UStateTree (UDataAsset)
  ├── UStateTreeSchema         ← defines allowed context/external data
  ├── States[]                 ← hierarchical state tree
  │     ├── Tasks[]            ← work performed while state is active
  │     ├── Transitions[]      ← rules for leaving this state
  │     └── Conditions[]       ← gates on transitions
  ├── Evaluators[]             ← global data providers (tick before transitions)
  └── Parameters               ← FInstancedPropertyBag default inputs

FStateTreeInstanceData InstanceData;  // persists across frames
// Each tick:
FStateTreeExecutionContext Context(Owner, *StateTree, InstanceData);
Context.Tick(DeltaTime);

UCLASS()
class UMyGameSchema : public UStateTreeSchema
{
    GENERATED_BODY()
public:
    virtual bool IsStructAllowed(const UScriptStruct* InStruct) const override;
    virtual bool IsExternalItemAllowed(const UStruct& InStruct) const override;
    virtual TConstArrayView<FStateTreeExternalDataDesc> GetContextDataDescs() const override;

#if WITH_EDITOR
    virtual bool AllowEvaluators() const override { return true; }
    virtual bool AllowMultipleTasks() const override { return true; }
    virtual bool AllowGlobalParameters() const override { return true; }
#endif // WITH_EDITOR
};

USTRUCT()
struct FMyTaskInstanceData
{
    GENERATED_BODY()
    float ElapsedTime = 0.f;
};

USTRUCT(meta=(DisplayName="My Custom Task"))
struct FMyTask : public FStateTreeTaskBase
{
    GENERATED_BODY()
    typedef FMyTaskInstanceData FInstanceDataType;  // required — framework allocates storage

    virtual EStateTreeRunStatus EnterState(FStateTreeExecutionContext& Context,
        const FStateTreeTransitionResult& Transition) const override
    {
        FInstanceDataType& Data = Context.GetInstanceData(*this);
        Data.ElapsedTime = 0.f;
        return EStateTreeRunStatus::Running;
    }

    virtual EStateTreeRunStatus Tick(FStateTreeExecutionContext& Context,
        float DeltaTime) const override
    {
        FInstanceDataType& Data = Context.GetInstanceData(*this);
        Data.ElapsedTime += DeltaTime;
        return Data.ElapsedTime >= Duration
            ? EStateTreeRunStatus::Succeeded : EStateTreeRunStatus::Running;
    }

    UPROPERTY(EditAnywhere, Category = "Parameter")
    float Duration = 2.0f;
};

USTRUCT(meta=(Hidden))
struct FStateTreeConditionBase : public FStateTreeNodeBase
{
    virtual bool TestCondition(FStateTreeExecutionContext& Context) const;  // default: false
    EStateTreeExpressionOperand Operand = EStateTreeExpressionOperand::And;
    int8 DeltaIndent = 0;  // indent level for logical grouping
    EStateTreeConditionEvaluationMode EvaluationMode = EStateTreeConditionEvaluationMode::Evaluated;
};

USTRUCT(meta=(Hidden))
struct FStateTreeEvaluatorBase : public FStateTreeNodeBase
{
    virtual void TreeStart(FStateTreeExecutionContext& Context) const;
    virtual void TreeStop(FStateTreeExecutionContext& Context) const;
    virtual void Tick(FStateTreeExecutionContext& Context, float DeltaTime) const;
    // Note: DeltaTime is 0 during preselection
};

// From outside via UStateTreeComponent
TreeComp->SendStateTreeEvent(FGameplayTag::RequestGameplayTag("AI.Alert"));

// With payload
FMyAlertData AlertData;
AlertData.ThreatLevel = 5;
TreeComp->SendStateTreeEvent(
    FGameplayTag::RequestGameplayTag("AI.Alert"),
    FConstStructView::Make(AlertData), TEXT("PerceptionSystem"));

// From inside a task via execution context
Context.SendEvent(Tag, FConstStructView::Make(ResultData), TEXT("MyTask"));

// Declare handles in your task/condition/evaluator struct
TStateTreeExternalDataHandle<FMyActorContext, EStateTreeExternalDataRequirement::Required> ActorHandle;
TStateTreeExternalDataHandle<FMySubsystemContext, EStateTreeExternalDataRequirement::Optional> SubsystemHandle;

// Link in the Link override
virtual bool Link(FStateTreeLinker& Linker) override
{
    Linker.LinkExternalData(ActorHandle);
    Linker.LinkExternalData(SubsystemHandle);
    return true;
}

// Access at runtime
auto& ActorCtx = Context.GetExternalData(ActorHandle);       // Required — reference
auto* SubsystemCtx = Context.GetExternalDataPtr(SubsystemHandle);  // Optional — pointer

void SetStateTree(UStateTree* NewStateTree);
void SetStateTreeReference(FStateTreeReference NewStateTreeRef);
void SendStateTreeEvent(const FStateTreeEvent& Event);
void SendStateTreeEvent(FGameplayTag Tag, FConstStructView Payload, FName Origin);
EStateTreeRunStatus GetStateTreeRunStatus() const;
// Delegate — fires when run status changes
FStateTreeRunStatusChanged OnStateTreeRunStatusChanged;  // BlueprintAssignable
bool bStartLogicAutomatically = true;  // EditAnywhere

FStateTreeReference TreeRef;
TreeRef.SetStateTree(MyStateTreeAsset);
TreeRef.SyncParameters();
TreeRef.GetParameters().SetValueFloat(TEXT("AggroRange"), 1500.f);

Overrides.AddOverride(Tag_CombatVariant, CombatTreeRef);
Overrides.RemoveOverride(Tag_CombatVariant);

AMyAIController::AMyAIController()
{
    StateTreeComp = CreateDefaultSubobject<UStateTreeComponent>(TEXT("StateTree"));
    StateTreeComp->bStartLogicAutomatically = true;
}

// WRONG — context is per-tick, NOT persistent
FStateTreeExecutionContext* SavedContext;  // dangling after tick

// RIGHT — reconstruct each tick from persistent instance data
FStateTreeExecutionContext Context(Owner, *StateTree, InstanceData);
Context.Tick(DeltaTime);

// WRONG — task structs are const at runtime
virtual EStateTreeRunStatus Tick(...) const override {
    Timer += DeltaTime;  // compile error — 'this' is const
}

// RIGHT — use FInstanceDataType typedef
typedef FMyInstanceData FInstanceDataType;
virtual EStateTreeRunStatus Tick(...) const override {
    auto& Data = Context.GetInstanceData(*this);
    Data.Timer += DeltaTime;
}