MAC OS X COCOA プログラミング 23章で dragImage が使われているところ を beginDraggingSessionWithItems で書く

BigLetterView.h

@interface BigLetterView : NSView <NSDraggingSource> {
    NSColor *bgColor;
    NSString *string;
    NSMutableDictionary *attributes;
    NSEvent *mouseDownEvent;
}

BigLetterView.m

- (void)mouseDragged:(NSEvent *)event {
    NSPoint down = [mouseDownEvent locationInWindow];
    NSPoint drag = [event locationInWindow];
    float distance = hypot(down.x - drag.x, down.y - drag.y);
    if (distance < 3) {
        return;
    }
    
    if ([string length] == 0) {
        return;
    }
    
    NSSize s = [string sizeWithAttributes:attributes];
    NSImage *anImage = [[NSImage alloc] initWithSize:s];
    NSRect imageBounds;
    imageBounds.origin = NSZeroPoint;
    imageBounds.size = s;
    
    [anImage lockFocus];
    [self drawStringCenteredIn:imageBounds];
    [anImage unlockFocus];
    
    NSPoint p = [self convertPoint:down fromView:nil];
    p.x = p.x - s.width / 2;
    p.y = p.y - s.height / 2;
    
    NSDraggingItem *dragItem = [[NSDraggingItem alloc] initWithPasteboardWriter:string];
    [dragItem setDraggingFrame:NSMakeRect(p.x, p.y, s.width, s.height) contents:anImage];

    NSDraggingSession *draggingSession = [self beginDraggingSessionWithItems:[NSArray arrayWithObject:dragItem] event:mouseDownEvent source:self];
    draggingSession.animatesToStartingPositionsOnCancelOrFail = YES;
    draggingSession.draggingFormation = NSDraggingFormationNone;
}

- (NSDragOperation)draggingSession:(NSDraggingSession *)session sourceOperationMaskForDraggingContext:(NSDraggingContext)context {
    return NSDragOperationCopy | NSDragOperationDelete;
}

- (void)draggingSession:(NSDraggingSession *)session endedAtPoint:(NSPoint)screenPoint operation:(NSDragOperation)operation {
    switch (operation) {
        case NSDragOperationDelete:
            [self setString:@""];
            break;
        default:
            break;
    }
}