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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 and .java

Just like you would do with GDScript, you can directly attach your Kotlin/Java 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 (doesn't apply to Kotlin code): 
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    var test_script: MyScript = load("res://pathToScript/MyScript.kt").new() // Wrong
var test_script: Node = load("res://pathToScript/MyScript.kt").new() // Correct
    The same applies if you use a Godot object with a .kt/.java 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 and Java 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 project.

You can however configure the destination folder inside your build.gradle.kts:

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godot {
    registrationFileBaseDir.set(<folder>)
}

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 and members 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 Kotlin types.

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

Godot's singletons are mapped as Kotlin objects.

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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

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

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

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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

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
}

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).

Signals and exposed methods

In GDScript, signals can have any number of arguments, this is not possible in Kotlin as it is a statically typed language. At the moment, you can create signals and expose them to Godot with at most 10 parameters.

If you need more than 10 parameters, you can either use the not typesafe function connect(signalAsString, targetObject, targetMethodAsString) and the corresponding emit function or you can write your own typesafe extension functions like we did, to further increase the supported arg count. Keep in mind that you pass in the converted function and signal names (snake_case) to the above mentioned functions.

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)

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 documentaion.

In Kotlin, global functions are available inside the GD object singleton. However, don't forget that some functions couldn't be reproduced in Kotlin. E.g., the load() function 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 {
  // ...
}
Currently this feature except abstract classes.

Caching

Several Godot functions take StringName or NodePath as a parameter.

Extension functions have been provided String.asStringName(), String.asNodePath() and StringName.asNodePath(), to allow for the easy creation of these. As an optimisation, these StringName and NodePath instances are cached to reduce the overhead of creating new instances unnecessarily.

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// This first call to the extension function creates the cache entry.
val firstCall = "Test".asStringName()

// This second call for the same String value, will return the previously cached instance.
val secondCall = "Test".asStringName()

// This third call will create a second entry in the cache due to the different key value.
val thirdCall = "OtherTest".asStringName()

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!")

Kotlin's print functions, on the other hand, will only print to CLI! They will print to Godot editor's output panel.