UICollectionViewFlowLayout and auto layout on iOS

UICollectionViewFlowLayout is layout object supplied by UIKit and enables showing items in grid. It allows customising spacings and supports fixed size cells and cells with different sizes. This time I am going to show how to build a collection view containing items with different sizes where sizes are defined by auto layout constraints.

Cell sizes in UICollectionViewFlowLayout

By default UICollectionViewFlowLayout uses itemSize property for defining the sizes of the cells. Another way is to use UICollectionViewDelegateFlowLayout and supplying item sizes by implementing collectionView(_:layout:sizeForItemAt:). Third way is to let auto layout for defining the size of the cell. This is what I am going to build this time.

Enabling auto layout based cell sizes

When setting UICollectionViewFlowLayout’s estimatedItemSize to UICollectionViewFlowLayout.automaticSize enables layout object to use auto layout.

	final class CollectionViewController: UICollectionViewController {
	let content: [String]
	init(content: [String]) {
	self.content = content
	let layout = UICollectionViewFlowLayout()
	layout.estimatedItemSize = UICollectionViewFlowLayout.automaticSize
	super.init(collectionViewLayout: layout)
	}
	}

view raw CollectionViewController.swift hosted with ❤ by GitHub

Setting up collection view cell with auto-layout constraints

When layout object is configured, second step is to create a cell. In the current prototype it is going to be a simple cell with a label and border around the label. Constraints are set up to have a 8 points space around the label. Constraints together with label’s intrinsicContentSize define the minimum size for the cell. If text is longer, intrinsicContentSize is wider.

	final class TextCollectionViewCell: UICollectionViewCell {
	let textLabel: UILabel
	override init(frame: CGRect) {
	textLabel = {
	let label = UILabel(frame: .zero)
	label.adjustsFontForContentSizeCategory = true
	label.font = UIFont.preferredFont(forTextStyle: .body)
	label.translatesAutoresizingMaskIntoConstraints = false
	return label
	}()
	super.init(frame: frame)
	contentView.addSubview(textLabel)
	NSLayoutConstraint.activate([
	textLabel.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 8),
	textLabel.trailingAnchor.constraint(equalTo: trailingAnchor, constant: -8),
	textLabel.topAnchor.constraint(equalTo: topAnchor, constant: 8),
	textLabel.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -8)
	])
	contentView.layer.borderColor = UIColor.darkGray.cgColor
	contentView.layer.borderWidth = 1
	contentView.layer.cornerRadius = 4
	}
	required init?(coder aDecoder: NSCoder) {
	fatalError("init(coder:) has not been implemented")
	}
	}

view raw TextCollectionViewCell.swift hosted with ❤ by GitHub

When putting all the things together, the end result is a collection view where every cell has its own size. Moreover, it supports dynamic type and cells will grow if user changes default text sizes.

Summary

UICollectionViewFlowLayout’s estimatedItemSize enables using auto-layout for defining cell sizes. Therefore creating cells where text defines the size of the cell is simple to do on iOS.

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Example project

FittingCellsInCollectionViewFlowLayout Xcode 10.2, Swift 5.0

Resources

• UICollectionViewFlowLayout