拖放 - 从我的模型创建一个 NSItemProvider

Question

让我们分部分进行吧！

我正在尝试在我的UICollectionViewController 中实现Drag and Drop。

UICollectionView 的数据源是我创建的自定义 Model Struct 的 array。

根据需要我设置了我的collectionView.dragDelegate = self，并通过这样做我实现了required protocol functionitemsForBeginning session: UIDragSession...

这是我的问题开始的地方：

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]
}

我无法创建dragItem，因为我的模型不符合NSItemProviderWriting 类型。

如果我强制数据源为 String 类型并将项目转换为 NSString 它可以工作，但不适用于我的 struct Model。

有人知道如何解决这个问题吗？

Answer 1

您应该为您的Model 使用class（不是结构），因为正如您所建议的，您必须符合NSItemProviderWriting（继承自NSObjectProtocol）：

您在类上实现的协议，以允许项目提供者从该类的实例中检索数据。

许多 API 需要 NSObject 的子类，因此您必须使用一个类， Apple blog: struct vs class

所以你的Model 应该是这样的：

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

这是我实现的完整示例

1- 模型类/结构

2-在viewDidload中调用setup方法

3- 实现拖放collectionview协议

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]

    }
}