The History

While we’ve offered an API to get the Destination of a DragItem, it has been returning Nil on macOS for quite some time. In fact, it likely hasn’t worked since our transition from Carbon to Cocoa way back in the day. Amid all the API changes that came with that transition, DragItem.Destination was unfortunately left behind.

A New Way to FolderItem Drag

To avoid breaking existing code, we chose not to overhaul our entire drag-and-drop system just to support this feature. Instead, we’ve made a few simple additions that are necessary to get FolderItem drags to return a valid DragItem.Destination

Here’s how a FolderItem drag would currently be setup (for example in the DesktopWindow.MouseDown event):

Var fileToCopy As FolderItem = SpecialFolder.Temporary.Child("Test")
Var item As New DragItem(Self, 0, 0, 100, 100)
item.FolderItem = fileToCopy
item.Drag

After dropping the item at its Destination, checking item.Destination would always return Nil:

If item.Destination = Nil Then
    MessageBox "Not a happy destination. 😔"
End If

By making one simple addition and using the RawData API, we can now initiate FolderItem drags in a way that correctly returns a valid FolderItem.Destination.

Var fileToCopy As FolderItem = SpecialFolder.Temporary.Child("Test")
Var item As New DragItem(Self, 0, 0, 100, 100)
item.FolderItem = fileToCopy
item.RawData("public.file-url") = ""
item.Drag

Now, when you check the Destination after the drag is complete, you’ll receive something much more meaningful:

If item.Destination <> Nil And item.Destination IsA FolderItem Then
    MessageBox(FolderItem(item.Destination).NativePath)
End If

An Added Benefit

While this change is a big improvement, why stop there? In the past, you could only drag FolderItems to the Destination, and the entire contents of that FolderItem would be copied over. With the addition of this new dragging method, you can now assign content to RawData("public.file-url"). Instead of copying the entire FolderItem to the Destination, we’ll copy the content from the RawData instead.

Here’s how you could set it up:

// The fileToCopy does not need to exist, we'll simply
// pull the file name from it and create the file at
// the destination with the new content that is assigned
// to the RawData.
Var fileToCopy As FolderItem = SpecialFolder.Temporary.Child("DummyFile.txt")
Var item As New DragItem(Self, 0, 0, 100, 100)
item.FolderItem = fileToCopy
item.RawData("public.file-url") = "Content that will be written to the destination file."
item.Drag

Conclusion

With these enhancements, dragging FolderItems on macOS has become more flexible and powerful than ever. Not only can you now easily retrieve a valid Destination, but you also have the power to control the content being copied, thanks to the integration with the RawData API.

So, go ahead—drag that FolderItem and show us your destination!

William Yu grew up in Canada learning to program BASIC on a Vic-20. He is Xojo’s resident Windows and Linux engineer, among his many other skills. Some may say he has joined the dark side here in the USA, but he will always be a Canadian at heart.

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