How to apply shadow effect on specific side of the parent view only in ios

Question

For applying shadow effect on view I am using the below code.

func addShadowImage(parentview:UIView){

    parentview.layer.shadowColor = UIColor.redColor().CGColor
    parentview.layer.masksToBounds = false

    //for bottom shadow on view
    parentview.layer.shadowOffset = CGSizeMake(0,1.0)
    parentview.layer.shadowOpacity = 0.7
    parentview.layer.shadowRadius = 1.0


    //for bottom and right sides shadow on view
    parentview.layer.shadowOffset = CGSizeMake(1.0,1.0)
    parentview.layer.shadowOpacity = 1
    parentview.layer.shadowRadius = 1.0


    //for empty shadow on view
    parentview.layer.shadowOffset = CGSizeMake(0,0)
    parentview.layer.shadowOpacity = 1
    parentview.layer.shadowRadius = 0

    //for bottom and right and left sides shadow on view
    parentview.layer.shadowOffset = CGSizeMake(0,2.0)
    parentview.layer.shadowOpacity = 1
    parentview.layer.shadowRadius = 2.0

    //for four sides shadow on view
    parentview.layer.shadowOffset = CGSizeMake(0,0)
    parentview.layer.shadowOpacity = 1.0
    parentview.layer.shadowRadius = 5.0
}

Now simply call the above function with parameter type is UIView self.addShadowImage(yourViewObject)

Then it's working fine, But this shadow effected on all its subviews.

So now I want to apply a shadow effect on specific view and specific side(i.e either left or right/bottom/top).