A subclass of UIButton that automatically stretches the background images for all button states.

AutoStretchButton.h

#import <UIKit/UIKit.h>

@interface AutoStretchButton : UIButton
// Call this if you modify the background images after initialization.
- (void)autoStretch;
@end

AutoStretchButton.m

#import "AutoStretchButton.h"

@implementation AutoStretchButton

- (id)initWithFrame:(CGRect)frame {
    self = [super initWithFrame:frame];
    if (self) {
        [self autoStretch];
    }
    return self;
}

- (id)initWithCoder:(NSCoder *)decoder {
    self = [super initWithCoder:decoder];
    if (self) {
        [self autoStretch];
    }
    return self;
}

- (void)autoStretch {
    for (UIControlState state = UIControlStateNormal; state <= UIControlStateSelected; state++) {
        UIImage *image = [self backgroundImageForState:state];
        if (image) {
            UIImage *stretchedImage = [image
                                       stretchableImageWithLeftCapWidth:image.size.width / 2
                                       topCapHeight:image.size.height / 2];
            [self setBackgroundImage:stretchedImage forState:state];
        }
    }
}

@end