Export using Drag & Drop to Finder?

I haven't done this, but that should be possible.

Look here how to handle custom types, and adapt to your Project Document Type.

h ttps://www.raywenderlich.com/1016-drag-and-drop-tutorial-for-macos

This should be possible. You will most likely want to do this through the pasteboard by creating a custom pasteboard type. In one of my apps, I have a custom "Blob" that is a dictionary containing various objects. I have that "Blob" as a custom readable and writable pasteboard type that allows me to copy and paste it from from one document to another document, save it on a stack for later, etc.

Once you figure out how to get your Blob on the pasteboard when your app is not running, it should be downhill thereafter. Register a service?

You can put a file promise on the pasteboard. If/when it is accepted, your code will be called to create the file at a particular URL.

To create the file promise, instantiate and configure an NSFilePromiseProvider object. You'll need to specify a delegate object and implement the methods of NSFilePromiseProviderDelegate. Then, write the file promise to the pasteboard with the writeObjects method.

There's sample code here.

> Once you figure out how to get your Blob on the pasteboard when your app is not running, it should be downhill thereafter.

My app will always be running, as they'll be dragging a UI element to the Finder.

Thanks Ken, I'll give that a look.