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Attributes and Effects In ARPG

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The first step to using the ability system in Action RPG (ARPG) is to enable the GameplayAbilities Plugin and then create an Attribute Set class. Abilities are only supported in a C++ game because Attribute Sets must be a C++ subclass of UAttributeSet. URPGAttributeSet defines attributes for current/max health and mana, attack and defense buffs, movement speed, and a temporary damage attribute used in the damage formula. Each of these attributes is defined as an FGameplayAttributeData structure, which stores a Base value that is only modified by permanent changes and a Current value which is modified by temporary buffs/debuffs. The class uses some macros to add boilerplate code to handle modification and replication of these attributes. Since ARPG is relatively straightforward there is only one AttributeSet however for some games it may make more sense to have a Core set that is shared by players and enemies and a Player set that inherits from Core and includes extra attributes only used by players.

Before attributes are modified, the PreAttributeChange function handles scaling the current health/mana with the max value. After attributes are modified, the PostGameplayEffectExecute function handles clamping and notifying other objects about the changes. In ARPG all characters inherit from an RPGCharacterBase class that offers Blueprint Events to handle things like taking damage. The constructor for RPGCharacterBase is responsible for creating the URPGAbilitySystemComponent and URPGAttributeSet subobjects that enables gameplay effects to work. Depending on the game it may make more sense to handle this in your game-specific AbilitySystemComponent subclass or controller. Also, you may wish to spawn AttributeSets or AbilitySystemComponents when an Actor is first interacted with to avoid object overhead.

Before damage can be applied, a character needs to have Health > 0. There are several ways to initialize default values for attributes, but for ARPG we decided to initialize them using a Stats GameplayEffect. The Stats gameplay effect is applied in  ARPGCharacterBase::AddStartupGameplayAbilities where it reads the list of PassiveGameplayEffects from the character Blueprint and applies them, at the current CharacterLevel. If CharacterLevel changes, it removes and re-adds them at the new level. Here is what the GE_StatsBase gameplay effect used for NPCs looks like inside the Unreal Engine 4 (UE4) editor: 

ARPG_AttributesAndEffects_01.png

The Instant duration means that this is applied permanently precisely one time. Then for each of the primary stats, there is an Attribute Modifier that overrides the value based on a CurveTable. StartingStats is imported from a CSV in Abilities/DataTables and has a row for each stat and a column for each level. In this case, it will look at the DefaultMaxHealth row, and the column will be CharacterLevel. The GE_PlayerStats effect inherits from this generic effect and changes all the rows to be PlayerMaxHealth and so forth. By using Curve Tables in this way, it is easy to rebalance attributes for the entire game at once, without having to modify each individual effect by hand. You can also set up scripts outside the game to create CSV or JSON files from external data sources like Excel and import those as needed.

Mana is changed using simple Modifiers with the Add operation, but to do damage, the RPGDamageExecution class is used. The execution calculations consist of two parts, a set of capture declarations and an execution function. The capture declaration macros register information with the UE4 Editor, so Gameplay Effects can use the execution in your project. For each captured attribute, the list of currently active temporary modifiers is captured along with their gameplay tags. Then in URPGDamageExecution::Execute_Implementation it applies only those modifiers that match the Gameplay Tags that were passed in at effect execution time. After combining those modifiers to get a “calculated” number for Damage, AttackPower, and DefensePower, it turns that into “final” damage using the formula SourceDamage * AttackPower / DefensePower. The final damage then turns into a Health modifier in URPGAttributeSet::PostGameplayEffectExecute. Here is what GE_DamageBase looks like:

ARPG_AttributesAndEffects_02.png

The Damage done comes from the DefaultAttack row in AttackDamage, but you can also apply a per-attack multiplier by changing the 1.0 scale to the left of the Curve Table reference. The Source/Target tags allow setting the Require/Ignore tags for filtering, in this case, the damage will not be applied if the target has the tag Status.DamageImmune. Each individual attack subclasses GE_DamageBase and modifies the tags or modifiers as needed.