Sub Anim Input Issue in UE 4.23, part II

As the previous part mentioned, 4.23 has brought some issue about the Sub Anim Instance node.

The Key Issue

The ultimate cause of this issue is about FAnimNode_SubInput::Initialize_AnyThread:

void FAnimNode_SubInput::Initialize_AnyThread(const FAnimationInitializeContext& Context)
{
     if(InputProxy)
     {
          FAnimationInitializeContext InputContext(InputProxy);
          InputPose.Initialize(InputContext);
     }
}

The InputPose of the FAnimNode_SubInput is the link to another Anim Graph.

And the old version is:

void FAnimNode_SubInput::Initialize_AnyThread(const FAnimationInitializeContext& Context)
{
     FAnimNode_Base::Initialize_AnyThread(Context);
}

As a result, while InitializeAnimation function get called, the whole animation blueprint get initialized from the root node.

As for initialization of the Sub Anim Instance node, it would traverse every node in the sub animation graph, and initialize them(of course, begin from its root node).

But, instead of stopping at the Sub Input node, like the old version. Now the node would try to call the Initialize_AnyThread function of the outer graph. And that’s the point.

Why is StateMachine Node Broken?

Let us check out FAnimInstanceProxy::InitializeRootNode again:

if(AnimClassInterface)
    {
        // cache any state machine descriptions we have
        for (UStructProperty* Property : AnimClassInterface->GetAnimNodeProperties())
        {

            if (Property->Struct->IsChildOf(FAnimNode_Base::StaticStruct()))
            {
                FAnimNode_Base* AnimNode = Property->ContainerPtrToValuePtr<FAnimNode_Base>(AnimInstanceObject);
                if (AnimNode)
                {
                    InitializeNode(AnimNode);

                    if (Property->Struct->IsChildOf(FAnimNode_StateMachine::StaticStruct()))
                    {
                    	FAnimNode_StateMachine* StateMachine = static_cast<FAnimNode_StateMachine*>(AnimNode);
                    	StateMachine->CacheMachineDescription(AnimClassInterface);
                    }
                }
            }
        }
    ...

When a `FAnimInstanceProxy initialize itself, it traverses its AnimNodeProperties and try to initialize this node. But the point is:

ORDER IS TRULY IMPORTANT!!!

But how are those node properties sorted? The answer is surprisingly simple: they are sorted by the creating order.

On the other hand, we came to know that a state machine description is cached after initialization.

Imagine if you create the Sub Anim Instance relatively earlier than your other State Machine nodes. And here comes the problem: Sub Anim Instance initialization may call initialize the State Machine node, which might not get cached yet!

So it would be a solution to cache state machine description in advance. And in fact, this is what epic do in the future release.

Of course, if you delete this node, and re-create a Sub Anim Instance node. It would work, too. Simply put the Sub Anim Instance node property to the tail of AnimNodeProperties .