Scenario: The following Swift code uses NSItemProviderWriting to load a file from some data source async (The code actually just uses a for loop to simulate the time it needs to load the file). It returns the file url then. This code is used for drag & drop behavior in a macOS app from an app to another app. As the other app does only support file URLs as drop types, I can't send over the data itself.
class AnyFile: NSObject, NSItemProviderWriting {
let url: URL
init(url: URL) {
self.url = url
}
static var writableTypeIdentifiersForItemProvider: [String] {
return [UTType.fileURL.identifier]
}
func loadData(withTypeIdentifier typeIdentifier: String, forItemProviderCompletionHandler completionHandler: @escaping @Sendable (Data?, Error?) -> Void) -> Progress? {
let count = 100000
let progress = Progress(totalUnitCount: Int64(count))
print("a")
Task {
print("b")
for i in 0...100000 {
progress.completedUnitCount = Int64(i)
print(i)
}
print("c")
completionHandler(url.dataRepresentation, nil)
print("d")
}
print("e")
return progress
}
}
Problem: Drag & drop works, but when I start to drag the view (SwiftUI view using .onDrag), the view gets stuck as long as the loop is running. It is not clear to me why this happens as the loop us running inside a task. The console output also shows that progress is returned before the loop as finished. But still, it is stuck until the end of the loop. So the user experience is like not using async and the task.
Log output (shortened)
a
e
b
0
...
100000
c
d
Actual Question: Why is this Swift code for dragging files not behaving asynchronously even though it uses Task, completionHandler, and async/await constructs?
Alternatives tried:
I'm not bound to NSItemProviderWriting in particular. I have tried other ways, like Transferable, .draggable(), and NSItemProvider.registerFileRepresentation. But either they won't return a URL, or do not allow async at all. I also tried AppKit instead of SwiftUI with similar issues.
