拖放异步数据获取

Question

我正在尝试在共享图像的应用程序中实现拖放功能。

我所有的图像都是高性能缩略图（即小尺寸），所以我不能将它们用作我的 UIDragItem，至少不能用作最终图像。

我正在寻找的是一种为我的原始图像提供 URL 并将其作为 UIDragItem 发送，然后让目的地异步获取图像的方法。当图像存储在 iCloud 中时，这是在照片应用程序中完成的，所以它一定是可能的，我似乎无法弄清楚如何。

Answer 1

原来解决方案非常简单，在本次 WWDC 的第 227 节 Data Delivery with Drag and Drop 中进行了描述。

你基本上让你想要拖动的任何对象符合 NSItemProviderWriting，然后实现两件事。

The interface for supporting initialization of an item provider based on an object, used by a source app when providing copied or dragged items.

第一步

实施writableTypeIdentifiersForItemProvider，这将使您的接收者了解您提供的对象类型。这是一组保真度递减的类型标识符（他们在视频中描述得很好）

第二步

实施 loadData(withTypeIdentifier typeIdentifier: String, forItemProviderCompletionHandler completionHandler: @escaping (Data?, Error?) -> Void) -> Progress? 来完成繁重的工作，这将在接收方尝试加载您提供的对象时被调用。

例子

您可以忽略下面的数据获取细节（我使用的是 firebase），但使用本机 URLSession API 的工作方式几乎相同。

extension Media: NSItemProviderWriting {
  //Provide the types you want you are supplying
  static var writableTypeIdentifiersForItemProvider: [String]  {
    return [(kUTTypeImage as String)]
  }


  func loadData(withTypeIdentifier typeIdentifier: String, forItemProviderCompletionHandler completionHandler: @escaping (Data?, Error?) -> Void) -> Progress? {
    print("Item provider would like to write item from path: \(metadata.path!)")
    guard let path = metadata.path else { return nil }
    //Allow a maximum of ~30mb to be downloaded into memory if images, 1GB if video.
    let maxSize:Int64 = (isVideo ? 1000 : 30) * 1024 * 1024

    let storage = Storage.storage().reference(withPath: path)
    let progress = Progress(totalUnitCount: 100)
    var shouldContinue = true
    //When the receiver cancels this block is called where we will set the `shouldContinue` to false to cancel the current task
    progress.cancellationHandler = {
      shouldContinue = false
    }
    let task = storage.getData(maxSize: maxSize) { data, error in
      //Once the data is fetched or we encounter an error, call the completion handler
      completionHandler(data, error)
    }

    if !shouldContinue {
      task.cancel()
    }

    task.observe(.progress) { snapshot in
      if let p = snapshot.progress {
        progress.completedUnitCount = Int64(p.fractionCompleted * 100)
      }
    }
    task.observe(.success) { snapshot in
      print(snapshot)
    }
    task.observe(.failure) { snapshot in
      print(snapshot)
    }
    return progress
  }
}

然后在我们的 DragDelegate 中：

@available(iOS 11, *)
extension GridViewDelegateDataSource: UICollectionViewDragDelegate {
  func collectionView(_ collectionView: UICollectionView, itemsForBeginning session: UIDragSession, at indexPath: IndexPath) -> [UIDragItem] {

    let mediaItem = media[indexPath.item]
    //You can now instantiate an NSItemProvider directly from your object because it conforms to the `NSItemProviderWriting` protocol
    let itemProvider = NSItemProvider(object: mediaItem)
    let dragItem = UIDragItem(itemProvider: itemProvider)
    return [dragItem]
  }
}

Answer 2

此代码用于拖动 PHAsset

extension PHAsset : NSItemProviderWriting {
    public static var writableTypeIdentifiersForItemProvider: [String] {
        return UIImage.writableTypeIdentifiersForItemProvider
    }
    public func loadData(withTypeIdentifier typeIdentifier: String,
                         forItemProviderCompletionHandler completionHandler: @escaping (Data?, Error?) -> Void) -> Progress? {
        PHImageManager.default().requestImageData(for: self, options: nil) { (data, _, _, _) in
            completionHandler(data, nil)
        }
        return nil
    }
}

使用：

let item = UIDragItem(itemProvider: NSItemProvider.init(object: yourasset))

拖放异步数据获取

Drag and Drop asynchronous data fetching

drag-and-drop

ios

ios11

uidragitem

第一步

第二步

例子