Drag and drop mail into OS X app - objective-c

Question

I would like to be able to drag and drop and email from Outlook for Mac and from Mail.app into an OS X app. If I drag and drop and email from my finder (drag a file), then the following is called:
- (NSDragOperation)draggingEntered:(id <NSDraggingInfo>)sender
However, if I drag and drop from Outlook for Mac or from Mail.app, the method isn't called. I'm a bit lost on how can I achieve this. Any ideas?

Answer 1

From the Dragging Destinations portion of Drag and Drop Programming Topics:
To receive drag operations, you must register the pasteboard types that your window or view will accept by sending the object a registerForDraggedTypes: message, defined in both NSWindow and NSView, and implement several methods from the NSDraggingDestination protocol. During a dragging session, a candidate destination receives NSDraggingDestination messages only if the destination is registered for a pasteboard type that matches the type of the pasteboard data being dragged. The destination receives these messages as an image enters, moves around inside, and then exits or is released within the destination’s boundaries.
In order to accept drags from Mail you'll need to know what pasteboard types to register for. You can use ClipboardViewer to discover what types of data Mail places on the dragging pasteboard (available in the Auxiliary Tools package for recent versions of Xcode). Launch ClipboardViewer and select Drag Clipboard from the combo box in the toolbar. Switch back to Mail and drag a message briefly, then return to ClipboardViewer. You should see a number of pasteboard types listed in the sidebar. Of particular interest will be the public.url and com.apple.pasteboard.promised-file-content-type types. The former indicates that a URL is on the pasteboard. The latter that a file promise is on the pasteboard. URLs tend to be a good place to start, but in this particular case we can see that the URL isn't something useful like a file URL, it's a rather opaque message URL. That means we need to deal with the file promise instead, and so when configuring our view to receive drags we should call registerForDraggedTypes: with NSFilesPromisePboardType.
The second part of the Dragging File Promises documentation outlines specifically how to deal with receiving promises. To summarize, you call -namesOfPromisedFilesDroppedAtDestination: on the sender of the drag from within performDragOperation: to have them write the dragged data to a location of your choosing (e.g., fulfill the promise). The originator of the drag will write the data to disk before AppKit invokes concludeDragOperation: on your object. At any point from concludeDragOperation: forwards you can load the dropped files from disk and process them as you wish.

Answer 2

I thought I would post my answer to this problem as I struggled with it for a while. This code handles a promise and simply copies the dropped mail into a folder called Drop Stuff in your user folder. It also works for any file too and seems to work for other apps including address book and reminders etc. It doesn't work for copying multiple files (or mail messages) yet.
import Cocoa
class DropArea: NSImageView, NSDraggingDestination
{
override func drawRect(dirtyRect: NSRect)
{
super.drawRect(dirtyRect)
}
required init?(coder: NSCoder)
{
let types = [NSFilenamesPboardType, NSURLPboardType, NSPasteboardTypeTIFF, NSFilesPromisePboardType]
super.init(coder: coder)
registerForDraggedTypes(types)
}
override func draggingEntered(sender: NSDraggingInfo) -> NSDragOperation
{
return .Copy
}
override func performDragOperation(sender: NSDraggingInfo) -> Bool
{
var error: NSError?
var folderPath = NSHomeDirectory()+"/Drop Stuff/"
if (!NSFileManager.defaultManager().fileExistsAtPath(folderPath))
{
NSFileManager.defaultManager().createDirectoryAtPath(folderPath, withIntermediateDirectories: true, attributes: nil, error: &error)
}
var folderURL = NSURL(fileURLWithPath: folderPath)
var f = sender.namesOfPromisedFilesDroppedAtDestination(folderURL!)
println("Copied to \(folderPath)")
return true
}
}
Any suggestions to improve this code are of course welcome :-)