Stretch PNGs smoothly in Cocoa AppKit

In questions like this one it shows how to use UIKit's

-(UIImage *)stretchableImageWithLeftCapWidth:(NSInteger)leftCapWidth topCap开发者_如何学CHeight:(NSInteger)topCapHeight

to stretch a PNG easily, e.g when inserting text in an iChat like bubble. I would like to do the same on regular AppKit/Cocoa/desktop app using CoreGraphics or some NSImage related API but couldn't find an equivalent method. Does anybody know how to do this in AppKit?

Use NSDrawThreePartImage or NSDrawNinePartImage.

Just use -[NSImage drawInRect:fromRect:operation:fraction:], and pass NSZeroRect as the fromRect; parameter.

Here is a function i whipped up that takes 1 image, and stretches just the middle...

-(NSImage *)image:(NSImage *)image leftCapWidth:(float)leftWidth middleWidth:(float)middleWidth rightCapWidth:(float)rightWidth {

    // Calculate the new images dimensions
    float imageWidth = leftWidth + middleWidth + rightWidth;
    float imageHeight = image.size.height;

    // Generate the left image
    NSRect rectLeft = NSMakeRect(0, 0, leftWidth, imageHeight);
    NSImage *imageLeft = [[NSImage alloc] initWithSize:rectLeft.size];
    if (imageLeft.size.width > 0) {
        [imageLeft lockFocus];
        [image drawInRect:rectLeft fromRect:rectLeft operation:NSCompositeCopy fraction:1.0];
        [imageLeft unlockFocus];
    }

    // Generate the middle image
    NSRect rectMiddle = NSMakeRect(0, 0, image.size.width - (rightWidth + leftWidth), imageHeight);
    NSImage *imageMiddle = [[NSImage alloc] initWithSize:rectMiddle.size];
    if (imageMiddle.size.width > 0) {
        [imageMiddle lockFocus];
        [image drawInRect:rectMiddle fromRect:NSMakeRect(leftWidth, 0, rectMiddle.size.width, imageHeight) operation:NSCompositeCopy fraction:1.0];
        [imageMiddle unlockFocus];
    }

    // Generate the right image
    NSRect rectRight = NSMakeRect(0, 0, rightWidth, imageHeight);
    NSImage *imageRight = [[NSImage alloc] initWithSize:rectRight.size];
    if (imageRight.size.width > 0) {
        [imageRight lockFocus];
        [image drawInRect:rectRight fromRect:NSMakeRect(image.size.width - rightWidth, 0, rightWidth, imageHeight) operation:NSCompositeCopy fraction:1.0];
        [imageRight unlockFocus];
    }

    // Combine the images
    NSImage *newImage = [[[NSImage alloc] initWithSize:NSMakeSize(imageWidth,  imageHeight)] autorelease];
    if (newImage.size.width > 0) {
        [newImage lockFocus];
        NSDrawThreePartImage(NSMakeRect(0, 0, imageWidth, imageHeight), imageLeft, imageMiddle, imageRight, NO, NSCompositeSourceOver, 1, NO); 
        [newImage unlockFocus];
    }

    // Release the images and return the new image
    [imageLeft release];
    [imageMiddle release];
    [imageRight release];

    return newImage;
}