Question

So I have this system set up with a base class DisplayObject.
It has a Render method and a list of other DisplayObjects as it’s children. It also contains data about various matrix transformations but I does not contain the data about the actual drawing. That’s why the Render method is virtual.

I then have 2 classes. ColoredShape and TexturedShape each inheriting from DisplayObject and overriding the Render method with a new method starting with

base.Render(); 

and then doing the drawing code.

What I want now is to call Render on all of the object’s children after the Render on the parent has finished. So there’s my problem. If I do

foreach (var child in Children) child.Render();

at the end of the Render method in the DisplayObject class it will be executed before the overrides are, since it’s a part of the base.Render() call, and if I put that code in the subclasses instead, the DisplayObject alone loses that functionality and it forces all the future subclasses inheriting from it to implement the feature manually.

Answer 1

What you want is the abstract method pattern. In other words, don’t make your render method virtual crate another method (virtual or abstract) that render calls., so render will look something like this:

public void Render()
{
    DoRender()

    foreach (var child in _children)
    {
        child.Render();
    }
}

protected virtual void DoRender()
{
}