State Tree

Resources

• Detailed explanation of State Trees (from Epic dev) (The content of this video was noted in General view)
• Talk on it (from a studio)
• Another one (from another studio)
• Your First 60 Minutes with State Tree
• More info about the issues and behaviors of state trees

General view

Note

Most of the context of what’s following is from the Unreal Fest 2024 videos. Meaning that some statements might be true only in 5.4 or higher

What is the state tree composed of?

A state tree is a tree of states (with a hierarchy). The intermediate states (called branches) acts like selectors, this helps data driven actions and abstraction. The nest states (called leaf’s) contains actions and activities

States

Active states

• When a given leaf state is selected, all its parent states are activated to.
• Things set up in a parent state are also available in all child states.
  

State selection

Child state selection behavior

Tasks

What are tasks ?

• The behavior of a state is set up using tasks
• A task is a piece of logic that will be active when a state is activated, examples are:
  • Set tags or values
  • Claim resources
  • Play animation
  • Move to

Active tasks

• Each state can contain multiple tasks, which are active while the state is active
• Tasks on all active states are running
• Intermediate states tend to have the configuration tasks
• Leaf states usually have the action tasks
• Task competition drives state completion

Useful default properties on task class

Global tasks

• Tasks that run for the duration of the whole tree, this can be useful for:
  • Exposing data to the state tree
  • Configuring event listeners
  • Doing clean up at exit

Tasks exposed variables

• To make a variable showed as CONTEXT, add the Context category.
• To make a variable showed as IN (for input), add the Input category. (When marked as input you are forced to bind the value, so if you want to type it yourself you must not mark it)
• To make a variable showed as OUT (for output), add the Output category.

Transitions

In general

• On completion transitions

  • When a state completes, we must transition to a new state
  • Failure handling

• On event transitions

  • When an event occurs, we may transition to a new state
  • Can also test on every tick, but more expensive

• Transitions can be refined with conditions

Transition hierarchy

• Transitions are triggered from leaf state towards the root
• A state is completed when any of the actives tasks complete
  • Transitions are looked from the state where the task completed towards the root

Conditions

• Used for state’s enter conditions and transition conditions
• Use data binding to provide data for conditions
• Users can create their own conditions
• Conditions are combined to expressions
  • AND & OR (no precedence)
  • Use indentation to create nested expressions

Implicit transitions

1. When a tree is initially started, we transition to the root state
2. If a state does not provide completion transition, we look on the parent
3. If the parent does not have one, we’ll transition to root
4. If all completion transitions fails we transitions to root

Events

• Event can be gameplay tag and/or a struct
• Events can be used to trigger transitions
• Events can be used as conditions for entering a state
  • The event is captured on state enter and available for tasks
• Events are the most effective way to do transitions
• Payload access and capture is great for reactions
• Events can be send by external logic or tasks

Data flow

Property binding

You can bind properties in various cases.

Categories

• Input
  • Value UI is hidden
  • Assumed to be always bound
• Output
  • Value UI is hidden
  • Not bindable, but can be bound to
• Context
  • Value UI is hidden
  • Assumed to be always bound
  • Automatically binds to similar context data
  • Auto binding can be overridden

All other properties as bindable if set to editable.

Data Sources

1. Context data
   • Data provided from the State Tree use context
   • Actor Component vs Smart Object can have different data provided
2. Tree parameters
   • Configuration data provided from when a specific State Tree is used
3. Global tasks (and Evaluators)
   • Globally activated
4. State Parameters
   • Current and earlier states
5. Tasks
   • Tasks before the current task
   • Enter conditions, Transitions a bit different

Binding functions

New in 5.5

• State Tree nodes that set property values
• Useful for:
  • Conversion functions
  • Combine values
  • Get value based another value

Property references

New in 5.4

• State Tree property references allows the binding to return a pointer to the data
• Allows writing back results
  • Tasks that modify data (counters, etc)
  • Use state parameters as blackboard
• Pass references to instance data outside the State Tree

Composition

General

• State can be built from other branches of State Tree in three different ways
  • Linked subtrees
  • Linked assets
  • Parallel State Trees
• Composition allows more reuse and more efficiently

Subtrees and linked states

• Subtree states allow to create reusable State Tree branches (Kind of like functions)
• Linked states allow you to link to the subtree branches
• Linked states can pass parameters to subtree states
• Subtrees can access global data

Linked asset states

New in 5.4

• Linked asset states allow you to replace a branch of the tree with another asset
• Reconfigure State Trees
• Allow multiple designers to build the behaviour
• Events shared between nested trees

Run parallel task

New in 5.5

• Run parallel task allows you to run another State Tree along with the current State Tree
• Parallel tree is run as a task, so it will start after state selection
• Events are shared between main and parallel tree task
• Good for secondary states, like weapon selection

Linked overrides

• The asset and parameters provided to linked asset and run parallel task can be overridden when the State Tree is used.
• This allows you to build template State Trees, where parts of the tree are configured per character.

Using State Trees

How to run a State Tree

• The off-the-shelf case is to use State Tree component
• Works on any actor
• The component has functions to start and stop the tree
• Create a State Tree using State Tree component schema

State Tree schema

• Define a specific State Tree use case
• What external data is available?
• What kind of nodes can be used to build the tree?

Extending State Trees

The users can create their own schemas, tasks and conditions to create their own building blocks to make State Trees.

Using State Tree in your custom context

• The State Tree has been created to be easy to embed for different use case.
• FStateTreeReference handles asset selection and parameters
• FStateTreeInstanceData stores the current runtime data for a State Tree instance.
• FStateTreeExecutionContext is a temporary object used as a helper to tick a State Tree
• Does not require an actor to tick but needs an owner UObject in case new objects are created

State Tree Debugger

You can find it at Window -> Debugger

• Traces the State Tree execution
  • See state changes on timeline
  • See log how the tasks and conditions were executed
• Break points
  • Task Enter/Exit
  • State Enter/Exit
  • On Transition
• Enable/Disable
  • States
  • Tasks
  • Transitions
• Force conditions state on/off

Miscs

Helpers

You can use custom colors for states to visualize better. Section with more details