Dragging source code for promising files from my never-released movie player app.

PRHFrameGrabber+DraggingSource.m

//PRHFrameGrabber grabs frame images from the movie.
//It wraps a couple of AVAssetImageGenerators, one of which is allowed to round to a more convenient time (e.g., a keyframe).
//Thus, the temporally lax generator will return an image first, while we wait for the temporally strict generator, which may take a significant fraction of a second.
//PRHFrameGrabber is also an NSPasteboardWriter. It's what the movie view feeds to the dragging session when the user tries to drag a frame; the frame grabber fulfills its NSPasteboardWriter duties by returning the grabbed frame.

@interface PRHFrameGrabber ()
@property(readwrite, copy) NSImage *image;
@end

@implementation PRHFrameGrabber
{
	AVAssetImageGenerator *_temporallyStrictImageGenerator;
	AVAssetImageGenerator *_temporallyLaxImageGenerator;
}

[boring constructor code]

- (NSArray *) writableTypesForPasteboard:(NSPasteboard *)pasteboard {
	NSImage *image = self.image ?: self.roughImage;
	NSArray *writableTypes = [image writableTypesForPasteboard:pasteboard];

	writableTypes = [writableTypes arrayByAddingObjectsFromArray:@[ /* (__bridge NSString *)kPasteboardTypeFileURLPromise,*/
		(__bridge NSString *)kPasteboardTypeFilePromiseContent ]];

	return writableTypes;
}

- (NSPasteboardWritingOptions) writingOptionsForType:(NSString *)type pasteboard:(NSPasteboard *)pasteboard {
	NSPasteboardWritingOptions options = UTTypeEqual(kPasteboardTypeFilePromiseContent, (__bridge CFStringRef)type) ? 0 : NSPasteboardWritingPromised;

	return options;
}

- (id) pasteboardPropertyListForType:(NSString *)type {
	if (UTTypeEqual(kPasteboardTypeFilePromiseContent, (__bridge CFStringRef)type)) {
		return (NSString *)kUTTypeJPEG;
	}

	NSImage *image = self.image ?: self.imageFromTemporallyLaxImageGenerator;
	NSData *data = [image pasteboardPropertyListForType:type];

	if (data == nil) {
		//Really, NSImage?
		NSRect rect = { NSZeroPoint, image.size };
		CGImageRef imageCG = [image CGImageForProposedRect:&rect
			context:nil
			hints:nil];
		CFMutableDataRef mutableData = CFDataCreateMutable(kCFAllocatorDefault, /*capacity*/ 0);
		if (mutableData != NULL) {
			CGImageDestinationRef dest = CGImageDestinationCreateWithData(mutableData, (__bridge CFStringRef)type, /*count*/ 1, /*options*/ NULL);
			if (dest != NULL) {
				CGImageDestinationAddImage(dest, imageCG, /*properties*/ NULL);
				if ( ! CGImageDestinationFinalize(dest) ) {
					CFRelease(mutableData);
					mutableData = NULL;
				}
				CFRelease(dest);
			}
			data = (__bridge_transfer NSData *)mutableData;
		}
	}

	return data;
}

@end

PRHMovieView+DraggingSource.m

//This is one of the main classes in a movie player app I wrote and nearly finished before I joined Apple.
//It's a direct subclass of NSView—not QTMovieView, and AVKit hadn't been released yet when I wrote this.
//It's pretty huge, but here's the drag-and-drop code.

@interface PRHMovieView () <NSDraggingSource>

@property(nonatomic) bool drawDropTargetHighlight;

@end

@implementation PRHMovieView
{
	bool _clickBeganInThisView;
	bool _dragBeganInThisView;
	NSEvent *_mouseDownEvent;
	NSDraggingSession *_draggingSession;
	CMTime _draggingFrameTime;
	NSURL *_promisedFrameImageFileURL;
	PRHFrameGrabber *_draggingFrameGrabber;
	NSTimer *_cursorHidingTimer;
	bool _mouseIsWithin;
	//PRHTimeFormatter formats movie timestamps. In one configuration, it does so appropriately for filenames.
	PRHTimeFormatter *_timeFormatterForDraggedFrameImageFilenames;
}

[lots of boring and/or irrelevant and/or ugly code—seriously, this is a *huge* class]

//This view supports both clicks and drags. The click logic isn't relevant here, but suffice to say that we start drag(ging session)s from mouseDragged:.
- (void) mouseDown:(NSEvent *)event {
	_mouseDownEvent = event;
	_clickBeganInThisView = true;
}

- (void) mouseDragged:(NSEvent *)event {
	//Ignore mouseDragged:s that come from the user beginning a drag in the controller and taking it through here.
	if (!_clickBeganInThisView)
		return;

	if (!_dragBeganInThisView) {
		// https://github.com/boredzo/PRHVector
		bool isDrag = [PRHVector isStartOfDragFromMouseDownEvent:_mouseDownEvent
			toMouseDraggedEvent:event];

		if (isDrag) {
			//Our immediate first drag image is simply a screenshot of the view. We can dish that up even before the temporally lax frame grabber comes through.
			NSRect frameInWindow = [self.superview convertRect:self.frame toView:nil];
			NSRect frameInScreen = [self.window convertRectToScreen:frameInWindow];
			NSScreen *mainMenuScreen = [[NSScreen screens] objectAtIndex:0UL];
			NSRect screenFrame = mainMenuScreen.frame;
			NSAffineTransform *transform = [NSAffineTransform transform];
			[transform translateXBy:0.0 yBy:screenFrame.size.height];
			[transform scaleXBy:1.0 yBy:-1.0];
			[transform translateXBy:0.0 yBy:frameInWindow.size.height];
			frameInScreen.origin = [transform transformPoint:frameInScreen.origin];

			CGImageRef screenshot = CGWindowListCreateImage(frameInScreen, kCGWindowListOptionIncludingWindow, (CGWindowID)self.window.windowNumber, kCGWindowImageBoundsIgnoreFraming);
			NSImage *image = [[NSImage alloc] initWithCGImage:screenshot size:PRHNaturalSizeOfAsset(self.player.currentItem.asset)];
			CGImageRelease(screenshot);

			_draggingFrameTime = self.player.currentTime;
			PRHFrameGrabber *frameGrabber = [PRHFrameGrabber frameGrabberForAsset:self.player.currentItem.asset];
			frameGrabber.roughImage = image;
			frameGrabber.desiredFrameTime = _draggingFrameTime;
			frameGrabber.desiredFrameSize = self.bounds.size;
			[frameGrabber startGrab];
			_draggingFrameGrabber = frameGrabber;

			PRHTimeFormatter *timeFormatter = [PRHTimeFormatter new];
			[timeFormatter configureForTimestamp];

			NSDraggingItem *item = [[NSDraggingItem alloc] initWithPasteboardWriter:frameGrabber];
			item.draggingFrame = self.bounds;
			//This entire block simply sets the drag image, which is the thumbnail with a timestamp beneath it.
			item.imageComponentsProvider = ^NSArray *{
				NSRect imageRect = { NSZeroPoint, image.size };

				NSDraggingImageComponent *imageComponent = [NSDraggingImageComponent draggingImageComponentWithKey:NSDraggingImageComponentIconKey];
				imageComponent.contents = image;
				imageComponent.frame = imageRect;
				frameGrabber.imageForExactTimeArrived = ^(NSImage *imageAtExactTime) {
					imageComponent.contents = imageAtExactTime;
				};

				NSString *string = [timeFormatter stringForTime:self->_draggingFrameTime];
				NSMutableParagraphStyle *centered = [[NSParagraphStyle defaultParagraphStyle] mutableCopy];
				centered.alignment = NSCenterTextAlignment;
				NSAttributedString *attrStr = [[NSAttributedString alloc]
					initWithString:[@[@" ", string, @" "] componentsJoinedByString:@""]
					attributes:@{
						NSFontAttributeName: [NSFont fontWithName:@"Helvetica" size:12.0],
						NSParagraphStyleAttributeName: centered,
						NSForegroundColorAttributeName: [NSColor whiteColor],
						NSBackgroundColorAttributeName: [[NSColor blackColor] colorWithAlphaComponent:0.75],
					}
				];
				NSSize textSize = [attrStr size];
				NSRect textRect = { { NSMidX(imageRect) - textSize.width / 2.0, NSMinY(imageRect) }, textSize };
				textRect.origin.y = -textRect.size.height;
				textRect.size.width += (20.0 - textRect.size.height) * 2.0;
				textRect.size.height = 20.0;

				NSImage *stringImage = [[NSImage alloc] initWithSize:textRect.size];
				[stringImage lockFocus];
				[attrStr drawAtPoint:NSZeroPoint];
				[stringImage unlockFocus];

				NSDraggingImageComponent *labelComponent = [NSDraggingImageComponent draggingImageComponentWithKey:NSDraggingImageComponentLabelKey];
				labelComponent.contents = stringImage;
				labelComponent.frame = textRect;

				return @[ imageComponent, labelComponent ];
			};
			NSArray *items = @[ item ];
			_draggingSession = [self beginDraggingSessionWithItems:items
				event:_mouseDownEvent
				source:self];
			_dragBeganInThisView = true;
		}
	}
}

- (void) mouseUp:(NSEvent *)theEvent {
	//Ignore mouseUp:s that come from the user beginning a drag in the controller and ending it here.
	if (!_clickBeganInThisView)
		return;
	_clickBeganInThisView = false;
	if (!_dragBeganInThisView)
		[NSApp sendAction:self.action to:self.target from:self];
	else
		_dragBeganInThisView = false;
}

[other event methods that aren't relevant]

- (NSDragOperation) draggingSession:(NSDraggingSession *)session
	sourceOperationMaskForDraggingContext:(NSDraggingContext)context
{
	switch (context) {
		case NSDraggingContextWithinApplication:
			return NSDragOperationNone;

		case NSDraggingContextOutsideApplication:
			return NSDragOperationCopy;
	}

	return NSDragOperationNone;
}

- (NSArray *)namesOfPromisedFilesDroppedAtDestination:(NSURL *)destURL {
	NSDocument *document = self.document;

	if (_timeFormatterForDraggedFrameImageFilenames == nil) {
		_timeFormatterForDraggedFrameImageFilenames = [PRHTimeFormatter new];
		[_timeFormatterForDraggedFrameImageFilenames configureForFilename];
		_timeFormatterForDraggedFrameImageFilenames.minimumNumberOfComponents = [PRHTimeFormatter numberOfComponentsInTime:self.player.currentItem.duration];
	}
	NSString *timeString = [_timeFormatterForDraggedFrameImageFilenames stringForTime:_draggingFrameTime];

	PRHFrameGrabber *frameGrabber = _draggingFrameGrabber;
	NSString *type = [frameGrabber pasteboardPropertyListForType:(__bridge NSString *)kPasteboardTypeFilePromiseContent];
	NSString *extension = (__bridge_transfer NSString *)UTTypeCopyPreferredTagWithClass((__bridge CFStringRef)type, kUTTagClassFilenameExtension);
	if ([extension isEqualToString:@"jpeg"]) {
		//ಠ_ಠ
		extension = @"jpg";
	}

	NSString *imageFilename = [NSString stringWithFormat:@"%@@%@.%@", document.displayName, timeString, extension];

	_promisedFrameImageFileURL = [destURL URLByAppendingPathComponent:imageFilename isDirectory:NO];

	return @[ imageFilename ];
}

- (void) draggingSession:(NSDraggingSession *)session
	endedAtPoint:(NSPoint)screenPoint
	operation:(NSDragOperation)operation
{
	if (operation == NSDragOperationCopy) {
		PRHFrameGrabber *frameGrabber = _draggingFrameGrabber;
		NSString *type = [frameGrabber pasteboardPropertyListForType:(__bridge NSString *)kPasteboardTypeFilePromiseContent];
		NSData *imageData = [frameGrabber pasteboardPropertyListForType:type];
		[imageData writeToURL:_promisedFrameImageFileURL options:NSDataWritingAtomic error:NULL];
	}
	_draggingFrameGrabber = nil;

	_clickBeganInThisView = _dragBeganInThisView = false;
	_draggingSession = nil;
}

[other stuff, including dragging *destination* logic]

@end