导航栏透明渐变

由于项目需求需要做到导航栏随tableView的滚动由透明到显示颜色。
所以在此做个笔记。

- (void)viewWillAppear:(BOOL)animated
{
    //去掉透明后导航栏下边的黑边
    self.navigationController.navigationBar.shadowImage = [[UIImage alloc] init];
    UIImage *image = [self createAImageWithColor:[UIColor clearColor] alpha:0.0];
    //设置导航栏背景图片为一个空的image，这样就透明了
    [self.navigationController.navigationBar setBackgroundImage:image forBarMetrics:UIBarMetricsDefault];
}

- (UIImage *)createAImageWithColor:(UIColor *)color alpha:(CGFloat)alpha{
    CGRect rect = CGRectMake(0.0f, 0.0f, 1.0f, 1.0f);
    UIGraphicsBeginImageContext(rect.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetFillColorWithColor(context, [color CGColor]);
    CGContextSetAlpha(context, alpha);
    CGContextFillRect(context, rect);
    UIImage *theImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return theImage;
}

tableView滚动时渐变导航栏颜色

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    CGFloat minAlphaOffset = - 64;
    CGFloat maxAlphaOffset = 200;
    CGFloat offset = scrollView.contentOffset.y;
    CGFloat alpha = (offset - minAlphaOffset) / (maxAlphaOffset - minAlphaOffset);
    UIImage *image = [self createAImageWithColor:[UIColor orangeColor] alpha:alpha];
    [self.navigationController.navigationBar setBackgroundImage:image forBarMetrics:UIBarMetricsDefault];
    NSLog(@"alpha %f",alpha);
}

到这里基本已经实现，不过当你需要push到其他页面的时候需要设置回来
视图即将消失的时候

- (void)viewWillDisappear:(BOOL)animated
{
    self.navigationController.navigationBar.shadowImage = nil;
    [self.navigationController.navigationBar setBackgroundImage:nil forBarMetrics:UIBarMetricsDefault];
}

The End！