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GameMaker tutorial: Elegant instance_change() in your state machine

In GameMaker, a commonly used technique is to build a system made up of a several objects to represent an entity in your game, such as the player or enemy, in various states, such as idle, dead, shooting, jumping, running, climbing, and so forth. This is what is known as a Finite State Machine pattern.

When the time is right in the game, we change an instance from one state object to another by using the powerful instance_change() function. Instance_change() takes the instance and transforms it into a new type of object. Its Event behaviors will change to those defined by the new object type, but its old properties (object variables) will remain the same as before, allowing the instance to retain its variables with their current values.

The instance_change() function takes two arguments: object, the object the instance will turn into, and perform_events, a boolean which controls whether the new object’s Create event will be performed or not.

Normally, the Create event is where an object initializes its variables and initiates its default behavior. When we’re dealing with a State Machine comprised of a number of objects, this can become problematic, however. Some code in the Create Event is initialization code that we may only want to execute one time, to set up the instance when a brand new instance is created, while other code in the Create event is behavioral and we may need to execute whenever an existing instance reverts back into that state again. Thus, the perform_events argument in the instance_change() function isn’t adequate for this situation — it’s too all or nothing.

For example, let’s say I have a generic object for an enemy, oEnemy. I want some visual variety to this enemy, so I’ve created a few different sprites for it. In the Create Event, I want to randomly choose one of those sprites to be the sprite for this instance. But if the instance changes into another state object, and then reverts back, if I call the Create Event, it will randomly choose a new sprite. I don’t want this, as it ruins the illusion of continuity — I need that instance to retain its sprite. But I do need the Create Event to run, whenever it re-enters this state, because I’m using it to set the instance in motion.

So, how can I elegantly select which lines of code I want to run in the Create Event?

Conditional blocks

This is the least elegant solution, but you could use if to check whether a variable exists or has a value. For example:

if sprite_index == -1 {sprite_index = choose(sprite1, sprite2, sprite3);}

This is inelegant because it adds lots of lines of code that only need to be run one time (when a brand new instance is created) but need to be checked potentially many times (any time that instance changes back into the object state). It also only checks certain, specific things, case by case. As I continue to build the state machine, I may end up introducing more features which require initialization, which would necessitate more checks, further bloating the code. I always want to write the least amount of code needed, both for reasons of performance and maintainability.

Move one-time code to an init state object.

The more elegant solution is to recognize that initialization is its own state, and we need to separate it out from the other states in the state machine. We can create an oEnemy_init object, put our one-time initialization code into it, and then the final step in the Create Event for the init object would be to change the object into the default state.

None of the other states in the state machine should put the instance back into the init state, thereby guaranteeing that the init code only executes once. Now your code is neatly separated, your states objects in your state machine are as simple as can be.

Game Maker Wave Motion Tutorial

Following up on my motion and position tutorial, I present a tutorial on wave-motion. This was something I wanted to include in the original article, but I realized that there’s enough complexity to this concept that it merited its own separate article.

Wave Motion

Wavelike motion is any motion that involves periodic oscillation, not just linear undulating motion. (Other types of wavelike motion include pulsing and concentric ripples, for example.) But we’ll talk mostly if not exclusively about linear undulation, since it is easiest to understand, simplest to implement, and the basis for many others.

(more…)

GameMaker Position and Motion Tutorial

Motion is critical to just about any video game. Nearly every game has moving things in it, and how they move is a vital part of the game. Learning how to program motion and control it effectively is one of the most important parts of a successful game. There are a number of possible approaches to handling position and movement. Learning how these work will help you make better games.

This isn’t absolutely everything there is to know about motion, but it’s a great overview to start with, and covers everything I’ve learned with respect to motion in GM:S.  (more…)

Tutorial: Building a Game Maker Extension

[Update 1/13/2014: See the official GameMaker documentation and this MSDN blog entry for how to build GM Extensions directly inside of GM:Studio. The GM Extension Builder tool that I explain how to use in this article will build GEX that can work with GM 7, 8, 8.1, and GM:Studio. As long as the GML used in the extension is compatible with the version of GameMaker that the extension is added to, the extension should work. ]

One of the nice things about Game Maker is that it is extensible. Developers can make their code more re-usable by converting their GML scripts to Game Maker Extensions. Once the .gml code is packaged in a .gex extension, you can import the extension into Game Maker and use the functions it provides in any project with ease. This means over time you can build up an entire library of re-usable functions that you can bring into your projects, saving you time and allowing you to focus on building new stuff instead of re-implementing the same basic things again and again. (more…)

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