Object system unification

[zackthehuman]

Summary

The current object system discourages code reuse because it is overly specific. The system should be simplified.

Tasks

• Create new id system, where IDs can be used in place of smart pointers
• Create new handle system, used in combination with the id system
• Update GameWorld to issue Handles and support lifecycle (Update GameWorld to support object Handles and Id versioning #244)
• Generalize Projectile into a GameObject + remove Projectile class (Generalize Projectile into a GameObject + remove Projectile class #245)
• Create an GODefinition object, which stores "type" information for different objects (Create an GODefinition object, which stores "type" information for different objects #246)
• Generalize "behaviors" and "brains" into a common concept (Generalize "behaviors" for all objects #248)

Description

I think that enough time has passed, and requirements have changed enough to necessitate an overhaul of the object system. There are very minute differences between the subclasses of Entity. The difference between a GameObject and an Entity is also questionable since there is basically no application of plain GameObject besides lookups, which then get casted.

By unifying the objects in the system with a simpler definition, it should make it easier to handle the kinds of features we haven't been able to implement due to complexity of interaction between objects.

At a high level, the notion of classes separating Enemies, Items, Projectiles, etc. no longer makes sense because most of these object types need to do the same kind of things.

There should just be "GameObject".

Additionally, scripting via Squirrel may not be necessary, and possibly can be replaced by behavior trees with special hooks for certain events (onDeath, onBirth, onParentDeath, etc).

Object Types / Attributes

Attribute	Item	Enemy	Particle	Projectile	Spawner
`id`	✓	✓	✓	✓	✓
`active` flag	✓	✓	✓	✓	✓
`onActivated` hook	✓	✓	✓	✓	✓
`onDeactivated` hook	✓	✓	✓	✓	✓
`onBirth` hook	✓	✘	✘	✓	✘
`onDeath` hook	✓	✘	✘	✓	✘
`onWake` hook	✓	✘	✘	✓	✓
`onSleep` hook	✓	✘	✘	✓	✓
`update` hook	✓	✓	✓	✓	✓
`reset` hook	✓	✓	✓	✓	✓
behavior	✓	✓	✘	✓	✘

A simple type of base object without subclasses should be able to cover all the cases we see in the game. "Optional" parts of an object can just be pointers to other structures. Overall the current system is over-engineered to its own detriment.

Definition

What is a GameObject, then? A GameObject is any object that can appear on-screen within the game world. For example:

• Enemies
• Items (drops, collectibles, etc.)
• Particles
• Projectiles

This means the following things are not GameObjects:

• Forces (wind, magnets, conveyors, etc.)
• Spawners
• Block Sequencers