Question

I am following the 23th chapter of Aaron’s Cocoa programming for Mac OS X.
I have to drag a letter from a view and copy it in another application like text edit.
This is the portion of code with which I find problems:

- (void) mouseDragged:(NSEvent *)theEvent
{
    NSPasteboard* pb=[NSPasteboard pasteboardWithName: NSDragPboard];
    NSPoint down=[mouseDownEvent locationInWindow];
    NSPoint drag=[theEvent locationInWindow],p;
    NSSize size=[string sizeWithAttributes: attributes];
    NSRect imageBounds;
    NSImage* image=[NSImage alloc];
    float distance;
    distance= hypot(down.x-drag.x, down.y-drag.y);
    if(distance<3 || [string length]==0)
    return;
    image=[image initWithSize: size];
    imageBounds.origin=NSZeroPoint;
    imageBounds.size=size;
    [image lockFocus];
    [self drawStringCenteredIn: imageBounds];
    [image unlockFocus];
    p=[self convertPoint: down fromView: nil];
    p.x= p.x - size.width/2;
    p.y= p.y - size.height/2;
    [self writeToPasteboard: pb];
    [self dragImage: image at: p offset: NSZeroSize event: mouseDownEvent pasteboard: pb source: self slideBack: YES];
}

Where:
– mouseDownEvent is an event previously saved, when the user has clicked on the lecter (mouseDown event);
– string is a NSMutableAttributesString, a string of max 1 lected that contains the lecter to be displayed into the view;

When the event occurs there is already a letter displayed on the view (so the string has length 1).If I forgot some important informations please ask.

The problem: the drag and drop operation works fine, but the problem is that while I drag the image, I don’t see the image displacing from it’s original position.
This is what I see while I drag the letter:

This is what I should see instead:

So the letter should displace, but this doesn’t happen.I don’t recognize the cause of this problem, I think that the method drawImageCenteredIn should work fine.That’s the code of this method:

- (void) drawStringCenteredIn: (NSRect) rect
{
    NSSize size=[string sizeWithAttributes: attributes];
    NSPoint origin;
    origin.x= rect.origin.x+ (rect.size.width+size.width)/2;
    origin.y=rect.origin.y + (rect.size.height+size.height)/2;
    [string drawAtPoint: origin withAttributes: attributes];
}

Answer 1

It’s simple. The sign is incorrect in these two lines:

  origin.x= rect.origin.x+ (rect.size.width-size.width)/2;
  origin.y= rect.origin.y + (rect.size.height-size.height)/2;

Here is the corrected method:

- (void) drawStringCenteredIn: (NSRect) rect
{
    NSSize size=[string sizeWithAttributes: attributes];
    NSPoint origin;
    origin.x= rect.origin.x+ (rect.size.width-size.width)/2;
    origin.y= rect.origin.y + (rect.size.height-size.height)/2;
    [string drawAtPoint: origin withAttributes: attributes];
}