Custom merchant handling

plugins/civmodcore-paper/src/main/java/vg/civcraft/mc/civmodcore/entities/merchant/CustomBukkitMerchant.java

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+package vg.civcraft.mc.civmodcore.entities.merchant;
+
+import io.papermc.paper.event.player.PlayerTradeEvent;
+import java.lang.reflect.Field;
+import java.util.logging.Level;
+import net.kyori.adventure.text.Component;
+import org.apache.commons.lang3.reflect.FieldUtils;
+import org.bukkit.Bukkit;
+import org.bukkit.craftbukkit.v1_16_R3.inventory.CraftMerchant;
+import org.bukkit.craftbukkit.v1_16_R3.inventory.CraftMerchantCustom;
+import vg.civcraft.mc.civmodcore.util.CivLogger;
+
+/**
+ * This is an alternative to {@link Bukkit#createMerchant(Component)} that re-adds
+ * {@link PlayerTradeEvent} emissions. Just do: {@code new CustomBukkitMerchant(Component)}
+ */
+public class CustomBukkitMerchant extends CraftMerchantCustom {
+
+	private static final CivLogger LOGGER;
+	private static final Field MERCHANT_FIELD;
+
+	static {
+		LOGGER = CivLogger.getLogger(CustomBukkitMerchant.class);
+		MERCHANT_FIELD = FieldUtils.getField(CraftMerchant.class, "merchant", true);
+		FieldUtils.removeFinalModifier(MERCHANT_FIELD);
+	}
+
+	public CustomBukkitMerchant(final Component title) {
+		super(title);
+		final var nmsMerchant = new CustomNMSMerchant(this, title);
+		try {
+			FieldUtils.writeField(MERCHANT_FIELD, this, nmsMerchant, true);
+		}
+		catch (final IllegalAccessException exception) {
+			LOGGER.log(Level.SEVERE,
+					"Could not re-set merchant to [" + nmsMerchant + "]",
+					exception);
+		}
+	}
+
+	public CustomNMSMerchant getNMSMerchant() {
+		return (CustomNMSMerchant) super.getMerchant();
+	}
+
+	/**
+	 * Use {@link #getNMSMerchant()} instead. The only reason why this is being kept is because
+	 * the super constructor calls this function and expects this type, not {@link CustomNMSMerchant}.
+	 */
+	@Deprecated
+	@Override
+	public CraftMerchantCustom.MinecraftMerchant getMerchant() {
+		return super.getMerchant();
+	}
+
+	@Override
+	public String toString() {
+		/** Stolen from {@link Object#toString()} because the super version is garbage */
+		return getClass().getName() + "@" + Integer.toHexString(hashCode());
+	}
+
+}

plugins/civmodcore-paper/src/main/java/vg/civcraft/mc/civmodcore/entities/merchant/CustomNMSMerchant.java

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+package vg.civcraft.mc.civmodcore.entities.merchant;
+
+import io.papermc.paper.event.player.PlayerTradeEvent;
+import java.lang.reflect.Field;
+import java.util.Objects;
+import net.kyori.adventure.text.Component;
+import net.minecraft.server.v1_16_R3.Entity;
+import net.minecraft.server.v1_16_R3.EntityExperienceOrb;
+import net.minecraft.server.v1_16_R3.EntityPlayer;
+import net.minecraft.server.v1_16_R3.EntityVillagerAbstract;
+import net.minecraft.server.v1_16_R3.EntityVillagerTrader;
+import net.minecraft.server.v1_16_R3.MerchantRecipe;
+import net.minecraft.server.v1_16_R3.MerchantRecipeList;
+import org.apache.commons.lang3.reflect.FieldUtils;
+import org.bukkit.craftbukkit.v1_16_R3.inventory.CraftMerchantCustom;
+import org.bukkit.entity.AbstractVillager;
+import org.bukkit.entity.ExperienceOrb;
+
+public class CustomNMSMerchant extends CraftMerchantCustom.MinecraftMerchant {
+
+	private static final Field TRADES_FIELD;
+
+	static {
+		TRADES_FIELD = FieldUtils.getField(CraftMerchantCustom.MinecraftMerchant.class, "trades", true);
+		FieldUtils.removeFinalModifier(TRADES_FIELD);
+	}
+
+	CustomNMSMerchant(final CustomBukkitMerchant merchant, final Component title) {
+		super(title);
+		this.craftMerchant = Objects.requireNonNull(merchant);
+	}
+
+	/**
+	 * @return Returns the raw offers object (which is mutable) via reflection.
+	 */
+	public MerchantRecipeList getRawOffers() {
+		try {
+			return (MerchantRecipeList) FieldUtils.readField(TRADES_FIELD, this, true);
+		}
+		catch (final IllegalAccessException exception) {
+			throw new RuntimeException(exception);
+		}
+	}
+
+	/**
+	 * This is based heavily on {@link EntityVillagerAbstract#a(MerchantRecipe)}. This method is called
+	 * by NMS when a trade is being purchased.
+	 */
+	@Override
+	public void a(final MerchantRecipe trade) {
+		if (getTrader() instanceof EntityPlayer) {
+			final var trader = (EntityPlayer) getTrader();
+			final var event = new PlayerTradeEvent(
+					trader.getBukkitEntity(),
+					(AbstractVillager) getCraftMerchant(),
+					trade.asBukkit(),
+					true,	// reward xp?
+					true);	// should increase uses?
+			event.callEvent();
+			if (event.isCancelled()) {
+				return;
+			}
+			final var eventTrade = event.getTrade();
+			if (event.willIncreaseTradeUses()) {
+				eventTrade.setUses(eventTrade.getUses() + 1);
+			}
+			if (event.isRewardingExp() && eventTrade.hasExperienceReward()) {
+				/** Based on {@link EntityVillagerTrader#b(MerchantRecipe)} */
+				final int xp = 3 + Entity.SHARED_RANDOM.nextInt(4);
+				final var world = trader.getWorld();
+				world.addEntity(new EntityExperienceOrb(
+						world, trader.locX(), trader.locY() + 0.5d, trader.locZ(),
+						xp, ExperienceOrb.SpawnReason.VILLAGER_TRADE, trader, null));
+			}
+			return;
+		}
+		super.a(trade);
+	}
+
+}