In this tutorial we will create simple storage block similar to a dispenser, explaining how to build a user interface with the ScreenHandler
API from Fabric and Vanilla Minecraft along the way.
Let us first explain some vocabulary:
Screenhandler:
A ScreenHandler
is a class responsible for synchronizing inventory contents between the client and the server. It can sync additional integer values like furnace progress as well, which will be showed in the next tutorial.
Our subclass will have two constructors here: one will be used on the server side and will contain the real Inventory
and another one will be used on the client side to hold the ItemStack
s and synchronize them.
Screen:
The Screen
class only exists on the client and will render the background and other decorations for your ScreenHandler
.
First we need to create the Block
and its BlockEntity
.
public class BoxBlock extends BlockWithEntity { protected BoxBlock(Settings settings) { super(settings); } @Override public BlockEntity createBlockEntity(BlockPos pos, BlockState state) { return new BoxBlockEntity(pos, state); } @Override public BlockRenderType getRenderType(BlockState state) { //With inheriting from BlockWithEntity this defaults to INVISIBLE, so we need to change that! return BlockRenderType.MODEL; } @Override public ActionResult onUse(BlockState state, World world, BlockPos pos, PlayerEntity player, Hand hand, BlockHitResult hit) { if (!world.isClient) { //This will call the createScreenHandlerFactory method from BlockWithEntity, which will return our blockEntity casted to //a namedScreenHandlerFactory. If your block class does not extend BlockWithEntity, it needs to implement createScreenHandlerFactory. NamedScreenHandlerFactory screenHandlerFactory = state.createScreenHandlerFactory(world, pos); if (screenHandlerFactory != null) { //With this call the server will request the client to open the appropriate Screenhandler player.openHandledScreen(screenHandlerFactory); } } return ActionResult.SUCCESS; } //This method will drop all items onto the ground when the block is broken @Override public void onStateReplaced(BlockState state, World world, BlockPos pos, BlockState newState, boolean moved) { if (state.getBlock() != newState.getBlock()) { BlockEntity blockEntity = world.getBlockEntity(pos); if (blockEntity instanceof BoxBlockEntity) { ItemScatterer.spawn(world, pos, (BoxBlockEntity)blockEntity); // update comparators world.updateComparators(pos,this); } super.onStateReplaced(state, world, pos, newState, moved); } } @Override public boolean hasComparatorOutput(BlockState state) { return true; } @Override public int getComparatorOutput(BlockState state, World world, BlockPos pos) { return ScreenHandler.calculateComparatorOutput(world.getBlockEntity(pos)); } }
Now we will create our BlockEntity
, it will use the ImplementedInventory
interface from the Inventory Tutorial.
public class BoxBlockEntity extends BlockEntity implements NamedScreenHandlerFactory, ImplementedInventory { private final DefaultedList<ItemStack> inventory = DefaultedList.ofSize(9, ItemStack.EMPTY); public BoxBlockEntity(BlockPos pos, BlockState state) { super(ExampleMod.BOX_BLOCK_ENTITY, pos, state); } //From the ImplementedInventory Interface @Override public DefaultedList<ItemStack> getItems() { return inventory; } //These Methods are from the NamedScreenHandlerFactory Interface //createMenu creates the ScreenHandler itself //getDisplayName will Provide its name which is normally shown at the top @Override public ScreenHandler createMenu(int syncId, PlayerInventory playerInventory, PlayerEntity player) { //We provide *this* to the screenHandler as our class Implements Inventory //Only the Server has the Inventory at the start, this will be synced to the client in the ScreenHandler return new BoxScreenHandler(syncId, playerInventory, this); } @Override public Text getDisplayName() { // for 1.19+ return Text.translatable(getCachedState().getBlock().getTranslationKey()); // for earlier versions // return new TranslatableText(getCachedState().getBlock().getTranslationKey()); } @Override public void readNbt(NbtCompound nbt) { super.readNbt(nbt); Inventories.readNbt(nbt, this.inventory); } @Override public NbtCompound writeNbt(NbtCompound nbt) { super.writeNbt(nbt); Inventories.writeNbt(nbt, this.inventory); return nbt; } }
public class ExampleMod implements ModInitializer { public static final Block BOX_BLOCK; public static final BlockItem BOX_BLOCK_ITEM; public static final BlockEntityType<BoxBlockEntity> BOX_BLOCK_ENTITY; // a public identifier for multiple parts of our bigger chest public static final Identifier BOX = new Identifier(MOD_ID, "box_block"); static { BOX_BLOCK = Registry.register(Registries.BLOCK, BOX, new BoxBlock(FabricBlockSettings.copyOf(Blocks.CHEST))); BOX_BLOCK_ITEM = Registry.register(Registries.ITEM, BOX, new BlockItem(BOX_BLOCK, new Item.Settings())); //The parameter of build at the very end is always null, do not worry about it // pre-1.17 // In 1.17 use FabricBlockEntityTypeBuilder instead of BlockEntityType.Builder } @Override public void onInitialize() { } }
As explained earlier, we need both a ScreenHandler
and a HandledScreen
to display and sync the GUI. ScreenHandler
classes are used to synchronize GUI state between the server and the client. HandledScreen
classes are fully client-sided and are responsible for drawing GUI elements.
public class BoxScreenHandler extends ScreenHandler { private final Inventory inventory; //This constructor gets called on the client when the server wants it to open the screenHandler, //The client will call the other constructor with an empty Inventory and the screenHandler will automatically //sync this empty inventory with the inventory on the server. public BoxScreenHandler(int syncId, PlayerInventory playerInventory) { this(syncId, playerInventory, new SimpleInventory(9)); } //This constructor gets called from the BlockEntity on the server without calling the other constructor first, the server knows the inventory of the container //and can therefore directly provide it as an argument. This inventory will then be synced to the client. public BoxScreenHandler(int syncId, PlayerInventory playerInventory, Inventory inventory) { super(ExampleMod.BOX_SCREEN_HANDLER, syncId); checkSize(inventory, 9); this.inventory = inventory; //some inventories do custom logic when a player opens it. inventory.onOpen(playerInventory.player); //This will place the slot in the correct locations for a 3x3 Grid. The slots exist on both server and client! //This will not render the background of the slots however, this is the Screens job int m; int l; //Our inventory for (m = 0; m < 3; ++m) { for (l = 0; l < 3; ++l) { this.addSlot(new Slot(inventory, l + m * 3, 62 + l * 18, 17 + m * 18)); } } //The player inventory for (m = 0; m < 3; ++m) { for (l = 0; l < 9; ++l) { this.addSlot(new Slot(playerInventory, l + m * 9 + 9, 8 + l * 18, 84 + m * 18)); } } //The player Hotbar for (m = 0; m < 9; ++m) { this.addSlot(new Slot(playerInventory, m, 8 + m * 18, 142)); } } @Override public boolean canUse(PlayerEntity player) { return this.inventory.canPlayerUse(player); } // Shift + Player Inv Slot @Override public ItemStack quickMove(PlayerEntity player, int invSlot) { ItemStack newStack = ItemStack.EMPTY; Slot slot = this.slots.get(invSlot); if (slot != null && slot.hasStack()) { ItemStack originalStack = slot.getStack(); newStack = originalStack.copy(); if (invSlot < this.inventory.size()) { if (!this.insertItem(originalStack, this.inventory.size(), this.slots.size(), true)) { return ItemStack.EMPTY; } } else if (!this.insertItem(originalStack, 0, this.inventory.size(), false)) { return ItemStack.EMPTY; } if (originalStack.isEmpty()) { slot.setStack(ItemStack.EMPTY); } else { slot.markDirty(); } } return newStack; } }
public class BoxScreen extends HandledScreen<BoxScreenHandler> { //A path to the gui texture. In this example we use the texture from the dispenser private static final Identifier TEXTURE = new Identifier("minecraft", "textures/gui/container/dispenser.png"); public BoxScreen(BoxScreenHandler handler, PlayerInventory inventory, Text title) { super(handler, inventory, title); } @Override protected void drawBackground(MatrixStack matrices, float delta, int mouseX, int mouseY) { RenderSystem.setShader(GameRenderer::getPositionTexProgram); RenderSystem.setShaderColor(1.0F, 1.0F, 1.0F, 1.0F); RenderSystem.setShaderTexture(0, TEXTURE); int x = (width - backgroundWidth) / 2; int y = (height - backgroundHeight) / 2; drawTexture(matrices, x, y, 0, 0, backgroundWidth, backgroundHeight); //in 1.20 or above,this method is in DrawContext class. } @Override public void render(MatrixStack matrices, int mouseX, int mouseY, float delta) { renderBackground(matrices); super.render(matrices, mouseX, mouseY, delta); drawMouseoverTooltip(matrices, mouseX, mouseY); } @Override protected void init() { super.init(); // Center the title titleX = (backgroundWidth - textRenderer.getWidth(title)) / 2; } }
As screens are a client-only concept, we can only register them on the client.
public class ExampleClientMod implements ClientModInitializer { @Override public void onInitializeClient() { HandledScreens.register(ExampleMod.BOX_SCREEN_HANDLER, BoxScreen::new); } }
Don't forget to register this entrypoint in fabric.mod.json
if you haven't done it yet:
/* ... */ "entrypoints": { /* ... */ "client": [ "tutorial.path.to.ExampleModClient" ] },
ScreenHandler
s exist both on the client and on the server and therefore have to be registered on both.
public class ExampleMod implements ModInitializer { [...] public static final ScreenHandlerType<BoxScreenHandler> BOX_SCREEN_HANDLER; [...] static { [...] //We use registerSimple here because our Entity is not an ExtendedScreenHandlerFactory //but a NamedScreenHandlerFactory. //In a later Tutorial you will see what ExtendedScreenHandlerFactory can do! BOX_SCREEN_HANDLER = ScreenHandlerRegistry.registerSimple(BOX, BoxScreenHandler::new); } @Override public void onInitialize() { } }
You have now created your own container Block, you could easily change it to contain a smaller or bigger Inventory. Maybe even apply a texture
- Syncing Custom Data with Extended ScreenHandlers when screen is opened
- Syncing Integers continuously with PropertyDelegates
- An example mod using the ScreenHandler
API: ExampleMod on Github.