扩展UINavigationBar，动态修改UINavigationBar的背景色

转载于：
http://www.cocoachina.com/ios/20150409/11505.html
来源：Glow 技术团队博客

我们来看看apple为我们提供了哪些API来设置navigationBar的颜色。
首先想到的是最常用的[UINavigationBar appearance]，我们一般会在AppDelegate中使用它对navigationBar进行统一的设置。但是如果试一下，会发现在scrollViewDidScrollView中调用它并不能动态地改变navigationBar的颜色，原因可以看一下Apple的doc：

Use the UIAppearance protocol to get the appearance proxy for a class. You can customize the appearance of instances of a class by sending appearance modification messages to the class’s appearance proxy.

但是：

iOS applies appearance changes when a view enters a window, it doesn’t change the appearance of a view that’s already in a window. To change the appearance of a view that’s currently in a window, remove the view from the view hierarchy and then put it back.

所以变通一下，我们的思路很简单，参照Apple的实现，在navigationBar的view hierarchy中插入一个view，通过它来控制在navigationBar的backgroundColor。考虑到继承UINavigationBar使用起来会非常不便，我们决定用Category来实现，首先定义我们的category：

#import 
@interface UINavigationBar (BackgroundColor)
- (void)lt_setBackgroundColor:(UIColor *)backgroundColor;
@end

实现：我们使用associatedObject将overlayView动态地绑定到UINavigationBar的instance上，当调用lt_setBackgroundColor的时候，我们只要更新这个overlayView就行啦～

@implementation UINavigationBar (BackgroundColor)

static char overlayKey;
 
- (UIView *)overlay
{    return objc_getAssociatedObject(self, &overlayKey);
}
 
- (void)setOverlay:(UIView *)overlay
{
    objc_setAssociatedObject(self, &overlayKey, overlay, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (void)lt_setBackgroundColor:(UIColor *)backgroundColor
{    if (!self.overlay) {
        [self setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
        [self setShadowImage:[UIImage new]];        // insert an overlay into the view hierarchy
        self.overlay = [[UIView alloc] initWithFrame:CGRectMake(0, -20, [UIScreen mainScreen].bounds.size.width, 64)];
        [self insertSubview:self.overlay atIndex:0];
     }    
     self.overlay.backgroundColor = backgroundColor;
}
@end

我们就可以动态地修改UINavigationBar的backgroundColor了：

[self.navigationController.navigationBar lt_setBackgroundColor:[color colorWithAlphaComponent:alpha]];