Question

I’m implementing drag & drop for a customView; this customView is a subclass of NSView and include some elements.
When I start drag operation on it, the dragImage it’s just an rectangular gray box of the same size of the customView.

This is the code I wrote:

-(void) mouseDragged:(NSEvent *)theEvent
{
    NSPoint downWinLocation = [mouseDownEvent locationInWindow];
    NSPoint dragWinLocation = [theEvent locationInWindow];

    float distance = hypotf(downWinLocation.x - dragWinLocation.x, downWinLocation.y - downWinLocation.x);
    if (distance < 3) {
        return;
    }

    NSImage *viewImage = [self getSnapshotOfView];
    NSSize viewImageSize = [viewImage size];
    //Get Location of mouseDown event
    NSPoint p = [self convertPoint:downWinLocation fromView:nil];

    //Drag from the center of image
    p.x = p.x - viewImageSize.width / 2;
    p.y = p.y - viewImageSize.height / 2;

    //Write on PasteBoard
    NSPasteboard *pb = [NSPasteboard pasteboardWithName:NSDragPboard];
    [pb declareTypes:[NSArray arrayWithObject:NSFilenamesPboardType]
               owner:nil];
    //Assume fileList is list of files been readed
    NSArray *fileList = [NSArray arrayWithObjects:@"/tmp/ciao.txt", @"/tmp/ciao2.txt", nil];
    [pb setPropertyList:fileList forType:NSFilenamesPboardType];

    [self dragImage:viewImage at:p offset:NSMakeSize(0, 0) event:mouseDownEvent pasteboard:pb source:self slideBack:YES];
}

And this is the function I use to create the snapshot:

- (NSImage *) getSnapshotOfView
{
    NSRect rect = [self bounds] ;

    NSImage *image = [[[NSImage alloc] initWithSize: rect.size] autorelease];

    NSRect imageBounds;
    imageBounds.origin = NSZeroPoint;
    imageBounds.size = rect.size;

    [self lockFocus];
    NSBitmapImageRep *rep = [[NSBitmapImageRep alloc] initWithFocusedViewRect:imageBounds];
    [self unlockFocus];

    [image addRepresentation:rep];
    [rep release];

    return image;
}

This is an image of a drag operation on my customView (the one with the icon and the label “drag me”)

Why my dragImage it’s just a gray box?

Answer 1

From the screenshot of IB in your comment, it looks like your view is layer backed. Layer backed views draw to their own graphics area that is separate from the normal window backing store.

This code:

[self lockFocus];
NSBitmapImageRep *rep = [[NSBitmapImageRep alloc] initWithFocusedViewRect:imageBounds];
[self unlockFocus];

Effectively reads pixels from the window backing store. Since your view is layer backed, its content is not picked up.

Try this without a layer backed view.