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I'm trying to implement a simple drag and drop of a String from ViewA to ViewB, on a macOS SwiftUI app, but my NSItemProvider.loadObject(ofClass:completionHandler:) always fails with the error "Could not coerce an item to class NSString".

ViewA(element: myElement)
    .onDrag { NSItemProvider(object: myElement.title as NSString) }

ViewB()
    .onDrop(of: [.plainText], isTargeted: nil, perform: { providers, location in
        _ = providers.first?.loadObject(ofClass: NSString.self, completionHandler: { text, error in
            print("Dropped \(text?.description ?? "?") with error: \(error?.localizedDescription ?? "")")
        })
        return true
    })

Some things to notice:

  • The same code works as expected on iOS/iPadOS.
  • If instead I try to drag and drop a URL by changing myElement.title to URL, and changing the specific item providers to handle the URL, the code also works as expected.
  • I found a Reddit post from 6 months ago with the exact same problem.

I have created a simple project with the issue: https://github.com/marcosatanaka/drag-and-drop-string

How can I load a string from NSItemProvider, so I can drag and drop it from one view to the other, on macOS? Is this a bug or am I doing something wrong?

Marcos Tanaka
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1 Answers1

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I think you can fill a bug on this, after some debugging I thinks the problem come from the creation of the provider item.

On iOS we receive this provider :

<UIItemProvider: 0x600000198000> {types = (
    "public.utf8-plain-text"
)}

When on macOS we receive this provider:

<NSItemProvider: 0x600000374540> {types = (
)}

As you can see the item providers is empty on types, that why you can't load it. Without the type the system can't load it.

Hikosei
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  • Thanks a lot for the details. Indeed, it appears to be a macOS bug. I'm going to open a feedback about it. – Marcos Tanaka Nov 17 '21 at 13:38
  • Hello @MarcosTanaka , I thinks a find a duplicate of you question , I tried and it worked : https://stackoverflow.com/a/69325742/2873694 please give it a try – Hikosei Nov 19 '21 at 19:12