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Fallbacks (Controls)

This family of nodes are known as "Selector" or "Priority" in other frameworks.

Their purpose is to try different strategies, until we find one that "works".

Currently the framework provides three kinds of nodes:

  • Fallback
  • AsyncFallback
  • ReactiveFallback

They share the following rules:

  • Before ticking the first child, the node status becomes RUNNING.

  • If a child returns FAILURE, the fallback ticks the next child.

  • If the last child returns FAILURE too, all the children are halted and the fallback returns FAILURE.

  • If a child returns SUCCESS, it stops and returns SUCCESS. All the children are halted.

Synchronous vs Asynchronous

Fallback and AsyncFallback share the same logic, but differ in how they handle the transition between children:

  • Fallback ticks all children in a single tick of the tree. When a child returns FAILURE, the next child is ticked immediately within the same call.
  • AsyncFallback yields execution back to the tree after each child fails, returning RUNNING and emitting a wake-up signal. This makes the fallback interruptible between children, allowing other parts of the tree (e.g., a ReactiveSequence parent) to re-evaluate conditions before the next child starts.

Comparison table

To understand how the three ControlNodes differ, refer to the following table:

Type of ControlNode Child returns RUNNING Yields between children
Fallback Tick again No
AsyncFallback Tick again Yes
ReactiveFallback Restart No

  • "Restart" means that the entire fallback is restarted from the first child of the list.

  • "Tick again" means that the next time the fallback is ticked, the same child is ticked again. Previous siblings, which returned FAILURE already, are not ticked again.

For the complete list of built-in nodes, see the other pages in this section and the source code on Github.

Fallback

In this example, we try different strategies to open the door. Check first (and once) if the door is open.

FallbackNode

AsyncFallback

AsyncFallback behaves like Fallback, but yields execution back to the tree after each child returns FAILURE. It returns RUNNING and emits a wake-up signal, allowing a reactive parent to re-evaluate conditions before the next child is ticked.

<ReactiveSequence>
    <IsRobotHungry/>
    <AsyncFallback>
        <FindFoodInBackpack/>
        <FindNearbyRestaurant/>
        <OrderFoodDelivery/>
    </AsyncFallback>
</ReactiveSequence>

In this example, IsRobotHungry is re-checked between each attempt of the AsyncFallback. If the robot is no longer hungry after FindFoodInBackpack fails, the fallback is interrupted before FindNearbyRestaurant starts.

ReactiveFallback

This ControlNode is used when we want to interrupt an asynchronous child if one of the previous Conditions changes its state from FAILURE to SUCCESS.

In the following example, the character will sleep up to 8 hours. If he/she has fully rested, then the node areYouRested? will return SUCCESS and the asynchronous nodes Timeout (8 hrs) and Sleep will be interrupted.

ReactiveFallback