Open Radar

 
I am using UIStackView to layout UILabels in my UICollectionViewCell subclass. I'm using the iOS SDK 9.2

The scrolling of the collection view is smooth if I don't update the labels' `text` when I dequeue them. However, if I update their `text` as I dequeue them, the scrolling is very slow. 

I made a very small demo to show the problem, to be ran on a device (not the simulator). You can create a new empty project and replace the contents of `ViewController.swift` with this:

    import UIKit

    class ViewController: UIViewController {

        override func loadView() {
            view = UIView()

            let layout = UICollectionViewFlowLayout()
            layout.itemSize = CGSize(width: 100, height: 200)
            let collectionView = UICollectionView(frame: CGRectZero, collectionViewLayout: layout)
            collectionView.registerClass(Cell.self, forCellWithReuseIdentifier: "Cell")
            collectionView.translatesAutoresizingMaskIntoConstraints = false
            collectionView.dataSource = self
            view.addSubview(collectionView)

            let constraints = ["H:|-[collectionView]-|",
                "V:|[collectionView]|"
            ].flatMap { NSLayoutConstraint.constraintsWithVisualFormat($0, options: [], metrics: nil, views: ["collectionView": collectionView])
            }
            NSLayoutConstraint.activateConstraints(constraints)

        }
    }

    extension ViewController: UICollectionViewDataSource {
        func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
            let cell = collectionView.dequeueReusableCellWithReuseIdentifier("Cell", forIndexPath: indexPath) as! Cell

            //comment out the line below to make the scrolling smoother: 
            cell.fillLabels()

            return cell
        }
        func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
            return 100
        }
    }

    class Cell: UICollectionViewCell {

        var labelArray = [UILabel]()

        func fillLabels() {
            for label in labelArray {
                label.text = "\(label.text!) yo"
            }
        }

        override init(frame: CGRect) {
            super.init(frame: frame)

            contentView.backgroundColor = UIColor.whiteColor()

            let stackView = UIStackView()
            stackView.axis = .Horizontal
            stackView.alignment = .Leading
            stackView.distribution = .EqualSpacing
            stackView.translatesAutoresizingMaskIntoConstraints = false
            contentView.addSubview(stackView)

            let leftStack = UIStackView()
            leftStack.axis = .Vertical

            let rightStack = UIStackView()
            rightStack.axis = .Vertical

            stackView.addArrangedSubview(leftStack)
            stackView.addArrangedSubview(rightStack)

            for index in 0...10 {
                let leftLabel = UILabel()
                leftLabel.text = "\(index)"
                leftStack.addArrangedSubview(leftLabel)

                labelArray.append(leftLabel)

                let rightLabel = UILabel()
                rightLabel.text = "\(index)"
                rightStack.addArrangedSubview(rightLabel)

                labelArray.append(rightLabel)
            }


            let constraints = [
                "H:|[stackView]|",
                "V:|[stackView]|"
                ].flatMap {
                    NSLayoutConstraint.constraintsWithVisualFormat($0, options: [], metrics: nil, views: ["stackView": stackView])
            }
            
            NSLayoutConstraint.activateConstraints(constraints)

        }

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

You'll notice the scrolling is smooth when you comment out the call to `fillLabels`. 

If you try to reproduce the same layout without `UIStackViews` and include the call `fillLabels`, you'll notice the scrolling is smooth too.

This suggests `UIStackView` suffers performance bottlenecks if it has recalculate its layout.