Ultimate Catalyst Guide

How do I implement drag and drop?

released Fri, 05 Jul 2019
Swift Version 5.1

Drag and drop is implemented via the same mechanisms that also support drag and drop on iOS. Below, you can see an example of a simple UIViewContoller that allows droping JSON files onto the application in order to import them.

/// This is the identifier of a JSON File
private let JSONTypeIdentifier = "public.json"

extension MyDragAndDropSupportingViewController: UIDropInteractionDelegate {
    func dropInteraction(_ interaction: UIDropInteraction,
                         canHandle session: UIDropSession) -> Bool {
    // We tell the drag and drop system that we support JSON
        return session.hasItemsConforming(toTypeIdentifiers: [JSONTypeIdentifier])

    func dropInteraction(_ interaction: UIDropInteraction, sessionDidUpdate session: UIDropSession) -> UIDropProposal {
        // If a drag comes in, we copy the file. We don't want to consume it.
        return UIDropProposal(operation: .copy)

    func dropInteraction(_ interaction: UIDropInteraction, performDrop session: UIDropSession) {
        // This is called with an array of NSURL
    session.loadObjects(ofClass: URL.self) { urls in
            for url in urls {
                importJSONData(from: url)

As with other Catalyst technologies, there is a lot of good Apple documentation for this as this is the same as on iOS.