Modification to EssentialsX Commandsell.java to provide hooking into when people sell items to the server

Commandsell.patch

diff --git a/Essentials/src/com/earth2me/essentials/commands/Commandsell.java b/Essentials/src/com/earth2me/essentials/commands/Commandsell.java
index e6c41a98..8897b6b5 100644
--- a/Essentials/src/com/earth2me/essentials/commands/Commandsell.java
+++ b/Essentials/src/com/earth2me/essentials/commands/Commandsell.java
@@ -11,6 +11,9 @@ import java.math.BigDecimal;
 import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
+import java.util.Map;
+import java.util.HashMap;
+import java.util.function.BiConsumer;
 import java.util.logging.Level;
 
 import static com.earth2me.essentials.I18n.tl;
@@ -21,6 +24,36 @@ public class Commandsell extends EssentialsCommand {
         super("sell");
     }
 
+	//DANNY PATCH BEGIN
+	private static int callback_id = 0;
+	private static Map<Integer, BiConsumer<ItemStack, BigDecimal>> sell_callbacks = new HashMap<>();
+
+	/**
+	 * Adds a callback
+	 * @return The id of the callback. Used to remove the callback.
+	 **/
+	public static int onSell(BiConsumer<ItemStack, BigDecimal> callback) {
+		sell_callbacks.put(callback_id, callback);
+		return callback_id++;
+	}
+	
+	/**
+	 * Removes a callback
+	 **/
+	public static void endCallback(int id) {
+		if(sell_callbacks.containsKey(id)) {
+			sell_callbacks.remove(id);
+		}
+	}
+
+	private void doCallbacks(ItemStack item, BigDecimal money) {
+		sell_callbacks.values()
+					  .stream()
+					  .filter(n -> n != null)
+					  .forEach(cb -> cb.accept(item, money));
+	}
+	//DANNY PATCH END
+
     @Override
     public void run(final Server server, final User user, final String commandLabel, final String[] args) throws Exception {
         BigDecimal totalWorth = BigDecimal.ZERO;
@@ -91,6 +124,11 @@ public class Commandsell extends EssentialsCommand {
             // This should never happen.
             throw new IllegalStateException("Trying to remove more items than are available.");
         }
+
+		//DANNY PATCH BEGIN
+		doCallbacks(ris, result);
+		//DANNY PATCH END
+
         user.getBase().getInventory().removeItem(ris);
         user.getBase().updateInventory();
         Trade.log("Command", "Sell", "Item", user.getName(), new Trade(ris, ess), user.getName(), new Trade(result, ess), user.getLocation(), ess);