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Registration files and attaching scripts

Godot Kotlin/JVM offers two different ways to attach scripts:

  • Source files
  • Registration files.

Source files .kt, .java and .scala

Just like you would do with GDScript, you can directly attach your Kotlin/Java/Scala files to Nodes as scripts. This is the most straightforward method to use Kotlin scripts but not the most flexible.

The limitations are the following:

  • Your files must be located inside a valid source set defined in your gradle configuration file.
  • Scripts written in a location outside the Godot project can't be used as the engine won't be able to find them. This applies to modules and libraries.
  • If several script classes are defined inside a single file, only one of them will be usable.
  • The script is nameless. You won't be able to write code in GDScript like: 
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    var test_script: MyScript = load("res://pathToScript/MyScript.gdj").new() # Wrong for direct source files
var test_script: Node = load("res://pathToScript/MyScript.kt").new() # Correct
    The same applies if you use a Godot object with a .kt/.java/.scala file attached to it.

If those limitations don't apply to you, feel free to use Kotlin source files directly.

Registration files .gdj

For each class you register to Godot, a corresponding registration file is generated (a gdj file) during compilation. Like the source files, you can attach them to Nodes. They have several benefits over source files:

  • .gdj can be placed wherever you want in your Godot project, you are not limited to the source set.
  • Each script get its own .gdj. This includes scripts in different modules and libraries.
  • If a source file contains several scripts. A different .gdj will be generated for each.
  • Registration files are language agnostic, they are generated for Kotlin, Java and Scala files with no difference.
  • When creating a script from code using its registered name. The module is going to use the registration file as the script. Therefore, registration files are treated as the default way to use scripts inside the module.

By default, these files are generated into a folder called gdj in the root of your Godot project.

You can however configure the Godot root and the base directory used for newly created registration files inside your build.gradle.kts:

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import godot.gradle.GodotLanguage

godot {
    // Optional: limit the initial compile pass to the JVM languages your project actually uses.
    languages.set(setOf(GodotLanguage.KOTLIN, GodotLanguage.JAVA))

    // Only needed when the Gradle project directory is not the Godot project root.
    godotProjectDirectory.set(file(".."))

    registrationFilesDirectory.set(<folder>)
}

During the sync step, the Gradle plugin scans the whole configured Godot project for existing .gdj files. Matching files are updated in place, obsolete ones are deleted, and only newly discovered registrations are copied into registrationFilesDirectory.

Reason

Contrary to GDScript, Kotlin is a compiled language. Hence, if you use a library which defines scripts you can not attach those to nodes anymore as the source files don't exist. You only have a jar of the library. While in GDScript you still have the sources when using an addon. With our registration files our compiler plugin is able to extract those from the libraries you use and provide them to you, so you can also attach scripts from libraries you use.

Class and member registration

Contrary to what you might be used to from GDScript or C#, this binding requires you to explicitly define which classes and which members of those classes should be exposed to Godot.
This requires a bit more code but enables to properly define visibility across languages and restrict access to values which you might not want to expose.
See the individual sections in the user-guide in this documentation to see how to register your classes, properties, and functions so you can use them from godot and other scripting languages.

Instance types and singletons

Creating a new instance of a Godot type can be done like any JVM object.

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val node3D = Node3D()
val vec = Vector3()

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Node3D node3D = new Node3D();
Vector3 vec = new Vector3();

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val node3D = new Node3D()
val vec = new Vector3()

Godot's singletons are exposed as static access points.

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Physics2DServer.areaGetTransform(area)

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Physics2DServer.areaGetTransform(area);

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Physics2DServer.areaGetTransform(area)

Core types

Godot's built-in types are passed by value (except for Dictionary and VariantArray - more on this later), so the following snippet won't work as expected.

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val node3D = Node3D()
node3D.rotation.y += 10f

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Node3D node3D = new Node3D();
node3D.getRotation().setY(node3D.getRotation().getY() + 10f);

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val node3D = new Node3D()
node3D.getRotation.setY(node3D.getRotation.getY + 10f)

You are actually mutating a copy of the rotation property, not a reference to it. To get the desired behaviour you have to re-assign the copy back.

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val rotation = node3D.rotation
rotation.y += 10f
node3D.rotation = rotation

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Vector3 rotation = node3D.getRotation();
rotation.setY(rotation.getY() + 10f);
node3D.setRotation(rotation);

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val rotation = node3D.getRotation
rotation.setY(rotation.getY + 10f)
node3D.setRotation(rotation)

This approach introduces a lot of boilerplate, so this binding provides a concise way of achieving the same behaviour. Only in Kotlin

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node3D.rotationMutate {
  y += 10f
}

The snippet above is functionally equivalent to the previous one.

Collection types

While VariantArray and Dictionary are passed by reference, the value returned by the retrieval methods (VariantArray.get(...) and Dictionary.get(...)) are not.

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array[index].y += 10f
dictionary["foo"].y += 5f

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array.get(index).setY(array.get(index).getY() + 10f);
dictionary.get("foo").setY(dictionary.get("foo").getY() + 5f);

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array.get(index).setY(array.get(index).getY + 10f)
dictionary.get("foo").setY(dictionary.get("foo").getY + 5f)

To get the desired behaviour, you can re-assign the copy back or in a similar fashion as before, this binding provides a better alternative.

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array.get(index) {
  y += 10f
}

dictionary.get("foo") {
  y += 5f
}

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Vector3 arrayValue = array.get(index);
arrayValue.setY(arrayValue.getY() + 10f);
array.set(index, arrayValue);

Vector3 dictionaryValue = dictionary.get("foo");
dictionaryValue.setY(dictionaryValue.getY() + 5f);
dictionary.set("foo", dictionaryValue);

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val arrayValue = array.get(index)
arrayValue.setY(arrayValue.getY + 10f)
array.set(index, arrayValue)

val dictionaryValue = dictionary.get("foo")
dictionaryValue.setY(dictionaryValue.getY + 5f)
dictionary.set("foo", dictionaryValue)

Enums and constants

Godot enums are mapped to Kotlin enums, the generated enum exposes a value property that represents the value in Godot. Constants in Godot classes that represent an enum value (such as Node.PAUSE_MODE_INHERIT) are not present in this module, please use the generated enum instead (Node.PauseMode.INHERIT).

Renamed symbols

To avoid confusion and conflict with Kotlin types, the following Godot symbol is renamed.

  • Array -> VariantArray (to avoid confusion with a built-in type in Kotlin)
  • PackedXArray::toByteArray -> PackedXArray::toPackedByteArray (to avoid confusion with a built-in type in Kotlin)
  • PackedByteArray::toXArray -> PackedByteArray::toPackedXArray (to avoid confusion with a built-in type in Kotlin)
  • All enum values are shortened, the name of the enum itself has been removed. Here are some examples:
  • Error.ERR_PARAMETER_RANGE_ERROR -> Error.PARAMETER_RANGE
  • MethodFlags.METHOD_FLAG_NORMAL -> MethodFlags.NORMAL
  • ProcessThreadMessages.FLAG_PROCESS_THREAD_MESSAGES_PHYSICS -> Error.FLAG_PHYSICS

Global functions

In GDScript, some functions are always available (such as mathematical or RNG functions). The complete list can be found on the following page of Godot's documentation.

In Kotlin, Java, and Scala, global functions are available through the GD singleton helpers. However, don't forget that some functions couldn't be reproduced exactly on the JVM side. For example, load() is available but preload() is not.

Additional functions

For comfort, some objects got some additional functions to enjoy some Kotlin syntax sugar. You can find them all in this folder.

Notifications

You can implement _notification and have class hierarchy notification call without using super call, as in GDScript and C++. However, the syntax is a bit different:

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@RegisterFunction
override fun _notification() = godotNotification {
  // ...
}

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@RegisterFunction
@Override
public GodotNotification _notification() {
    return godotNotification((self, notification) -> {
        // ...
    });
}

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@RegisterFunction
override def _notification(): GodotNotification = {
  godotNotification((self, notification) => {
    // ...
  })
}

Currently this feature except abstract classes.

StringName and NodePath

Several Godot functions take StringName or NodePath as a parameter. It's often more convenient to directly use a String and convert it.

This kind of operation can be costly so we provide extension functions which cache the result of the conversion for later calls:

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    val stringName = "myString".asCachedStringName() // Cache the string for faster future calls.
    val nodePath = "myNode/myChildNode".asCachedNodePath() // Cache the string for faster future calls.
    val snakeCaseStringName = "myString".toGodotName() // Convert the string to snake_case and cache it for faster future calls.

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    StringName stringName = StringNames.asCachedStringName("myString");
    NodePath nodePath = NodePaths.asCachedNodePath("myNode/myChildNode");
    StringName snakeCaseStringName = StringNames.toGodotName("myString");

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    val stringName = StringNames.asCachedStringName("myString")
    val nodePath = NodePaths.asCachedNodePath("myNode/myChildNode")
    val snakeCaseStringName = StringNames.toGodotName("myString")

You can also use the non-cached version of them if you simply want ease of conversion:

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    val stringName = "myString".asStringName()
    val nodePath = "myNode/myChildNode".asNodePath()

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    StringName stringName = StringNames.asStringName("myString");
    NodePath nodePath = NodePaths.asNodePath("myNode/myChildNode");

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    val stringName = StringNames.asStringName("myString")
    val nodePath = NodePaths.asNodePath("myNode/myChildNode")

Logging

If you want logs to appear both in CLI and in the Godot Editor you will have to use the print functions inside the GD singleton like:

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GD.print("Hello There!")

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GD.print("Hello There!");

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GD.print("Hello There!")

Kotlin's print functions, on the other hand, will only print to CLI! They won't print to Godot editor's output panel.