In the image the distance is shown as an arrow (this is the constraint, that I want to add).

Hope someone can help me out.

Thanks in advance.

Answer 1

Assuming the label is a subview from the CollectionViewController , it can be done like:

label.topAnchor.constraint(equalTo: cell.bottomAnchor, constant: someConstant).isActive = true

The above code works for me at least with an UITableViewController and its cells, so it should also work with a UICollectionView .

Answer 2

First at all you should know what constraints you have to set. In this case I guess it is top, trailing and leading if you want an infinite label. Else you have to set height constraint too.

Example of code for adding constraints for label in collectionViewCell:

    label = UILabel()
    self.addSubview(label)

    label.translatesAutoresizingMaskIntoConstraints = false

    // creating a top constraint related to cell content view top with constant 16
    let topConstraint = NSLayoutConstraint(item: label, attribute: .top, relatedBy: NSLayoutConstraint.Relation.equal, toItem: self, attribute: .top, multiplier: 1, constant: 16)
    let leadingConstraint = NSLayoutConstraint(item: label, attribute: .leading, relatedBy: NSLayoutConstraint.Relation.equal, toItem: self, attribute: .leading, multiplier: 1, constant: 16)
    let trailingConstraint = NSLayoutConstraint(item: label, attribute: .trailing, relatedBy: NSLayoutConstraint.Relation.equal, toItem: self, attribute: .trailing, multiplier: 1, constant: 16)

    // optional constraint
    //let heightConstaint = NSLayoutConstraint(item: label, attribute: .height, relatedBy: NSLayoutConstraint.Relation.equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 44)

    self.addConstraints([topConstraint, leadingConstraint, trailingConstraint])

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If I understood your question right, you want to have similar relations between your collectionView and label.

So you should write the same code as I wrote above but relate your label constraints to collectionView.