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：

@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];        // insert an overlay into the view hierarchyself.overlay = [[UIView alloc] initWithFrame:CGRectMake(0, -20, [UIScreen mainScreen].bounds.size.width, self.bounds.size.height + 20)];      self.overlay.autoresizingMask = UIViewAutoresizingFlexibleWidth|UIViewAutoresizingFlexibleHeight;[self insertSubview:self.overlay atIndex:0];}    self.overlay.backgroundColor = backgroundColor;
}
@end

最后在scrollViewDidScroll中，我们就可以动态地修改UINavigationBar的backgroundColor了：

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

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-(void)scrollViewDidScroll:(UIScrollView *)scrollView{

CGFloat alpha=scrollView.contentOffset.y/90.0f>1.0f?1:scrollView.contentOffset.y/90.0f;

[self.navigationController.navigationBar setBackgroundImage:[self getImageWithAlpha:alpha] forBarMetrics:UIBarMetricsDefault];

}

#pragma handle image -mark

//合成图片

-(UIImage *)getImageWithAlpha:(CGFloat)alpha{

UIColor *color=[UIColor colorWithRed:1 green:0 blue:0 alpha:alpha];

CGSize colorSize=CGSizeMake(1, 1);

UIGraphicsBeginImageContext(colorSize);

CGContextRef context = UIGraphicsGetCurrentContext();

CGContextSetFillColorWithColor(context, color.CGColor);

CGContextFillRect(context, CGRectMake(0, 0, 1, 1));

UIImage *img=UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

return img;

}