API differences

Instance types and singletons

Creating a new instance of a Godot type can be done like any Kotlin types.

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val spatial = Spatial()
val vec = Vector3()

Godot 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 spatial = Spatial()
spatial.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 = spatial.rotation
rotation.y += 10f
spatial.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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spatial.rotation {
  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.get(index).asVector3().y += 10f
dictionary.get("foo").asVector3().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<Vector3>(index) {
  y += 10f
}

dictionary.get<Vector3>(index) {
  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 binding, please use the generated enum instead (Node.PauseMode.INHERIT).

Signals and exposed methods

In GDScript, signals and methods 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 methods to Godot with at most 10 parameters.

Additionally, signals are mapped to properties of type Signal and must start with a prefix signal (check Signals section for more details). The prefix is dropped during registration, so the signal signalReverseChanged is known in Godot as reverseChanged. This is done to avoid naming conflicts with other members of a class. There is no signal type in GDScript, signals are only referenced by name so they can have the same name as methods and/or properties in the same class.

Renamed symbols

To avoid confusion and conflict with Kotlin types, the following Godot symbols are renamed.

  • Array -> VariantArray (to avoid confusion with a built-in type in Kotlin)
  • PoolRealArray -> PoolFloatArray (for naming consistency)
  • Variant.asReal() -> Variant.asFloat() (for naming consistency)