//必须设置为UIBarStyleBlack 否则毛玻璃效果不是黑色的
    [self.navigationBar setBarStyle:UIBarStyleBlack];
    [self.navigationController.navigationBar setBackgroundImage:nil forBarMetrics:UIBarMetricsDefault];
    UIView *bgView = [[UIView alloc] init];
    bgView.frame = ({
        CGRect frame = self.navigationBar.bounds;
        frame.size.height += 20;
        frame.origin.y -= 20;
        frame;
    });
    CAGradientLayer *gradientLayer = [[CAGradientLayer alloc] init];
    gradientLayer.frame = bgView.bounds;
    [UIColor colorWithRed:4/255.0 green:0 blue:18/255.0 alpha:0.76];
    gradientLayer.colors = @[(__bridge id)[UIColor colorWithRed:4/255.0 green:0 blue:18/255.0 alpha:0.76].CGColor,(__bridge id)[UIColor colorWithRed:4/255.0 green:0 blue:18/255.0 alpha:0.28].CGColor];
    gradientLayer.startPoint = CGPointMake(0, 0);
    gradientLayer.endPoint = CGPointMake(0, 1.0);
    [bgView.layer addSublayer:gradientLayer];
    bgView.userInteractionEnabled = NO;
    //保证bgView 在最底部
    bgView.layer.zPosition = -1;
    [self.navigationBar addSubview:bgView];
    [self.navigationBar sendSubviewToBack:bgView];