Registration files and attaching scripts
Contrary to GDScript and C# you cannot attach scripts (aka source files) directly to nodes. Godot Kotlin JVM uses a different approach: registration files. These files have the extension
gdj and are generated during compilation. These are the files you attach to nodes.
Have a look at the classes documentation for more information.
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.
Godot singletons are mapped as Kotlin objects.
Godot's built-in types are passed by value (except for
VariantArray - more on this later), so the following snippet won't work as expected.
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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This approach introduces a lot of boilerplate, so this binding provides a concise way of achieving the same behaviour.
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The snippet above is functionally equivalent to the previous one.
Dictionary are passed by reference, the value returned by the retrieval methods (
Dictionary.get(...)) are not.
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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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 (
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.
To avoid confusion and conflict with Kotlin types, the following Godot symbol is renamed.
VariantArray(to avoid confusion with a built-in type in Kotlin)
In GDScript, some functions are always available like mathematical functions or RNG functions. The complete list is here: https://docs.godotengine.org/en/stable/classes/class_%40gdscript.html.
In Kotlin, they are available inside the
GD singleton. Don't forget that some functions couldn't be reproduced in Kotlin.
load() is available but
preload() is not.
For comfort, some Objects got some additional functions to enjoy some Kotlin syntax sugar. You can find them all in this folder
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:
Kotlin's print functions on the other hand will only print to CLI! They will print to Godot editor's output panel.