DragAndDropTableViewController.swift

// Demonstrating a bug with what the table view reports as the destination index path when dragging within
// the same section

import UIKit

class TableViewController: UITableViewController, UITableViewDragDelegate, UITableViewDropDelegate {
    var data: [(String, [String])] = [("One", ["A", "B", "C"]), ("Two", ["1", "2", "3"])]

    override func viewDidLoad() {
        super.viewDidLoad()
        tableView.register(UITableViewCell.self, forCellReuseIdentifier: "cell")
        tableView.dragInteractionEnabled = true
        tableView.dragDelegate = self
        tableView.dropDelegate = self
    }

    override func numberOfSections(in tableView: UITableView) -> Int {
        return data.count
    }

    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return data[section].1.count
    }

    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "cell")!
        cell.textLabel?.text = data[indexPath.section].1[indexPath.row]
        return cell
    }

    override func tableView(_ tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
        return data[section].0
    }

    // DRAG

    func tableView(_ tableView: UITableView, itemsForBeginning session: UIDragSession, at indexPath: IndexPath) -> [UIDragItem] {
        return [UIDragItem(itemProvider: NSItemProvider(object: DragItem(data[indexPath.section].1[indexPath.row])))]
    }

    // DROP

    func tableView(_ tableView: UITableView, dropSessionDidUpdate session: UIDropSession, withDestinationIndexPath destinationIndexPath: IndexPath?) -> UITableViewDropProposal {
        guard let indexPath = destinationIndexPath else {
            info("Forbidden")
            return UITableViewDropProposal(operation: .forbidden)
        }

        // When reordering within the same section, unexpected results are shown here.

        info("Insert at \(indexPath)?")
        return UITableViewDropProposal(operation: .move, intent: .insertAtDestinationIndexPath)
    }

    func tableView(_ tableView: UITableView, performDropWith coordinator: UITableViewDropCoordinator) {
        guard let indexPath = coordinator.destinationIndexPath else {
            info("Nowhere to drop")
            return
        }
        info("Dropping at \(indexPath)")
    }

    func info(_ string: String) {
        navigationItem.title = string
    }
}

class DragItem: NSObject, NSItemProviderWriting {
    static var writableTypeIdentifiersForItemProvider: [String] = ["tech.skagedal.test"]

    func loadData(withTypeIdentifier typeIdentifier: String, forItemProviderCompletionHandler completionHandler: @escaping (Data?, Error?) -> Void) -> Progress? {
        return nil
    }

    let string: String
    init(_ string: String) {
        self.string = string
    }
}