In this lesson, you will learn how to leverage Unreal’s Gameplay Ability System (GAS) prediction mechanisms to ensure that abilities feel responsive to the local player while maintaining authoritative consistency across the network.
The Challenge of Latency
In a multiplayer environment, waiting for the server to acknowledge an input before playing an animation or effect causes "input lag." If you press a key to fire a spell, you expect the visual feedback to happen immediately. GAS solves this through Prediction. When you trigger an ability on the client, GAS executes the logic locally and assigns a "prediction key." This key acts as a bookmark, allowing the server to reconcile the client’s actions once the network packet eventually arrives.
Implementing Predictive Ability Tasks
To synchronize execution, we use AbilityTasks—specifically those that support prediction, like WaitTargetData or PlayMontageAndWait. When an ability is activated, the GameplayAbility class automatically manages the prediction window.
Consider a simple ability that triggers a projectile:
void UMyGameplayAbility::ActivateAbility(...)
{
// The task handles the logic locally while the server validates
UAbilityTask_PlayMontageAndWait* Task = UAbilityTask_PlayMontageAndWait::CreatePlayMontageAndWaitProxy(
this, NAME_None, MontageToPlay);
Task->OnCompleted.AddDynamic(this, &UMyGameplayAbility::OnCompleted);
Task->ReadyForActivation();
}
Why Prediction Matters
When the client calls ReadyForActivation(), the AbilityTask attempts to run the logic immediately. If the server eventually validates that the ability was allowed (based on your tags, cooldowns, and costs), the state remains synchronized. If the server denies the action (e.g., you were stunned at the exact moment of activation), GAS automatically "rolls back" the client-side state to match the server’s authoritative truth, preventing cheating and state divergence.
By using built-in AbilityTasks, you offload the complex bookkeeping of state synchronization to the engine. You don't need to manually replicate variables for every ability interaction; instead, you rely on the ability’s lifecycle to handle the local prediction and subsequent server-side validation.
Key Takeaway: Use built-in AbilityTasks to trigger gameplay actions, as they automatically handle prediction keys to mask latency while ensuring the server remains the authoritative source of truth.