A category to draw an NSImage using 9 slice stretching

NSImage+TMStretchable.h

//
//  NSImage+TMStretchable.h
//  ThinkMessenger
//
//  Created by David Beck on 1/11/12.
//  Copyright (c) 2012 ThinkUltimate. All rights reserved.
//

#import <AppKit/AppKit.h>

typedef struct {
    CGFloat top, left, bottom, right;
} TMEdgeInsets;

#define UIEdgeInsetsZero { 0.0, 0.0, 0.0, 0.0 }

TMEdgeInsets TMEdgeInsetsMake (CGFloat top,
			       CGFloat left,
			       CGFloat bottom,
			       CGFloat right);


@interface NSImage (TMStretchable)

- (void)drawInRect:(CGRect)rect withCapInsets:(TMEdgeInsets)capInsets;

@end

NSImage+TMStretchable.m

//
//  NSImage+TMStretchable.m
//  ThinkMessenger
//
//  Created by David Beck on 1/11/12.
//  Copyright (c) 2012 ThinkUltimate. All rights reserved.
//

#import "NSImage+TMStretchable.h"


TMEdgeInsets TMEdgeInsetsMake (CGFloat top,
							   CGFloat left,
							   CGFloat bottom,
							   CGFloat right)
{
	TMEdgeInsets insets;
	insets.top = top;
	insets.left = left;
	insets.bottom = bottom;
	insets.right = right;
	
	return insets;
}


@implementation NSImage (TMStretchable)

- (void)drawInRect:(CGRect)rect withCapInsets:(TMEdgeInsets)capInsets
{
	rect.origin.x = round(rect.origin.x);
	rect.origin.y = round(rect.origin.y);
	rect.size.width = round(rect.size.width);
	rect.size.height = round(rect.size.height);
	
	//bottom left
	[self drawInRect:NSMakeRect(rect.origin.x, rect.origin.y,
								capInsets.left, capInsets.bottom)
			fromRect:NSMakeRect(0.0, 0.0,
								capInsets.left, capInsets.bottom)
		   operation:NSCompositeSourceOver fraction:1.0];
	
	//top left
	[self drawInRect:NSMakeRect(rect.origin.x, rect.origin.y + rect.size.height - capInsets.top,
								capInsets.left, capInsets.top)
			fromRect:NSMakeRect(0.0, self.size.height - capInsets.top,
								capInsets.left, capInsets.top)
		   operation:NSCompositeSourceOver fraction:1.0];
	
	//top right
	[self drawInRect:NSMakeRect(rect.origin.x + rect.size.width - capInsets.right, rect.origin.y + rect.size.height - capInsets.top, 
								capInsets.right, capInsets.top)
			fromRect:NSMakeRect(self.size.width - capInsets.right, self.size.height - capInsets.top,
								capInsets.right, capInsets.top)
		   operation:NSCompositeSourceOver fraction:1.0];
	
	//bottom right
	[self drawInRect:NSMakeRect(rect.origin.x + rect.size.width - capInsets.right, rect.origin.y, 
								capInsets.right, capInsets.bottom)
			fromRect:NSMakeRect(self.size.width - capInsets.right, 0.0,
								capInsets.right, capInsets.bottom)
		   operation:NSCompositeSourceOver fraction:1.0];
	
	
	//bottom center
	[self drawInRect:NSMakeRect(rect.origin.x + capInsets.left, rect.origin.y, 
								rect.size.width - capInsets.right - capInsets.left, capInsets.bottom)
			fromRect:NSMakeRect(capInsets.left, 0.0,
								self.size.width - capInsets.right - capInsets.left, capInsets.bottom)
		   operation:NSCompositeSourceOver fraction:1.0];
	
	//top center
	[self drawInRect:NSMakeRect(rect.origin.x + capInsets.left, rect.origin.y + rect.size.height - capInsets.top, 
								rect.size.width - capInsets.right - capInsets.left, capInsets.top)
			fromRect:NSMakeRect(capInsets.left, self.size.height - capInsets.top,
								self.size.width - capInsets.right - capInsets.left, capInsets.top)
		   operation:NSCompositeSourceOver fraction:1.0];
	
	//left center
	[self drawInRect:NSMakeRect(rect.origin.x, rect.origin.y + capInsets.bottom, 
								capInsets.left, rect.size.height - capInsets.top - capInsets.bottom)
			fromRect:NSMakeRect(0.0, capInsets.bottom,
								capInsets.left, self.size.height - capInsets.top - capInsets.bottom)
		   operation:NSCompositeSourceOver fraction:1.0];
	
	//right center
	[self drawInRect:NSMakeRect(rect.origin.x + rect.size.width - capInsets.right, rect.origin.y + capInsets.bottom, 
								capInsets.right, rect.size.height - capInsets.top - capInsets.bottom)
			fromRect:NSMakeRect(self.size.width - capInsets.right, capInsets.bottom,
								capInsets.right, self.size.height - capInsets.top - capInsets.bottom)
		   operation:NSCompositeSourceOver fraction:1.0];
	
	
	//center center
	[self drawInRect:NSMakeRect(rect.origin.x + capInsets.left, rect.origin.y + capInsets.bottom, 
								rect.size.width - capInsets.right - capInsets.left, rect.size.height - capInsets.top - capInsets.bottom)
			fromRect:NSMakeRect(capInsets.left, capInsets.bottom,
								self.size.width - capInsets.right - capInsets.left, self.size.height - capInsets.top - capInsets.bottom)
		   operation:NSCompositeSourceOver fraction:1.0];
}

@end