Behavior graph example

This section provides an example to illustrate how to create a behavior graph. The scenario involves an agent (GameObject) that randomly makes a choice between two actions, showcasing different functionalities of behavior graph. The agent has the following two options:

Think about what to do and wait for 5 s.
Talk to itself and search for a random target (GameObject) that talks or calls out a message for the agent. Consequently, the agent selects that target and moves towards it.

This example illustrates the implementation of decision-making processes and action sequences within behavior graphs.

By the end of this example, you will create a behavior graph that looks like:

Final behavior graph

To create this behavior graph, perform the following steps:

Right-click any empty area of the Project window.
Select Create > Behavior > Behavior Graph.
Enter a name for the new behavior graph.
Double-click the new behavior graph to open it in the Unity Behavior editor. You're now ready to implement the logic of your artificial intelligence (AI).

The start node is already provided on the Unity Behavior graph editor.
To create a control flow node, right-click any empty area of the Unity Behavior graph editor and select Add.

The Add Node window appears.
To add a Talk node, select Action > Talk.

Unity Behavior adds the Talk node to the graph.

Note

When you add your first node, the Unity Behavior tutorial displays a message to help you with the next step. To proceed with the tutorial, select Continue.
Join the On Start node to the new Talk node.
1. Drag the Talk node so it's aligned below the On Start node.
2. Hover your cursor over the bottom of the On Start node until a connector arrow appears.
3. Drag the connector arrow from the On Start node to the Talk node.
Create the variables required in the behavior graph. In this example, you need three variables: Self, Target, and Speed. By default, the Self variable is already available on the Blackboard.

To create the remaining variables, perform the following steps on the Blackboard.
1. Click the plus () icon.
2. Select Object.
3. Type a name for the new variable, for example Target.
4. Click the plus icon again and select Float.
5. Name the new float variable as Speed and specify a value. For example, 2.
Select the link icon in the Agent field of the action node to link it with a variable on the Blackboard and select Self.
Enter the relevant text in the field besides Self says.

This is the dialog or the sentence that your character will speak. For example, What should I do?.
Add a Random flow node. To do this, right-click any empty area of the Unity Behavior graph editor and select Add.

The Add Node window appears.
Select Flow > Random.

Unity Behavior adds the Random node to the graph.
Join the previously created Talk node to the new Random node.

Now create two branches that the agent can execute at random. In the first branch, the agent will talk to itself and then wait for 0.5 seconds. In the second branch, the agent will talk to itself, select a random target, and then move towards the target.

First branch

Create a Talk node, as described in the previous procedure.

(Right-click > Add > Action > Talk).
Select the link icon in the Agent field of the action node to link it with a variable on the Blackboard and select Self.
Type I'll think about it! in the field beside Self says.
To add a Wait node, right-click any empty space of the Unity Behavior graph editor and select Add.
Select Action > Wait.

Unity Behavior adds the Wait node to the graph.
Enter 0.5 as the seconds value.
Group both of these actions so they run sequentially. To do this, drag the Talk node to the top half of the Wait node until a blue line appears. Then, drop the node to create a group.
Join this group to the Random node created in step 12 of the first procedure.

Second branch

Create a Talk node using the same steps from the first procedure.

(Right-click > Add > Action > Talk).
Select the link icon in the Agent field of the action node to link it with a variable on the Blackboard and select Self.
Type I'll go to a random target! in the field beside Self says.
To assign a target to a random object, right-click any empty space of the Unity Behavior graph editor and select Add.
Select Action > Set Random Target.

Unity Behavior adds the Set Random Target node to the graph.
Type Targets in the field beside Tag. The Unity Behavior sample includes a scene where its GameObjects are already tagged with the Targets value.

This corresponds to the Tag field values shown in the Unity Editor's Inspector window.

Note

If you're creating your own scene (rather than using the SampleScene included with the Unity Behavior sample), be sure to tag some of your GameObjects with the Targets value. To do this, use the Tag drop-down menu in the Unity Editor's Inspector window (not Unity Behavior editor's Inspector window). For more information on creating and assigning tags, refer to Tags.
Create another Talk node (Right-click > Add > Action > Talk) and link the Agent field to the Target variable.
Type Pick me! Pick me!! in the field beside Target says field.
Add a Move to Target node to move the agent to the target. To do this, right-click an empty area of the Unity Behavior graph and select Add > Action > Move > Move To Target.

Unity Behavior adds the Move To Target node to the graph.
Link the Agent field to the Self variable. To do this, select the link icon in the Agent field and select Self.
Group these four actions so they run sequentially. To do this, drag the Talk node to the top half of the Set Random Target node until a blue line appears. Then, drop the node to create a group. Similarly, add the other Talk node and Move to Target node at the bottom of the Set Random Target node to create a group of four nodes.
Select the four nodes in your group by using your cursor to drag a bounding box around the nodes. A blue line surrounds the group.
Move the group so it's near the bottom right of the Random node.
Join this group to the Random node created in step 12 of the first procedure.

You have successfully created the sample behavior graph.

Test your behavior graph in a scene

Now that your behavior graph is complete, you can observe it in Play mode.

If you haven't installed the Unity Behavior sample, install it by referring to Install Unity Behavior with the Package Manager.
From the Project window, drag SampleScene from Assets/Samples/Behavior/[Version_Number]/Unity Behavior Example to the Hierarchy window.
If you have multiple scenes, select SampleScene and set it as the active scene.
Enter Play mode and observe the behavior of the GameObjects.

Because your behavior graph has multiple branches that run randomly, you can play your scene multiple times to check the different behaviors.

Update variables in real time

In Unity Behavior, the C# code updates most variables, such as Agent and Target. However, if you're testing a feature or iterating on a variable, you need to change their values in real time. For example, when debugging in Play mode, you might need to increase an agent's speed in real time.

To update the variables available on the Blackboard in real time, perform the following steps:

Click the Hierarchy window of the Unity Editor.
Select SampleScene > Agent.
In the Inspector window of the Unity Editor, scroll down to the Behavior Graph Agent (Script) section.

The Inspector window displays all the variables used in the current scene.

The example graph described in this section displays the following Blackboard variables:
- Self
- Target
- Speed
Select a different GameObject or input a new value.

If you change the value of a variable, Unity will append Override next to the variable name as shown in the following image.
To change the value of the variable to the original one, right-click the variable and select Revert Variable.