Loading URL previews using LinkPresentation framework in Swift

iOS and macOS got a new framework in WWDC’19 named LinkPresentation. LinkPresentation enables fetching URL previews.

Adding LPLinkView for presenting preview

LPLinkView is a view subclass meant for rendering LPLinkMetadata. LPLinkMetadata contains information about the link: title, icon, image, video.

	import LinkPresentation
	let linkView = LPLinkView(metadata: LPLinkMetadata())
	linkView.translatesAutoresizingMaskIntoConstraints = false
	stackView.insertArrangedSubview(linkView, at: 0)

view raw ViewController.swift hosted with ❤ by GitHub

Fetching previews with LPMetadataProvider

Instances of LPLinkMetadata are fetched using LPLinkMetadataProvider for a given url. LPLinkMetadata is conforming to NSSecureCoding what enables a way of converting it to Data and storing the metadata on disk. Next time we need metadata for this url, we can use a locally cached data instead. Archiving and unarchiving is done with help of NSKeyedArchiver and NSKeyedUnarchiver and in the example archived data is stored in UserDefaults. In real apps it makes sense to store the data in separate files instead and not polluting UserDefaults with preview data.

	private let metadataStorage = MetadataStorage()
	private lazy var metadataProvider = LPMetadataProvider()
	private weak var linkView: LPLinkView?
	@IBAction func loadPreview(_ sender: UIButton) {
	if let text = textField.text, let url = URL(string: text) {
	// Avoid fetching LPLinkMetadata every time and archieve it disk
	if let metadata = metadataStorage.metadata(for: url) {
	linkView?.metadata = metadata
	return
	}
	metadataProvider.startFetchingMetadata(for: url) { [weak self] (metadata, error) in
	if let error = error {
	print(error)
	}
	else if let metadata = metadata {
	DispatchQueue.main.async { [weak self] in
	self?.metadataStorage.store(metadata)
	self?.linkView?.metadata = metadata
	}
	}
	}
	}
	}

view raw ViewController.swift hosted with ❤ by GitHub

	struct MetadataStorage {
	private let storage = UserDefaults.standard
	func store(_ metadata: LPLinkMetadata) {
	do {
	let data = try NSKeyedArchiver.archivedData(withRootObject: metadata, requiringSecureCoding: true)
	var metadatas = storage.dictionary(forKey: "Metadata") as? [String: Data] ?? [String: Data]()
	while metadatas.count > 10 {
	metadatas.removeValue(forKey: metadatas.randomElement()!.key)
	}
	metadatas[metadata.originalURL!.absoluteString] = data
	storage.set(metadatas, forKey: "Metadata")
	}
	catch {
	print("Failed storing metadata with error \(error as NSError)")
	}
	}
	func metadata(for url: URL) -> LPLinkMetadata? {
	guard let metadatas = storage.dictionary(forKey: "Metadata") as? [String: Data] else { return nil }
	guard let data = metadatas[url.absoluteString] else { return nil }
	do {
	return try NSKeyedUnarchiver.unarchivedObject(ofClass: LPLinkMetadata.self, from: data)
	}
	catch {
	print("Failed to unarchive metadata with error \(error as NSError)")
	return nil
	}
	}
	}

view raw MetadataStore.swift hosted with ❤ by GitHub

Summary

LinkPresentation framework adds a easy way of fetching previews for web pages. It provides a LPLinkView class making it extremely easy to render the preview.

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Example Project

LinkPresentationView (Xcode 11 GM)