swift - 在 SwiftUI AppKit 项目中，没有触发 DropDelegate 的 dropEntered

Question

来自https://github.com/onmyway133/blog/issues/594的代码，并进行一些更改。

很简单，创建一个 SwiftUI + Appkit项目，将下面的代码复制粘贴到项目中。

import SwiftUI

struct SelectFileView: View {
  let buttonTitle: String
  @State var isDrop: Bool = false
  
  var body: some View {
    VStack(alignment: .leading) {
      Button(action: {}) {
        Text(buttonTitle)
      }
      .offset(x: -16)
      Text("Alternatively, you can drag and drop file here")
        .font(.footnote)
        .foregroundColor(Color.gray)
    }
    .border(isDrop ? Color.orange : Color.clear)
    .onDrop(of: ["public.image"], delegate: self)
    .padding(32)
  }
}

extension SelectFileView: DropDelegate {
  func dropEntered(info: DropInfo) {
    print("dropEntered")
    self.isDrop = true
  }
  
  func dropExited(info: DropInfo) {
    print("dropExited")
    self.isDrop = false
  }
  
  func performDrop(info: DropInfo) -> Bool {
    guard
      let itemProvider = info.itemProviders(for: ["public.image"]).first
    else { return false }
    
    itemProvider.loadItem(forTypeIdentifier: "public.image", options: nil) { item, error in
      guard
        let data = item as? Data,
        let url = URL(dataRepresentation: data, relativeTo: nil)
      else { return }
      
    }
    
    return true
  }
}

struct ContentView: View {
  var body: some View {
    SelectFileView(buttonTitle: "Drop Test")
  }
}

在 Xcode 12.0 beta 2 上构建并运行后(12A6163b)，它总是 print dropExited，这就是问题所在。

dropEntered(info:)未触发，但dropExited(info:)有效