Commands and Functions

Defining Commands

You can define your own commands, which allow the scripts you write in Yarn Spinner to control parts of the game that you've built.

In Unity, there are two ways to add new commands to Yarn Spinner: automatically, via the YarnCommand attribute, or manually, using the DialogueRunner's AddCommandHandler method.

The YarnCommand attribute

The YarnCommand attribute lets you expose methods on a MonoBehaviour to Yarn Spinner.

When you add the YarnCommand attribute to a method, you specify what name the command should have in Yarn scripts. You can then use that name as a command.

If the method is not static, you call it with the name of the game object you want the command to run on.

For example, if you have a script called CharacterMovement that has a method Leap, you can add a YarnCommand attribute to it to make it available to your Yarn scripts:

public class CharacterMovement : MonoBehaviour {

    [YarnCommand("leap")]
    public void Leap() {
        Debug.Log($"{name} is leaping!");
    }
}

If you save this in a file called CharacterMovement.cs, create a new game object called MyCharacter, and attach the CharacterMovement script to that game object, you can run this code in your Yarn scripts like this:

<<leap MyCharacter>>
// will print "MyCharacter is leaping!" in the console

If the method is static, you call it directly, without providing a game object name. For example:

// Note that we aren't subclassing MonoBehaviour here; 
// static commands can be on any class.
public class FadeCamera {

    [YarnCommand("fade_camera")]
    public static void FadeCamera() {
        Debug.Log("Fading the camera!");
    }
}

If you save this in a file called FadeCamera.cs, you can run this code in your Yarn scripts like this:

<<fade_camera>>
// will print "Fading the camera!" in the console

You can also use methods that take parameters. Yarn Spinner will take the parameters that you provide, and convert them to the appropriate type.

Methods that are used with YarnCommand may take the following kinds of parameters:

Method parameters may be optional.

For example, consider this method:

[YarnCommand("walk")]
public void Walk(GameObject destination, bool dancing = false) {
    var position = destination.transform.position;

    // If the second parameter is used in the command,
    // and it's "true" or "dancing", use a dance 
    // animation
    if (dancing) {
        // set animation to a dance
    } else {
        // set animation to a regular walk
    }

    // walk the character to 'position'
}

This command could be called like this:

<<walk MyCharacter StageLeft>> // walk to the position of the object named 'StageLeft'

<<walk MyOtherCharacter StageRight dancing>> // walk to StageRight, while dancing

Finding Yarn Commands in your Project

You can see a list of commands that you've registered with the YarnCommand attribute by opening the Window menu and choosing Yarn Spinner -> Commands.

This will show a list of every command that you can call in your Yarn script, what parameters they take, and which parameters are optional.

Adding commands through code

You can also add new commands directly to a Dialogue Runner, using the AddCommandHandler method.

AddCommandHandler takes two parameters: the name of the command as it should be used in Yarn Spinner, and a method to call when the function is run.

If you want to add a command using AddCommandHandler that takes parameters, you must list the types of those parameters.

For example, to create a command that makes the main camera look at an object, create a new C# script in Unity with the following code:

public class CustomCommands : MonoBehaviour {    

    // Drag and drop your Dialogue Runner into this variable.
    public DialogueRunner dialogueRunner;

    public void Awake() {

        // Create a new command called 'camera_look', which looks at a target. 
        // Note how we're listing 'GameObject' as the parameter type.
        dialogueRunner.AddCommandHandler<GameObject>(
            "camera_look",     // the name of the command
            CameraLookAtTarget // the method to run
        );
    }

    // The method that gets called when '<<camera_look>>' is run.
    private void CameraLookAtTarget(GameObject target) {
        if (target == null) {
            debug.Log("Can't find the target!");
        }
        // Make the main camera look at this target
        Camera.main.transform.LookAt(target.transform);
    }    
}

Add this script to any game object, and it will register the camera_look in the Dialogue Runner you attach.

You can then call this method like this:

<<camera_look LeftMarker>> // make the camera look at an object named LeftMarker

YarnCommand vs AddCommandHandler

We provide two different means of handling commands in Yarn Spinner: the AddCommandHandler method and the YarnCommand attribute. Both of these provide effectively the same functionality, and under-the-hood the YarnCommand attribute is even a wrapper around the AddCommandHandler call. So if there are two different ways to achieve the same thing when should you use each one?

• The YarnCommand attribute allows you to tag specific methods as being a command, Yarn Spinner will then automatically handle the binding and connection of the the command in text to the method call in C#.

• AddCommandHandler method allows you to manually connect a method in C# to a command in Yarn, letting you set the name of the command and which method it connect to, giving you the control over the binding.

Most of the time, we feel that the YarnCommand attribute is the better option, because it is easier to use, and maps well to how we find most people use commands - that is, calling specific methods on specific GameObjects.

This convenience, however, does come at a cost of flexibility, because your YarnCommands either need to be on static methods, or follow specific calling conventions, which may not be what you need or want.

The YarnCommand attribute works best in our opinion when your commands are calling into specific GameObjects in your scene, which means that it works very well for moving, animating, or changing characters and items in a scene.

For larger gameplay changing moments, such as loading new scenes, moving between dialogue and the rest of your game, or for more global events like saving the game or unlocking an achievement, the AddCommandHandler method is better.

Making Commands Using Coroutines

Coroutines can be commands. If you register a command, either using the YarnCommand attribute, or the AddCommandHandler method, and the method you're using it with is a coroutine (that is, it returns IEnumerator, and yields objects like WaitForSeconds), Yarn Spinner will pause execution of your dialogue when the command is called.

Additionally, if your method returns a Coroutine object, Yarn Spinner will wait for that coroutine to complete. You can create and return a Coroutine by using the StartCoroutine method.

For example, here's how you'd write your own custom implementation of <<wait>>. (You don't have to do this in your own games, because <<wait>> is already added for you, but this example shows you how you'd do it yourself.)

public class CustomWaitCommand : MonoBehaviour {    

    [YarnCommand("custom_wait")]
    static IEnumerator CustomWait() {

        // Wait for 1 second
        yield return new WaitForSeconds(1.0);
        
        // Because this method returns IEnumerator, it's a coroutine. 
        // Yarn Spinner will wait until onComplete is called.
    }    
}

This new method can be called like this:

<<custom_wait>> // Waits for one second, then continues running

Defining Functions

Functions are units of code that Yarn scripts can call to receive a value.

In addition to the built-in functions that come with Yarn Spinner, you can create your own.

To create a function, you use the YarnFunction attribute, or the AddFunction method on a Dialogue Runner. These work very similarly to commands, but with two important distinctions:

1. Functions must return a value.

2. Functions are required to be static.

For example, here's a custom function that adds two numbers together:

public class AdderFunction {
   [YarnFunction("add_numbers")]
   public static int AddNumbers(int first, int second)
   {
       return first + second;
   }
}

When this code has been added to your project, you can use it in any expression, like an if statement, or inside a line:

<<if add_numbers(1,1) == 2>>
   One plus one is {add_numbers(1, 1)}
<<endif>>

Yarn functions can return the following types of values:

• string

• int

• float

• bool