拖放异步数据获取

我正在尝试将Drag and Drop实现到我共享图像的应用程序中.

我的所有图像都是高性能缩略图(即小尺寸),因此我不能将它们用作我UIDragItem的图像,至少不能用作最终图像.

我正在寻找的是一种方法,为我的原始图像提供URL并将其发送为UIDragItem,然后使目标异步获取图像.当图像存储在iCloud中时,这在照片应用程序中完成,所以它必须以某种方式可能,我似乎无法弄清楚如何.

事实证明,该解决方案非常简单,并且在此WWDC期间的会话227 数据传输与拖放中进行了描述.

您基本上将要拖动的任何对象符合NSItemProviderWriting,然后实现两件事.

NSItemProviderWriting:

用于支持基于对象初始化项目提供程序的接口,在提供复制或拖动项目时由源应用程序使用.

第一步

实施writableTypeIdentifiersForItemProvider,让您的接收器知道您提供什么类型的对象.这是一个具有降低保真度的类型标识符数组(他们在视频中很好地描述了这一点)

第二步

执行loadData(withTypeIdentifier typeIdentifier: String, forItemProviderCompletionHandler completionHandler: @escaping (Data?, Error?) -> Void) -> Progress?繁重的工作,当接收器尝试加载您提供的对象时,将调用它.

例

您可以忽略下面提取的数据的具体细节(我正在使用firebase),但使用本机URLSession API将以相同的方式工作.

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]
  }
}