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 in 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 static, you call it directly. 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

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:

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

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

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, it is easier to use and maps well to how we find most people use commands, that is to say calling specific methods on specific GameObjects. This convenience however does come at a cost of flexibility as 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 works best in our opinion when your commands are calling into specific GameObjects in your scene, so 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, or moving between dialogue and the rest of your game, or for more global events like declaring a save should happen or an achievement has been unlocked the AddCommandHandler method is better.

Finally the YarnCommand attribute performs a search on your project to locate and bind commands to specific method calls which the AddCommandHandler does not have to do. While for the most part this lookup will go by unnoticed, on larger projects you may find this adds a noticeable performance penalty.

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.

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 additon 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 like this:

One plus one is {add_numbers(1, 1)}

Yarn functions can return the following types of values:

• string

• int

• float

• bool

Commands, Functions and Assembly Definitions

Yarn Spinner searches your code for methods that have the YarnCommand and YarnFunction attributes when your game first starts up, as well as when a Dialogue Runner is told to run a Yarn Project.

If the Yarn Project's "Search All Assemblies" option is turned on, every assembly definition is searched; if it's turned off, only the assembly definitions specified in the "Assemblies To Search" option is searched. Code that is not in an assembly definition is always included.

By default, Yarn Spinner searches every assembly definition. If you have a large codebase, putting all of the code that contains commands and functions in an assembly definition can reduce the amount of time Yarn Spinner needs to take to find all of the commands and functions.