Mixin Injects

Mixin is a powerful tool used for bytecode transformation - specifically modifying the Minecraft source code in a way that prevents as many conflicts as possible between mods. Let's take a look at an example.

Mixin triggered whenever a block is broken

@Mixin(Block.class)
public class MixinBlock {
    @Inject(at = @At("HEAD"), method = "onBreak")
    public void onBreak(World world, BlockPos pos, BlockState state, PlayerEntity player, CallbackInfo info) {
        player.sendMessage(new LiteralText("You broke " + Registry.BLOCK.getId(state.getBlock())), false);
    }
}

This is considerably more complex than anything shown so far. But what does it do?

@Mixin(Block.class)

This tells the compiler that this is a mixin class, targeting the class net.minecraft.block.Block.

@Inject(at = @At("HEAD"), method = "onBreak")

@Inject is a Mixin annotation. It tells the Mixin runtime to "inject" the logic of our method into the onBreak method we specified with the method argument. The at = @At("HEAD") part means that our code will be injected at the top of the method; making it the first thing to run run when that method is called.

We then have a method with the same arguments as our target method, plus a CallbackInfo object.

Registering Mixins

When you add a mixin, you must put the mixin class in <your_package>.mixin. Then, you should add an entry to the server array in your <your_modid>.mixins.json file with the name of your mixin class. Any mixins listed in the server array will only be loaded on a dedicated server.

Fabric Wiki

Confused? The fabric wiki has a very extensive guide to mixins

Mixin cheatsheet

The Mixin cheatsheet has guides and examples for every type of mixin. If you want to learn how to do more than injecting at the top of a method, I would advise checking it out.

Game code used in the example

LiteralText: new LiteralText(String text) - this creates a Text object with the value of the string passed.

Text: a class representing a message that can be sent to a player through chat. Contains colour information, the string literal, or potentially a translation key.

PlayerEntity#sendMessage(Text text, boolean actionBar): Sends the player a message with the text component provided in the arguments. actionBar should nearly always be false