Gameplay Ability System for Unity (or GAS for short) is a Unity framework for creating games which contain any kind of abilities (e.g. DotA, Skyrim). The framework helps to simplify the interaction between components in a unified manner.
This project is heavily inspired by the Unreal Gameplay Ability System, and uses similar terminology, but the implementation is specific to Unity.
There are three main sub-projects here (TODO: Unity UPM package list):
- Attribute System
- Gameplay Tags
- Ability System and Gameplay Effects
- 1. Unity Gameplay Ability System
The Attribute System is a collection of code for defining attributes specific to a game (e.g. Strength, Health, etc.). It also manages how these attributes mutate over the course of a game - for example, taking damage reduces health, or levelling up increases strength.
Attributes have a Base Value and a Current Value, and currently active modifiers which may be additive, multiplicative, or overriding.
The Current Value is calculated as:
Current Value = (Base Value + Additive) * Multiplicative
Attribute assets are created through the Create/Gameplay Abiliy System/Attribute asset menu.
The behaviour of the default AttributeScriptableObject class can be modified by creating a child class derived from AttributeScriptableObject. There is an in-built child class included in this sample (LinearDerivedAttributeScriptableObject) which demonstrates how to create "derived attributes" - for example, a "Max Health" attribute which is linearly related to a "Strength" attribute.
This project contains the following attributes (see Assets/My Gameplay Ability System/Attributes/Base):
- Strength
- Agility
- Intelligence
All other attributes derive from these, using instances of LinearDerivedAttributeScriptableObject (see Assets/My Gameplay Ability System/Attributes/Derived):
- Armour
- Attack Speed
- Health Regen
- Mana Regen
- Max Health
- Max Mana
In addition, there are also Health and Mana attributes for tracking the current health.
Gameplay Tags are a way of assigning tags to objects in the Ability System. They're similar in concept to the way default Unity tags work, but the implementation here is using Scriptable Objects.
A parent can be defined for each tag, allowing you to create hierarchical gameplay tags.
Gameplay Tags assets are created through the Create/Gameplay Abiliy System/Tag asset menu.
The Ability System is the most complex of the three, and ties together the Attribute System and Gameplay Tags.
The Ability System allows you to define how abilities in the game work, and how they interact with characters, characters' attributes, and each other.
Abilities, at some point of their execution, create Gameplay Effects, which are applied to a character, resulting in a change to the character's attributes.
Gameplay Effects (GE) are at the heart of the ability system. They are created through the Create/Gameplay Ability System/Gameplay Effect Definition asset menu.
When creating a new GE, the following data needs to be defined:
The duration policy controls how the GE is applied, and whether it needs to be removed at a later point in time (e.g. a buff/debuff). The Duration Policy can be one of:
- Instant
- Infinite
- Durational
Instant GE affect an attribute's Base Value. Infinite and Duration GE are similar to Instant GE, but they modify an attribute's Current Value, and references to the effect are stored so the effect can be reverted. As their name implies, Infinite GE remain applied indefinitely, whereas Duration GE are applied for a set duration, and then automatically expire, reverting their change (like a temporary buff or debuff).
Generally, you would use Instant GE for base stats, Infinite GE for pseudo-temporary stat changes, such as extra damage from equipping a weapon, and Durational GE for timed duration stat changes.
The Duration Modifier and Duration Multiplier property controls the base value of the duration (in seconds). This is only applicable for Durational GE:
Duration = Duration Modifier * Duration Multiplier
A custom scriptable object inheriting from ModifierMagnitudeScriptableObject can be used to create new behaviours, such as using another attribute on the character as a base - see the AttributeBackedModifierMagnitude class, and it's instantiated Scriptable Objects MaxHealthAttributeBackedModifier and MaxManaAttributeBackedModifier in Asset/My Gameplay Ability System/Ability System/Gameplay Effects/Modifiers. These particular modifiers are used to initialise the Health and Mana attributes to the same value as that of the Max Health and Max Mana attributes, respectively.
The Modifiers collection defines what attributes this GE modifies, and how it modifies them.
The Attribute property defines what attributes this modifier affects. The Modifier Operator property, which can be one of Add, Multiply, or Override, determines how this modifier interacts with other modifiers on this attribute. The Modifier Magnitude property is used to calculate the magnitude of the effect, which is then multiplied by the Multiplier to give the final modification value:
Effect Magnitude = Modifier Magnitude * Multiplier
WIP
Tags are used to describe the GE, as well as dictate how it interacts with other GEs.
The Asset Tag is used to identify (usually uniquely, but not always) the GE. The intent is instead of checking if the GE is an instance of some class, we can compare asset tags to determine if any two GE are the same.
These tags are added to the character while the GE is applied.
These tags determine if the GE is active or temporarily disabled. For a GE to remain active, all tags in the Require Tags collection must be present on the character, and none of the tags in the Ignore Tags must be present.
These tags determine if the GE can be applied or not. Since Instant GE are never applied, these tags only affect Infinite and Durational GE. For an Infinite or Durational GE to be successfully applied, all tags in the Require Tags must be present on the character, and none of the tags in the Ignore Tags must be present.
These tags determine if the GE should be removed prematurely after application. Since Instant GE are never applied, these tags only affect Infinite and Durational GE. For an Infinite or Durational GE to be removed, all tags in the Require Tags must be present on the character, and none of the tags in the Ignore Tags must be present.
Any existing GE on the character which have an Asset Tag contained in this list are prematurely removed.
WIP
You can contribute to this project by:
- Posting issues
- Creating PRs with bug fixes or new features
- Testing this in your own games and providing feedback
- Adding to the Wiki
- Helping with documentation
- Telling your friends!
Gameplay Ability System for Unity is release under the MIT license. You are free to use, modify, and distribute this software, as long as the copyright header is left intact.
Thanks to @tranek for providing a comprehensive overview of the functionality of Unreal's Gameplay Ability System (https://github.com/tranek/GASDocumentation). This resource is used heavily in understanding how Unreal's Gameplay Ability System works, so it can be recreated for Unity.