Drag and Drop - create an NSItemProvider from my Model

Question

Let's go by parts!

I'm trying to implement Drag and Drop in my UICollectionViewController.

The datasource for the UICollectionView is an array of a custom Model Struct I've created.

As required I have set my collectionView.dragDelegate = self and by doing so I've implemented the required protocol function itemsForBeginning session: UIDragSession...

Here's where my problem starts:

struct Model {
    // some variables
    // Some initializations
}

var myModelDatasource: [Model] = [model1, model2, model3, ...] // it's a simple case example

func collectionView(_ collectionView: UICollectionView, itemsForBeginning session: UIDragSession, at indexPath: IndexPath) -> [UIDragItem] {
    let item = myModelDatasource[indexPath.row]

    let itemProvider = NSItemProvider(object: item)
    let dragItem = UIDragItem(itemProvider: itemProvider) // <-- ERROR HERE, Even If i force cast as NSItemProviderWriting
    dragItem.localObject = item

    return [dragItem]
}

I cannot create a dragItem because of my model doesn't conform to type NSItemProviderWriting.

If I force a datasource to be of type String and cast the item to NSString it works, but not with my struct Model.

Does anyone know how to resolve this issue?

Answer 1

You should use a class (not a struct) for your Model, because as you suggested you have to be conform to NSItemProviderWriting (which inherits from NSObjectProtocol):

The protocol you implement on a class to allow an item provider to retrieve data from an instance of the class.

Many APIs expect subclasses of NSObject, hence you have to use a class, Apple blog: struct vs class

So your Model should be something like:

class Model : NSObject, NSItemProviderWriting {
    public static var writableTypeIdentifiersForItemProvider: [String] {
        return [] // something here
    }

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

Answer 2

Here is complete example which I have implemented

1- model class/struct

2- call setup method in viewDidload

3- implement drag drop collectionview protocols

class ImageRequestModel {
    var uuid = UUID().uuidString
    var filename: String?
    var url: String? // displayable
    var caption:String?
    var image: UIImage? // its mean new or modifiable
  
}

 func setupCollectionView(){
        self.collectionView?.registerCell(id: PhotoCVC.className)
        self.collectionView?.collectionViewLayout = UICollectionViewLayout.createTwoColumnLayout()
        self.collectionView?.dataSource = self
        self.collectionView?.delegate = self
        self.collectionView?.dragInteractionEnabled = true
        self.collectionView?.dragDelegate = self
        self.collectionView?.dropDelegate = self
        self.setupRefreshControl()
    
    }


extension InspectionPhotosView: UICollectionViewDropDelegate {


    func collectionView(_ collectionView: UICollectionView, performDropWith coordinator: UICollectionViewDropCoordinator) {
        var destinationIndexPath: IndexPath
        if let indexpath = coordinator.destinationIndexPath {
            destinationIndexPath = indexpath
        } else {
            guard let row = self.collectionView?.numberOfItems(inSection: 0) else { return }
            destinationIndexPath = IndexPath(item: row - 1, section: 0)
        }
        if coordinator.proposal.operation == .move {
            Logger.debug(message: "\(destinationIndexPath.row)")
            self.reorderItems(coordinater: coordinator, destinationIndexPath: destinationIndexPath, collectionView: collectionView)
        }
    }


    func collectionView(_ collectionView: UICollectionView, dropSessionDidUpdate session: UIDropSession, withDestinationIndexPath destinationIndexPath: IndexPath?) -> UICollectionViewDropProposal {
//      if session.localDragSession != nil {
//        return UICollectionViewDropProposal(operation: .forbidden)
//      } else {
//        return UICollectionViewDropProposal(
//          operation: .copy,
//          intent: .insertAtDestinationIndexPath)
//      }

        if collectionView.hasActiveDrag {
            return UICollectionViewDropProposal(operation: .move, intent: .insertAtDestinationIndexPath)
        }
        return UICollectionViewDropProposal(operation: .forbidden)
    }

    fileprivate func reorderItems(coordinater: UICollectionViewDropCoordinator, destinationIndexPath: IndexPath, collectionView: UICollectionView) {
        if let item = coordinater.items.first, let sourceIndexPath = item.sourceIndexPath {
            collectionView.performBatchUpdates ({


                let object = imageList.remove(at: sourceIndexPath.item)
               // object.order = destinationIndexPath.row
                self.imageList.insert(object, at: destinationIndexPath.item)
                self.updateCollectionView(imageList)
                self.addPhotoView?.isImageSelected = true

                collectionView.deleteItems(at: [sourceIndexPath])
                collectionView.insertItems(at: [destinationIndexPath])


            }, completion: nil)

        }
    }
}



extension InspectionPhotosView: UICollectionViewDragDelegate {
    func collectionView(_ collectionView: UICollectionView, itemsForBeginning session: UIDragSession, at indexPath: IndexPath) -> [UIDragItem] {
        let item = self.imageList[indexPath.row]
        let itemProvider = NSItemProvider(object: item.uuid as NSString)
        let dragItem = UIDragItem(itemProvider: itemProvider)
           return [dragItem]

    }
}