Avoid blocking the main thread in a NSDraggingSession using a NSPasteboardItemDataProvider

Question

In a Mac OS X app (Cocoa), I'm copying some images from my app to others using a NSDraggingSession. The NSDraggingItem makes use of an object that implements the protocol NSPasteboardItemDataProvider, to provide the data when the user drops it.

As I'm dealing with images, the types involved are: NSPasteboardTypePNG, kPasteboardTypeFileURLPromise, kUTTypeFileURL, com.adobe.photoshop-image and public.svg-image. These images are in a remote location, so before I can provide them to the pasteboard, I have to download them from the Internet.

I implement the method - pasteboard(pasteboard:item:provideDataForType:) doing something like this:

• If the type requested is kPasteboardTypeFileURLPromise, I get the paste location and build and set in the pasteboard the URL string with the location where the file is supposed to be written in the future.

• If the type requested is kUTTypeFileURL, I download the file, specify a temporal location and write the downloaded file to that location. Then, I set in the pasteboard the URL string of the location.

• If the type requested is one of the others, I download the file and set the plain NSData in the pasteboard.

All these operations are performed on the main thread, producing some lags that I want to get rid of.

I've tried to perform these operations on a background thread, and come back to the main thread to set the final data in the pasteboard, but this doesn't work because the method finishes before.

Does anyone know a way to achieve it?

Answer 1

Promises of pasteboard types are usually meant to be an alternative format of data that you already have, where you want to avoid the expense in time and memory of converting before it's necessary. I don't think it's really appropriate to use it to defer downloading any of the data, at all. For one thing, the download could fail when it's ultimately requested. For another, it could take an arbitrarily long time, as you're struggling with now.

So, I think you should download the data in advance. Either keep it in memory or save it to a temporary file. Use promised types, if appropriate, to deliver it in different forms, but have it on hand in advance.