Use SaveCachedPose inside State Machine in UE4

What is a SaveCachedPose node?

We can always use SaveCachedPose node to cache the output pose, which can be used in the main tree, to avoid endless copy&paste of anim graph in an Animation Blueprint.

It is this node that can reduce complexity of an Anim Graph, except you cannot use it inside a state machine.

Digging into code

It seems that using a Save Cached Pose node inside a state machine requires a lot of work to grab. I’ve seen comment in the code like this:

else if (UAnimGraphNode_SaveCachedPose* SavePoseRoot = Cast<UAnimGraphNode_SaveCachedPose>(SourceNode))
{
	//@TODO: Ideally we only add these if there is a UseCachedPose node referencing them, but those can be anywhere and are hard to grab
	SaveCachedPoseNodes.Add(SavePoseRoot->CacheName, SavePoseRoot);
	RootSet.Add(SavePoseRoot);
}

But frankly speaking it is not hard at all, for animation systems that have been refactored over and over again. We only need to modify serveral lines of code to implement this feature.

How to implement

Enable SaveCachedPose node inside a state machine

First of all, we should make SaveCachedPose appear in the state machine node list. Thus, we should modify UAnimGraphNode_SaveCachedPose::IsCompatibleWithGraph function.

This function is like:

bool const bIsNotStateMachine = TargetGraph->GetOuter()->IsA(UAnimBlueprint::StaticClass());
return bIsNotStateMachine && Super::IsCompatibleWithGraph(TargetGraph);

We simply make variable bIsNotStateMachine always true:

bool const bIsNotStateMachine = true;
return bIsNotStateMachine && Super::IsCompatibleWithGraph(TargetGraph);

As a result, we can add a SaveCachedPose node inside a state machine.

Fix compile issue of the Animation Blueprint

Things cannot simply go so well. Some compile error arose when we actually use this node inside a state machine like this:

The first compile error

After digging into the call stack, it seems that something is wrong in ExpandGraphAndProcessNodes function:

// Process any animation nodes
{
	TArray<UAnimGraphNode_Base*> RootSet;
	RootSet.Add(TargetRootNode);
	ProcessAnimationNodesGivenRoot(AnimNodeList, RootSet);
}

As a SaveCachedPose node shall also be viewed as a so-called root. Thus we need to add these node to the RootSet as well like this:

// Process any animation nodes
{
	TArray<UAnimGraphNode_Base*> RootSet;
	RootSet.Add(TargetRootNode);
	RootSet.Append(SaveCachedPoseNodesInStateGraph);
	ProcessAnimationNodesGivenRoot(AnimNodeList, RootSet);
}

And it seems that the first compile error just gone. And we still have one more to deal with.

The second compile error

According to the call stack, it turned out that the SaveCachedPoseNodes does not contain a node that our UseCachedPose node referred.

Still very easy to deal with. We just need to add this SaveCachedPose node to FAnimBlueprintCompilerContext::SaveCachedPoseNodes in ExpandGraphAndProcessNodes function.

And it just works!