Class tweakers, formerly known as access wideners before gaining further functionality, provide transformation tools complementary to Mixin bytecode manipulation. They also allow some runtime modifications to be accessible within the development environment.

WARNING

Class tweakers are not specific to a given Minecraft version, but are only available starting from Fabric Loader 0.18.0, and may only target Vanilla Minecraft classes.

Setup

File Format

Class tweaker files are conventionally named after your modid, example-mod.classtweaker, to help IDE plugins recognize them. They should be stored in resources.

The file must have the following header as its first line:

classTweaker v1 named

Class tweaker files can have blank lines and comments starting with #. Comments can start at the end of a line.

Whilst the specific syntax depends on the feature, modifications are each declared on separate lines. An entry's elements can be separated using any whitespace, including tabs.

Specifying The File Location

The class tweaker file's location must be specified in your build.gradle and fabric.mod.json files. Remember that you must also depend on Fabric Loader 0.18.0 or above to use class tweakers.

The specifications are still named after access wideners to preserve backwards compatibility.

build.gradle

loom {

	accessWidenerPath = file("src/main/resources/example-mod.classtweaker")

}

fabric.mod.json

...

"accessWidener": "example-mod.classtweaker",

...

After specifying the file location in your build.gradle file, make sure to reload your Gradle project in the IDE.

Validating the File

By default, class tweaker will ignore entries referencing modification targets that cannot be found. To check if all the classes, fields and methods specified in the file are valid, run the validateAccessWidener Gradle task.

Errors will point out any invalid entry, but they can be about which part of an entry is invalid.