NSImage+NinePartDrawing.h

/*
 *  NSImage+NinePartDrawing.h
 *
 *  Copyright (c) 2011 Noel R. Cower
 *
 *  This software is provided 'as-is', without any express or implied
 *  warranty. In no event will the authors be held liable for any damages
 *  arising from the use of this software.
 *
 *  Permission is granted to anyone to use this software for any purpose,
 *  including commercial applications, and to alter it and redistribute it
 *  freely, subject to the following restrictions:
 *
 *  1. The origin of this software must not be misrepresented; you must not
 *  claim that you wrote the original software. If you use this software
 *  in a product, an acknowledgment in the product documentation would be
 *  appreciated but is not required.
 *
 *  2. Altered source versions must be plainly marked as such, and must not be
 *  misrepresented as being the original software.
 *
 *  3. This notice may not be removed or altered from any source
 *  distribution.
 *
 */

#import <Cocoa/Cocoa.h>

extern const CGFloat NNoCap;

@interface NSImage (NinePartDrawing)

- (void)drawInRect:(NSRect)dstSpacePortionRect
         operation:(NSCompositingOperation)op
          fraction:(CGFloat)requestedAlpha
    respectFlipped:(BOOL)respectContextIsFlipped
             hints:(NSDictionary *)hints
            topCap:(CGFloat)top
         bottomCap:(CGFloat)bottom
           leftCap:(CGFloat)left
          rightCap:(CGFloat)right;

- (void)drawInRect:(NSRect)dstSpacePortionRect
            topCap:(CGFloat)top
         bottomCap:(CGFloat)bottom
           leftCap:(CGFloat)left
          rightCap:(CGFloat)right;

- (void)drawInRect:(NSRect)dstSpacePortionRect
         heightCap:(CGFloat)heightCap
          widthCap:(CGFloat)widthCap;

- (void)drawInRect:(NSRect)dstSpacePortionRect
           withCap:(CGFloat)cap;

@end

NSImage+NinePartDrawing.m

/*
 *  NSImage+NinePartDrawing.m
 *
 *  Copyright (c) 2011 Noel R. Cower
 *
 *  This software is provided 'as-is', without any express or implied
 *  warranty. In no event will the authors be held liable for any damages
 *  arising from the use of this software.
 *
 *  Permission is granted to anyone to use this software for any purpose,
 *  including commercial applications, and to alter it and redistribute it
 *  freely, subject to the following restrictions:
 *
 *  1. The origin of this software must not be misrepresented; you must not
 *  claim that you wrote the original software. If you use this software
 *  in a product, an acknowledgment in the product documentation would be
 *  appreciated but is not required.
 *
 *  2. Altered source versions must be plainly marked as such, and must not be
 *  misrepresented as being the original software.
 *
 *  3. This notice may not be removed or altered from any source
 *  distribution.
 *
 */

#import "NSImage+NinePartDrawing.h"

const CGFloat NNoCap = -1;

@implementation NSImage (NinePartDrawing)

- (void)drawInRect:(NSRect)dstSpacePortionRect
            topCap:(CGFloat)top
         bottomCap:(CGFloat)bottom
           leftCap:(CGFloat)left
          rightCap:(CGFloat)right {
    [self drawInRect:dstSpacePortionRect
           operation:NSCompositeSourceOver
            fraction:1.0
      respectFlipped:YES
               hints:nil
              topCap:top
           bottomCap:left
             leftCap:bottom
            rightCap:right];
}

- (void)drawInRect:(NSRect)dstSpacePortionRect
         heightCap:(CGFloat)heightCap
          widthCap:(CGFloat)widthCap {
    [self drawInRect:dstSpacePortionRect
           operation:NSCompositeSourceOver
            fraction:1.0
      respectFlipped:YES
               hints:nil
              topCap:heightCap
           bottomCap:heightCap
             leftCap:widthCap
            rightCap:widthCap];
}

- (void)drawInRect:(NSRect)dstSpacePortionRect
           withCap:(CGFloat)cap {
    [self drawInRect:dstSpacePortionRect
           operation:NSCompositeSourceOver
            fraction:1.0
      respectFlipped:YES
               hints:nil
              topCap:cap
           bottomCap:cap
             leftCap:cap
            rightCap:cap];
}

- (void)drawInRect:(NSRect)dstSpacePortionRect
         operation:(NSCompositingOperation)op
          fraction:(CGFloat)requestedAlpha
    respectFlipped:(BOOL)respectContextIsFlipped
             hints:(NSDictionary *)hints
            topCap:(CGFloat)top
         bottomCap:(CGFloat)bottom
           leftCap:(CGFloat)left
          rightCap:(CGFloat)right {
    
    top = fmax(top, 0);
    bottom = fmax(bottom, 0);
    left = fmax(left, 0);
    right = fmax(right, 0);
    
    NSSize size = [self size];
    NSRect srcRect = NSZeroRect;
    NSRect dstRect = dstSpacePortionRect;
    
    srcRect.size.width = left;
    srcRect.size.height = bottom;
    dstRect.size = srcRect.size;
    
    if (bottom > 0) {
        if (left > 0) {
            [self drawInRect:dstRect
                    fromRect:srcRect
                   operation:op
                    fraction:requestedAlpha
              respectFlipped:respectContextIsFlipped
                       hints:hints];
        }
        
        dstRect.origin.x += (srcRect.origin.x = left);
        dstRect.size.width = dstSpacePortionRect.size.width - (left + right);
        srcRect.size.width = size.width - left - right;

        [self drawInRect:dstRect
                fromRect:srcRect
               operation:op
                fraction:requestedAlpha
          respectFlipped:respectContextIsFlipped
                   hints:hints];
    
        dstRect.origin.x = (dstSpacePortionRect.origin.x + dstSpacePortionRect.size.width) - right;
        srcRect.origin.x = size.width - right;
        dstRect.size.width = srcRect.size.width = right;
    
        if (right > 0) {
            [self drawInRect:dstRect
                    fromRect:srcRect
                   operation:op
                    fraction:requestedAlpha
              respectFlipped:respectContextIsFlipped
                       hints:hints];
        }
    }
    
    srcRect.size.height = size.height - (bottom + top);
    dstRect.size.height = dstSpacePortionRect.size.height - (bottom + top);
    dstRect.origin.y = dstSpacePortionRect.origin.y + (srcRect.origin.y = bottom);
    
    if (right > 0) {
        [self drawInRect:dstRect
                fromRect:srcRect
               operation:op
                fraction:requestedAlpha
          respectFlipped:respectContextIsFlipped
                   hints:hints];
    }
    
    if (left > 0) {
        srcRect.origin.x = 0;
        dstRect.origin.x = dstSpacePortionRect.origin.x;
        dstRect.size.width = srcRect.size.width = left;
        
        [self drawInRect:dstRect
                fromRect:srcRect
               operation:op
                fraction:requestedAlpha
          respectFlipped:respectContextIsFlipped
                   hints:hints];
    }
    
    dstRect.origin.x = dstSpacePortionRect.origin.x + (srcRect.origin.x = left);
    dstRect.size.width = dstSpacePortionRect.size.width - (left + right);
    srcRect.size.width = size.width - left - right;
    
    [self drawInRect:dstRect
            fromRect:srcRect
           operation:op
            fraction:requestedAlpha
      respectFlipped:respectContextIsFlipped
               hints:hints];
    
    if (top > 0) {
        srcRect.origin.y = size.height - top;
        dstRect.origin.y = (dstSpacePortionRect.origin.y + dstSpacePortionRect.size.height) - top;
        dstRect.size.height = srcRect.size.height = top;
        
        [self drawInRect:dstRect
                fromRect:srcRect
               operation:op
                fraction:requestedAlpha
          respectFlipped:respectContextIsFlipped
                   hints:hints];
        
        if (right > 0) {
            srcRect.origin.x = size.width - right;
            dstRect.origin.x = (dstSpacePortionRect.origin.x + dstSpacePortionRect.size.width) - right;
            srcRect.size.width = dstRect.size.width = right;
            
            [self drawInRect:dstRect
                    fromRect:srcRect
                   operation:op
                    fraction:requestedAlpha
              respectFlipped:respectContextIsFlipped
                       hints:hints];
        }
        
        if (left > 0) {
            srcRect.origin.x = 0;
            dstRect.origin.x = dstSpacePortionRect.origin.x;
            srcRect.size.width = dstRect.size.width = left;
            
            [self drawInRect:dstRect
                    fromRect:srcRect
                   operation:op
                    fraction:requestedAlpha
              respectFlipped:respectContextIsFlipped
                       hints:hints];
        }
    }
}

@end

Props to you good sir; works well!