In this lesson, you will learn how to ensure that locally predicted gameplay effects stay consistent with the server’s final authority during attribute modifications.
The Problem: Prediction Divergence
When you trigger a GameplayEffect locally within an ability, the client applies the effect immediately to keep the game feeling responsive. However, the server also runs the logic independently. If the server applies the effect and calculates a different result (due to latency or external modifiers), the client’s local attribute value will "snap" or jitter when the server sends the definitive state update. This is where Prediction Windows become critical.
Ensuring Synchronization
When you use MakeEffectContext inside an ability, you are already creating a foundation for prediction. To synchronize your effects, the UGameplayAbility must have the AbilityTask context that supports prediction. When you call ApplyGameplayEffectToTarget (or ToSelf) within an ability that was activated via a prediction-supported path, GAS creates a Prediction Key.
GAS keeps track of this key to "replay" or reconcile your changes. If the server verifies the effect application, the client acknowledges that the prediction was successful. If the values differ, GAS automatically rolls back the local attribute to match the server’s authoritative result.
Implementation Pattern
Ensure your effect application happens within the ActivateAbility flow, which is wrapped in a prediction scope.
// Inside your Gameplay Ability
void UMyAbility::ActivateAbility(...)
{
// The activation itself is predicted; subsequent effects
// triggered here are automatically linked to the prediction window.
FGameplayEffectSpecHandle SpecHandle = MakeOutgoingGameplayEffectSpec(EffectClass, 1.0f);
if (SpecHandle.IsValid())
{
// Because this is inside the ability's activation scope,
// GAS handles the synchronization key for this effect application.
ApplyGameplayEffectSpecToOwner(SpecHandle);
}
}
By keeping your effect application strictly inside the ActivateAbility logic (and avoiding side effects outside of the ability's scope), you allow GAS to manage the reconciliation process automatically, preventing "rubber-banding" of your character's health, stamina, or mana values.
Key Takeaway: By triggering Gameplay Effects directly within the ability’s activation scope, you enable GAS to use prediction keys to seamlessly reconcile local attribute changes with the authoritative server state.