问题: 改背景仅需在UINavigationBar的子视图中加入UIImageView,但如果想在应用中修改背景又应该怎么办呢?
解决: 有些第三方的框架或者类都要派生UINavigationBar,它们有些“坏味道”!
绝妙解决: 我们会创建一个UINavigationBar的类别,并在UIApplicationDelegate中创建一个UIImage属性。
AppDelegate.h
@interface AppDelegate : NSObject
{
    UIImage *navImage;
    //其他略
}
@property (nonatomic, retain) UIImage *navImage;
//其他略
AppDelegate.m
@implementation AppDelegate;
@synthesize navImage;
//其他略
- (BOOL)application:(UIApplication *)application
didFinishLaunchingWithOptions:
(NSDictionary *)launchOptions
{
    self.navImage = [UIImage
    imageNamed:@"navigation-bar-cooler.png"];
    //其他略
    return YES;
}
UINavigationBar (Helpers).h
@interface UINavigationBar (Helpers)
@end
UINavigationBar (Helpers).m
#import "AppDelegate.h"
#import "UINavigationBar (Helpers).h"
@implementation UINavigationBar (Helpers)
- (void)drawRect:(CGRect)rect
{
    AppDelegate *delegate = (AppDelegate *)
    [[UIApplication sharedApplication] delegate];
    [delegate.navImage drawInRect:
    CGRectMake(0, 0, self.frame.size.width,
    self.frame.size.height)];
}
@end
一旦完成了这些,就可以按如下代码改变UINavigationBar的背景了:
- (void)viewWillAppear:(BOOL)animated
{
    delegate.navImage = [UIImage imageNamed:
    @"navigation-bar-other.png"];
    [self.navigationController.navigationBar
    setNeedsDisplay];
    [super viewWillAppear:animated];
}
 
原文链接:http://iphonedevblog.com/code/dynamically-change-a-uinavigationbars-background/