What i did

Prepare the MOD development environment

I will omit this because the procedure will come out if you google.

Add an item

Follow the steps below to add a new item.

Write the item registration process in the MOD main program

The actual code is shown below. (Import statement omitted)

MountWhistle.java

@Mod(modid = MountWhistle.MODID, name = MountWhistle.NAME, version = MountWhistle.VERSION)
public class MountWhistle {
    public static final String MODID = "mountwhistle";
    public static final String NAME = "Mount Whistle";
    public static final String VERSION = "1.0.2";
    public static final String PAKETNAME = "MountWhistle";

    public static Item mountWhistle;

    @Mod.EventHandler
    public void construct(FMLConstructionEvent event) {
        MinecraftForge.EVENT_BUS.register(this);
    }

    @SubscribeEvent
    public void registerItems(RegistryEvent.Register<Item> event) {
    	mountWhistle = new Whistle()
    			.setCreativeTab(CreativeTabs.TRANSPORTATION)
    			.setUnlocalizedName("mountwhistle")
    			.setRegistryName("mountwhistle")
    			.setMaxStackSize(1);
        event.getRegistry().register(mountWhistle);
    }

    @SubscribeEvent
    @SideOnly(Side.CLIENT)
    public void registerModels(ModelRegistryEvent event) {
        ModelLoader.setCustomModelResourceLocation(mountWhistle, 0, new ModelResourceLocation(new ResourceLocation("mountwhistle", "mountwhistle"), "inventory"));
    }
}

--In the registerItems (RegistryEvent.Register <Item> event) method, write the item registration process.
Item information is described here. --In the registerModels (ModelRegistryEvent event) method, write the item model registration process. This will reference the json file under src.main.resources.assets.MOD_ID.models.items.

Describe the information about the item texture in the json file

Create a src / main / resources / assets / MOD_ID / models / items package and write a json file under it that specifies the image file with item name.json.

mountwhistle.json

{
	"parent": "item/generated",
	"textures": {
		"layer0": "mountwhistle:items/whistle"
	}
}

Add item texture to project

Create a src / main / resources / assets / MOD_ID / textures / items package and save the image file under it with item name.png.

Implement the behavior when using the item

From here is the production. First, create a class of mount whistle items. Since the base class of Minecraft items is net.minecraft.item.Item, we will implement it by overriding this.

First, the implemented source code is shown below, and then the explanation is started.

Whistle.java source code

Whistle.java

public class Whistle extends Item {

	@Override
	public void setDamage(ItemStack stack, int damage) {
		return;
	}

	@Override
	public ActionResult<ItemStack> onItemRightClick(World worldIn, EntityPlayer playerIn, EnumHand handIn) {
		Entity mount = playerIn.getRidingEntity();
		ItemStack itemStack = playerIn.getHeldItem(handIn);
		//Are you riding
		if (mount != null) {
			//Register the mount you are currently riding
			NBTTagCompound nbtTag = itemStack.getTagCompound();
			if (nbtTag == null) {
				nbtTag = new NBTTagCompound();
				itemStack.setTagCompound(nbtTag);
			}

			if (nbtTag.hasKey("MountId") &&
					!UUID.fromString(nbtTag.getString("MountId")).equals(mount.getUniqueID())) {
				return new ActionResult<ItemStack>(EnumActionResult.PASS, playerIn.getHeldItem(handIn));
			}

			String name = mount.getName();

			if (worldIn.isRemote) {
				Minecraft.getMinecraft().ingameGUI.getChatGUI().printChatMessage(new TextComponentString("On the mount whistle" + name + "I registered!"));
			} else {
				nbtTag.setString("MountId", mount.getUniqueID().toString());
				nbtTag.setString("Name", name);
				itemStack.setStackDisplayName(name);

				//Make the mount immortal
				mount.setEntityInvulnerable(true);
			}
		} else {
			NBTTagCompound nbtTag = itemStack.getTagCompound();
			if (nbtTag == null || !nbtTag.hasKey("MountId")) {
				if (worldIn.isRemote) {
					Minecraft.getMinecraft().ingameGUI.getChatGUI().printChatMessage(new TextComponentString("You can register the mount by using it while riding."));
				}
				return new ActionResult<ItemStack>(EnumActionResult.PASS, playerIn.getHeldItem(handIn));
			}

			UUID mountId = UUID.fromString(nbtTag.getString("MountId"));

			MinecraftServer server =  worldIn.getMinecraftServer();

			if(server == null) {
				Minecraft.getMinecraft().ingameGUI.getChatGUI().printChatMessage(new TextComponentString("I tried to summon it."));
			} else {
				//Find mounts from all entities
				mount = server.getEntityFromUuid(mountId);
				mount.setLocationAndAngles(playerIn.posX, playerIn.posY, playerIn.posZ, 0.0F, 0.0F);

				if (worldIn.isRemote) {
					if (mount != null) {
						String name = mount.getName();
						Minecraft.getMinecraft().ingameGUI.getChatGUI().printChatMessage(new TextComponentString(name + "Was summoned."));
					}
				}
			}
		}

		return new ActionResult<ItemStack>(EnumActionResult.PASS, playerIn.getHeldItem(handIn));
	}
}

Commentary

Avoid consuming items

Items usually disappear after use, but mount whistle is an item you want to use again and again. The method that handles consumption when used is setDamage (ItemStack stack, int damage), so I override this and immediately return to prevent anything from happening.

Behavior when using items

The method called when using an item by right-clicking is ʻActionResult onItemRightClick (World worldIn, EntityPlayer playerIn, EnumHand handIn) `. By overriding this, you can implement the right-click event.

Here, we first determine if the player is riding on the mount. If you are riding, register the mount with the NBT of the mount whistle and process the mount to be immortal. If you are not riding, summon the mount registered in the mount whistle NBT to the player's coordinates.

What is NBT

When giving information to an item, we use a mechanism called NBT. NBT is an abbreviation of Named Binary Tag, and Minecraft stores various data in the data format called NBT. If the item does not have an NBT tag, create an instance of NBTTagCompound, firstsetTagCompound (nbtTag), and then register the UUID (unique identifier of the entity object) of the mount. Please note that the item will not retain any information unless you do the first setTagCompound (nbtTag). One thing to keep in mind when assigning NBT tags to items is that NBT tags cannot be attached to items, and the actual target to be attached is ʻItemStack`. ItemStack is a bundle of items and refers to "one frame in the inventory".

Things to be aware of when playing multiplayer

The MOD program fires on both the client and the server. However, I would like this process to work only on the client, or vice versa. As a handling at that time, you can branch by looking at the value of ʻisRemote of the Worldobject. True on the client side and false` on the server side. Here, the chat message is displayed only when the client side is executed, and the mount coordinates are changed only when the server side is executed. If you do not handle the server and client, the mount will move to you for a moment, and then the mount will start walking from the coordinates on the server side, so it will return to its original position instantly.

What was made

I uploaded it on GitHub. https://github.com/inokinn/MountWhistle

I will actually move it

First, get the mount whistle. By default, you can craft with 9 items "Gisar's Vegetables" added by the mod Chococraft . I will. By using the mount whistle when riding the mount, registration is successful if the item name becomes the name of the mount. After that, it is OK if you use the mount whistle when not riding and the mount comes to the player.

• Due to the specifications of Minecraft that can only refer to on-memory entities, it is not possible to call a mount that is too far away.

in conclusion

Minecraft MOD development related, there are few information, it takes time to build and check the operation, the behavior is different between single play and multiplayer, etc. As a result of repeating trial and error, from the start of development to the completion 2 It took about a day. In particular, the Japanese documentation was old and rare, so I felt like I had to read the actual source code. I hope Minecraft MOD development will be more exciting in Japan as well.