Writing an Addon

An addon can provide compatibility work that is too specific for Retromod core. It is a normal mod containing implementations of Retromod’s public extension interfaces.

Version Shims

Implement com.retromod.core.VersionShim to register a verified API change:

public final class ExampleShim implements VersionShim {
    public String getShimName() { return "Example mod fix"; }
    public String getSourceVersion() { return "1.20.1"; }
    public String getTargetVersion() { return "1.21"; }
    public String getModLoaderType() { return "fabric"; }

    public void registerRedirects(RetromodTransformer transformer) {
        transformer.registerClassRedirect(
                "net/minecraft/old/Thing",
                "net/minecraft/new/Thing");
    }
}

Keep redirects owner- and descriptor-specific when working with methods.

Implement com.retromod.core.AuxiliaryVersionShim when getSourceVersion() and getTargetVersion() describe library API releases. Retromod selects that provider by loader. It does not compare those values with Minecraft or use the provider as a graph edge.

Implement com.retromod.core.MinecraftVersionedApiShim when those values describe Minecraft host versions for an API repair. Retromod keeps that provider outside the transition graph, but it registers the provider only when the selected host reaches getTargetVersion(). Explicit same-version transforms can apply both provider types.

Polyfills

Implement com.retromod.polyfill.PolyfillProvider when an API was removed and needs a small replacement. Redirect the old name to a class in your addon’s own package. Do not add classes to loader-owned packages.

Service Registration

List implementations in standard service files:

META-INF/services/com.retromod.core.VersionShim
META-INF/services/com.retromod.polyfill.PolyfillProvider

Each file contains one implementation class per line. Install the addon beside Retromod.

Compile against VersionShim, PolyfillProvider, and documented RetromodTransformer registration methods. Treat other implementation classes as internal.

Licensing

Your addon uses your license. Permissively licensed fixes may be adopted into Retromod core with attribution. Pull requests to this repository are contributed under MIT.

See Contributing for core changes.