CAGradientLayer *gradient = [CAGradientLayer layer];
    gradient.frame = self.gradientView.bounds;
    gradient.colors = [NSArray arrayWithObjects:(id)[[UIColor clearColor] CGColor], [(id)[[UIColor blackColor] colorWithAlphaComponent:0.6] CGColor], nil];
    if (self.gradientView.layer.sublayers.count == 0) {
        [self.gradientView.layer insertSublayer:gradient atIndex:0];
    }