UINavigationBar和新的iOS 5+外观API - 如何提供两个背景图像？

#import <objc/runtime.h>

@implementation UINavigationBar (Background)

- (void)drawRect:(CGRect)rect {
    UIImage *image = [UIImage imageNamed:@"navigationBar_background.png"];
    [image drawInRect:rect];
}

+ (void)load {
    Method originalMethod = class_getInstanceMethod(self, @selector(drawRect:));
    Method swizzledMethod = class_getInstanceMethod(self, @selector(swizzled_drawRect:));
    method_exchangeImplementations(originalMethod, swizzledMethod);
}

- (void)swizzled_drawRect:(CGRect)rect {
    [self swizzled_drawRect:rect];

    UIImage *image2 = [UIImage imageNamed:@"navigationBar_background2.png"];
    [image2 drawInRect:rect];
}

@end