NavigationBar 渐隐效果
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 hierarchy
self.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;
}