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Design Patterns: Renderer

This post about design patterns will be a little unusual. To this day, I was going through a generally recognized set of design patterns that was introduced by the Gang of Four in Design Patterns: Elements of Reusable Object-Oriented Software. But today I want to introduce to you a useful design bit I came up with, while I was working on my bachelor’s thesis. I call it Renderer.

The problem I had was simple – unbelievable mess in my application’s source code. I was working on a procedural approach to rendering cities. And believe me, a city is kind of big-ass model to draw. There’s awful lot of rendering of different things on different places. And when it comes together it’s a giant blob of instructions. So I needed some way of structuring this rendering code and making it readable and if-I-got-lucky also extensible (don’t judge me, the due date was really haunting me in my sleep at the time). Finally I came up with a tree-like data structure.

What is a Renderer?

Glad you asked! It’s a class that renders stuff. The concept is reeeeeally simple, but it’s very powerful when you need to structure your code properly. The definition of the class is as simple like this

class Renderer
{
    public:
        virtual void render();
};

It’s actually more like interface. Every Renderer must implement this interface. The render() routine renders the content of the current renderer. The beauty of this concept is in the fact, that you can organize your renderers into a tree. The top-level renderer will render the object by delegating rendering of different parts to other renderers. This makes your code nicely structured as well as modular and reusable. Take for instance rendering of a car.

Cars can be visually relatively complex objects and it wouldn’t be nice to have all the rendering code in one class. If you wanted to draw a different car you’d have to write everything again even though that wheels are virtually the same in both models. But with using the renderer pattern, things would look like this

class CarRenderer : public Renderer
{
    CarBodyRenderer *body;
    WheelRenderer *wheels[4];
    WindowRenderer windshield;

    public:
        void render()
        {
            body->render();
            windshield->render();

            for (int i = 0; i < 4; i++) {
                wheels[i]->render();
            }
        }
};

class CarBodyRenderer : public Renderer
{
    SpoilerRenderer* spoiler;
    HoodRenderer* hood;
    SkeletonRenderer* skeleton;
    DoorRenderer* doors[5];
    public:
         void render();
};

I guess you get the idea. You decompose the object into a set of smaller entities and render them instead. This decomposition can go on virtually forever, in extreme cases you could be able to render a car from pixels using this design pattern and still be able to look at your code and understand it. Anyway, let me know if you find this pattern useful!

Sources