Decoupling Patterns

The Cardinal Rule - no Crimson plugin depends on another, except CrimsonCommon - is enforced by four patterns. Every cross-plugin link in the suite is one of these. The shared piece always lives in CrimsonCommon; the two plugins never include each other.

1. Interface bridge (primary)

Define an interface in CrimsonCommon. Plugin A holds a reference and casts to the interface; plugin B's type implements it. Neither knows the other's concrete types. The ability-set bridge is the canonical example - note the implementer owns its ASC and grants on it directly (there is no GetAbilitySystemComponent() on the interface).

// CrimsonCommon defines the contract:
class ICrimsonAbilitySystemOwner
{
public:
    virtual void GrantAbilitySet(UCrimsonAbilitySet* Set, UObject* SourceObject,
                                 FCrimsonAbilitySet_GrantedHandles* OutHandles) = 0;
    virtual void RemoveGrantedAbilitySet(FCrimsonAbilitySet_GrantedHandles& Handles) = 0;
};

// CrimsonAbilitySystem: UCrimsonAbilitySystemComponent implements it.
// CrimsonEquipment: uses it with zero CrimsonAbilitySystem includes -
Cast<ICrimsonAbilitySystemOwner>(OwnerActor)->GrantAbilitySet(Set, Item, &Handles);

2. Epic standard interfaces

When Unreal already defines the contract, use it. CrimsonInput discovers an ASC through Epic's IAbilitySystemInterface::GetAbilitySystemComponent() - so it binds ability input with only the GameplayAbilities engine module, and no dependency on CrimsonAbilitySystem.

3. GameplayMessageSubsystem

For fire-and-forget events, broadcast a struct on a tag channel via UCrimsonMessageSubsystem. Receivers subscribe independently; sender and receiver never reference each other. See How-To: Broadcast & Listen for Messages.

4. Independent save support

Persistence is data-in, data-out. A plugin exposes a plain save struct (getter/setter) and never depends on CrimsonSaveSystem or GameplayStatics. Each plugin that wants saving implements ICrimsonSaveableSystem (which lives in CrimsonSaveSystem, not here) on its own terms - no plugin is aware of another's save fragment.