ExampleScreenHandler.java

package com.example.examplemod;

import net.minecraft.entity.player.PlayerEntity;
import net.minecraft.entity.player.PlayerInventory;
import net.minecraft.inventory.Inventory;
import net.minecraft.inventory.SimpleInventory;
import net.minecraft.item.ItemStack;
import net.minecraft.registry.Registries;
import net.minecraft.registry.Registry;
import net.minecraft.screen.ScreenHandler;
import net.minecraft.screen.ScreenHandlerType;
import net.minecraft.screen.slot.Slot;
import net.minecraft.util.Identifier;
import net.minecraft.world.World;

/**
 * A minimal 3-slot ScreenHandler (server-side GUI logic) you'd open from a
 * block entity. Pair with a matching HandledScreen on the client for visuals.
 * ScreenHandler API is unchanged across 1.21.7 - 1.21.11.
 */
public class ExampleScreenHandler extends ScreenHandler {

    public static final ScreenHandlerType<ExampleScreenHandler> TYPE = Registry.register(
            Registries.SCREEN_HANDLER,
            Identifier.of(ExampleMod.MOD_ID, "example_screen_handler"),
            new ScreenHandlerType<>(ExampleScreenHandler::new, net.minecraft.feature.FeatureFlags.VANILLA_FEATURES)
    );

    private final Inventory inventory;

    // Client-side constructor (called when server tells client to open screen)
    public ExampleScreenHandler(int syncId, PlayerInventory playerInventory) {
        this(syncId, playerInventory, new SimpleInventory(3));
    }

    // Server-side constructor
    public ExampleScreenHandler(int syncId, PlayerInventory playerInventory, Inventory inventory) {
        super(TYPE, syncId);
        this.inventory = inventory;
        checkSize(inventory, 3);
        inventory.onOpen(playerInventory.player);

        int slotIndex = 0;

        // Our 3 custom slots (e.g. input/fuel/output style layout)
        for (int col = 0; col < 3; col++) {
            this.addSlot(new Slot(inventory, slotIndex++, 44 + col * 18, 20));
        }

        // Standard player inventory + hotbar slots below
        for (int row = 0; row < 3; row++) {
            for (int col = 0; col < 9; col++) {
                this.addSlot(new Slot(playerInventory, col + row * 9 + 9, 8 + col * 18, 51 + row * 18));
            }
        }
        for (int col = 0; col < 9; col++) {
            this.addSlot(new Slot(playerInventory, col, 8 + col * 18, 109));
        }
    }

    @Override
    public boolean canUse(World world, PlayerEntity player) {
        return inventory.canPlayerUse(player);
    }

    @Override
    public ItemStack quickMove(PlayerEntity player, int slotIndex) {
        // Simplified shift-click handling; real mods implement full slot transfer logic here.
        Slot slot = this.slots.get(slotIndex);
        if (!slot.hasStack()) return ItemStack.EMPTY;
        ItemStack stack = slot.getStack().copy();
        slot.setStack(ItemStack.EMPTY);
        return stack;
    }

    @Override
    public void onClosed(PlayerEntity player) {
        super.onClosed(player);
        inventory.onClose(player);
    }
}