在表格视图单元格中添加自动布局约束。

Question

I am trying to add a UIImageView to a cell and programmatically add auto layout constraints. However, it is giving me this following error: The view hierarchy is not prepared for the constraint... When added to a view, the constraint's items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled. The view hierarchy is not prepared for the constraint... When added to a view, the constraint's items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled. I looked at these following posts: Swift, Constraint, The view hierarchy is not prepared for the constraint , Why is my layout constraint returning an error in this case? (Swift) , and Swift, Constraint, The view hierarchy is not prepared for the constraint . One thing that I could not add to my code that these posts were suggesting me to add is setTranslatesAutoresizingMaskIntoConstraints . When I try to add this function to my imageView , I get an error.

Here is my code:

    func cellTwo(indexPath: NSIndexPath) -> UITableViewCell {

        let cell = tableView.dequeueReusableCellWithIdentifier("cellTwo", forIndexPath: indexPath) as! CathyTaskLogTwoTableViewCell

        let imageView:UIImageView = UIImageView(image: UIImage(named: "defaultPicture"))
        imageView.frame.size.width = 100
        imageView.frame.size.height = 31
                let horizonalContraints = NSLayoutConstraint(item: imageView, attribute:
            .LeadingMargin, relatedBy: .Equal, toItem: cell,
            attribute: .LeadingMargin, multiplier: 1.0,
            constant: 20)
        imageView.addConstraint(horizonalContraints)

        cell.addSubview(imageView)

        return cell

    }

Thank you so much in advance for your help :)

Answer 1

1. The sequence is:

   • instantiate the subview (eg the image view);
   • set translatesAutoresizingMaskIntoConstraints to false ;
   • call addSubview to add it to the view hierarchy; and
   • add the constraints.

2. Do not set the frame at all if you're using constraints. Everything should be defined by constraints.

3. It is probably not prudent to call dequeueReusableCellWithIdentifier and then add a subview. What if the cell is being reused? You'll be adding the same subview multiple times. The programmatic adding of subview is probably best put in the cell subclass implementation of awakeFromNib , if from storyboard or NIB, or init(style;, reuseIdentifier) if building it programmatically. Or, easiest, don't programmatically create cells at all and use storyboard or NIB.

Answer 2

Before adding constraints to any view programmatically you should add it as subview and turn translatesAutoresizingMaskIntoConstraints to false and then add your required constraints. Like

 cell.addSubview(imageView)
 imageView.translatesAutoresizingMaskIntoConstraints = false

 //now add your required constraints