On Game Loops

Hi all!

Today I want to talk to you about something that is a must-know in the world of game programming: the game loop.

The game loop is the sequence of actions that are run by the machine during the execution of the game and that are repeated until the game finishes. In C++-style, the game loop looks something like this:

while (!finished()) {

      finished = getInput(&input);

      update(input);

      draw();

      clearScreen(); 

}

The first thing the game does is to check the input provided by the player: keyboard presses/releases, screen touches, etc. Then, it updates, probably according to the input, the world: player, enemies, platforms, score and other variables, collisions, etc. Then, it draws (or more technically, it renders) the scene onto the screen, and then it clears out the screen to allow the rendering of the next loop execution, also known as frame. 

I think that (almost) any game in the world can be organized by using this structure. How explicit this structure is from the viewpoint of the developer, is, however, another story, and it depends on the level of abstraction that the developer is using. 

For example, in the game I'm developing for PC/Mac (that famous platform game I mentioned in earlier posts), I began almost from scratch. Therefore, all my class hierarchy and methods are set so as to follow the game loop structure, and the actual code for my main method matches almost perfectly the previous one.

However, I've recently started developing a small, puzzle game for iOS (more in future posts), and for this, I'm using the Cocos2D framework. When you use high-level libraries or frameworks, many of the gory details are hidden in order to abstract away unnecessary complexities and let the developer focus on the gameplay. In the case of frameworks, furthermore, it is usual the so-called inversion of control. This means that the framework takes the role of the main routine, and is the framework that is in charge of calling certain components/classes developed by the programmer, taking a leading role.

I'll give you a very simple concrete example: Cocos2D uses the notion of Node (CCNode class) and Layer (CCNode layer). Layers can contain nodes, and nodes can be rendered. Therefore you can write something like [layer addChild: node], which adds a node to a layer. Once you've added the node to a layer, the framework itself ensures that the draw method of the node is called every frame, so you cannot see the explicit call to this method because it is hidden inside the framework implementation and it's not exposed as an API (actually, you can override the draw method, but this is an advanced feature for more experienced developers). 

The moral is this: even when you don't see the game loop, you must be aware that it's there, observing each of your movements... So be aware of it! Once you really understand the game loop and concept of frame, everything becomes nicer and easier.

See you!