12 September 2011

MVC for games

Second thoughts on MVC (following a first article about the use of MVC by game developers). Let's assume that the user inputs are first caught by a controller, and a view is displayed to the user at the end. What are the communication possibilities in-between?

Vanilla MVC

As seen on Wikipedia, the theoretical 'vanilla' MVC refers to an architecture where the controller updates the model, then notifies the view, which in turn fetches the data to display from the model.

Application for games: the system is event-based: it is only awakened by the actions of the user. If the world has NPCs, they can not act independently of the user. It may be OK for turn-based games like Chess, but not so much for games with living worlds like Morrowind or Harvest Moon.


In web development, the "logic" is often split into multiple controllers. To determine which controller the user's action is targeted to, web frameworks such as ASP.NET or Spring use a front controller pattern. The front controller maps URLs to actions to execute (and which controllers to call). I'm not entirely sure, but compared to the vanilla MVC, it seems that the (front) controller is now giving the model to the view, instead of having the view fetching the model by itself. Therefore, the model to be displayed is carried from M to C, and from C to V, twice more data going around than in vanilla MVC (where it used to be moved only from M to V).

In games, the model can get quite large, and moving it around can get costly. It would not be so costly if only resulting differences to the model (instead of the whole model) were exchanged, but it depends on the modernity of the rendering. Alpha compositing makes it possible to render variations of the game state (ie the model) by layering image elements. Many games/game frameworks such as pygame still use bit blitting, which requires to erase and reprint the whole screen.

Game MVC

This nice article about MVC for games explains:

  • The controller handles the input and flow of the game logic. This is akin to the states your game can be in. My primary two controllers are main menu controller and an in-game controller. They are responsible for converting input into something the game world (model) can understand. Eg "Create unit x at base y".
  • The model handles all game logic. It has nothing to do with input, rendering or networking. It is a pure view into the game world. Designers need only worry about what’s in the model.
  • The view DOES know about the model. The [view] has read-only access to the model. This model can be anything. For my main menu it’s a list of UI controls the controller has built. For the in game view, it’s the game world. Rather than "reusing views in different controllers" what is more important is "having multiple views on a single controller". For example there might be a 2D and a 3D view of the game world.

What is still missing from the picture is the game loop. In games, the model is rendered by the view 60 times per second, and the inputs of the user are processed at the same frequency. As mentioned in the quotes above, some component, be it a controller or something else, still has to determine which view should be active at any given frame, and which controller it is associated with.


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