Now that all of the abilities have been created, the last step is actually to execute them. This part will be extremely game specific, and the Action RPG (ARPG) implementation is just one example of how to do this. ARPG uses two methods: Tags and Item Slots. Skills are activated via tags, with the general Engine function ActivateAbilitiesWithTags. This works because each skill ability has Ability.Skill in the AbilityTags. When equipping a Skill item, it ends up calling AddSlottedGameplayAbilities, which adds the ability for that skill to the AbilitySystemComponent.When calling ActivateAbilitiesWithTags, it looks for any abilities tagged with Ability.Skill and executes it.
Weapon and potion abilities are executed using an ARPG specific function, ActivateAbilitiesWithItemSlot. When abilities are added in AddSlottedGameplayAbilities, it stores their FGameplayAbilitySpecHandle into a map indexed by Item Slot. Abilities end up in item slots via two different ways for enemies and players: For enemies, they get added from the DefaultSlottedAbilities array, while for players they get filled in from the player’s actual inventory. There is some reasonably complicated logic in ARPGCharacterBase to handle adding/removing the right abilities after an inventory or character level change. But, giving the enemies a “fake inventory” enables putting most of the ability logic in a shared base class for both enemy and player characters. The IRPGInventoryInterface native interface is used to allow the base class to acquire its slot information from any source, so if we wanted to go back and give NPCs “real inventory” it would not require changing the ARPGCharacterBase code at all.
The other part of executing abilities is interacting with the rest of the movement and gameplay systems. For ARPG this is mostly handled via a set of functions like CanUseAnyAbility on the BP_Character Blueprint. These functions check for global state or currently active abilities and will prevent the player or AI from executing new abilities or doing other things while they should be busy or locked down. Every game will need to implement their own rules for allowing abilities to execute or not, ActivationRequiredTags and ActivationBlockedTags on ability are often beneficial for this. Finally, the UI will need to query the ability system for execution information. Here is an example from WB_OnSCreenControls where it uses GetCooldownRemainingForTag to display the cooldown timer after using a skill:
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