reznok/gmcabilitysystem/gmas-rules

Behavior below was verified against GMC 2.3.x and GMAS 1.4. Confirm signatures in this project's own headers (paths are relative to each plugin's root, e.g. Source/GMCCore/Public/Components/GMCReplicationComponent.h); GMC's source is not part of this plugin.

GMAS (GMC Ability System) is an ability system built on GMC's predicted move cycle instead of on RPCs. UGMC_AbilitySystemComponent (Source/GMCAbilitySystem/Public/Components/GMCAbilityComponent.h, "the ASC") binds its attributes, tags and operation queue into the pawn's move, so abilities and effects are predicted, validated and replayed by the same machinery as movement. Read gmas:gmc-prediction first if moves, replays and bound values are new to you; this skill holds the rules every other gmas:* skill links to. GMAS paths below are relative to Source/GMCAbilitySystem/.

How GMAS rides GMC

The ASC is not a GMC component and does nothing on its own: the pawn's movement component (UGMC_MovementUtilityCmp or a subclass) forwards five of its hooks to it.

void UMyMovementCmp::BindReplicationData_Implementation()
{
    Super::BindReplicationData_Implementation();
    BindSinglePrecisionFloat(DashCooldown, /* modes */);      // the game's own values first
    AbilitySystem->BindReplicationData();                     // GMAS binds LAST
}
 
void UMyMovementCmp::GenPredictionTick_Implementation(float DeltaTime)
{
    Super::GenPredictionTick_Implementation(DeltaTime);
    AbilitySystem->MatchTagToBool(StunnedTag, StunTimer > 0.f); // bound tag follows bound state
    AbilitySystem->GenPredictionTick(DeltaTime);               // then abilities, effects, attributes
}
 
void UMyMovementCmp::GenAncillaryTick_Implementation(float DeltaTime, bool bLocalMove, bool bCombinedClientMove)
{
    Super::GenAncillaryTick_Implementation(DeltaTime, bLocalMove, bCombinedClientMove);
    AbilitySystem->GenAncillaryTick(DeltaTime, bCombinedClientMove);
}

GenSimulationTick and PreLocalMoveExecution forward the same way. The ASC binds last by convention: it reads the movement component and binds a tag-sorted, variable-length block (one pair of floats per bound attribute, plus containers) after the game's fixed ones. The order of the ASC call relative to the game's own logic inside each hook is a design choice; keep it identical on every machine.

Where does this state live?

Decide this before writing a UPROPERTY. "Predicted logic" is anything that runs inside GenPredictionTick on either side: movement, ability ticks, effect ticks, activation checks.

Two rules follow:

• Never `DOREPLIFETIME` (or make an unbound attribute of) something predicted logic reads. A replicated property arrives between moves, is never restored by a replay, and gives client and server two different values for the same move: every read is a correction waiting to happen.
• Never bind something only the HUD or a cosmetic reads. A bound value costs bandwidth in every move and state, splits combined moves whenever it changes (CombineIfUnchanged), and corrects the client every time the two sides disagree about a number nobody acts on. Replicate it, or derive it from bound values on the display side.

Worked example, UMyAbility_Dash on AMyPawn:

Attributes

Attributes are declared in UGMCAttributesData assets (Public/Attributes/GMCAttributesData.h, FAttributeData) listed in the ASC's AttributeDataAssets; the runtime struct is FAttribute (Public/Attributes/GMCAttributes.h).

• Data before binding. BindReplicationData() reads AttributeDataAssets and binds the attributes sorted by tag. Fill AttributeDataAssets (and StartingEffects, StartingAbilities, StartingTags) in the constructor, in class defaults, or in PostInitializeComponents before the call that runs the GMC bind, identically on every machine: a different attribute list on client and server corrupts every state they exchange. Initial values can be overridden per attribute in SetAttributeInitialValue / OnInitializeAttributeInitialValue.
• Change bound attributes inside the move, on both sides. Through effects (gmas:gmas-effect) or ApplyAbilityAttributeModifier(const FGMCAttributeModifier&) (Op Add, Set, SetReplace (1.4+), …) called from predicted logic on the owning client and the server alike; the result is validated like any bound value. Unbound attributes change on the server only.
• `SetAttributeValueByTag(Tag, NewValue, bResetModifiers)` is deprecated and inert (removed in 1.5). It changes nothing; 1.4.0 returned true, 1.4.1+ returns false and logs once. Do not use it; initial values come from the data asset or SetAttributeInitialValue, changes from a Set modifier.
• Read with GetAttributeValueByTag (with temporal modifiers) or GetAttributeRawValue (permanent part only). OnAttributeChanged(Tag, Old, New) fires from the ancillary and simulation ticks once per real change, on every machine: the hook for HUD and cosmetics, never for gameplay decisions.

Abilities under replay