Add ability to register custom item transformers.

Essentials/src/main/java/com/earth2me/essentials/MetaItemStack.java

@@ -1,5 +1,6 @@
 package com.earth2me.essentials;
 
+import com.earth2me.essentials.items.transform.PluginItemTransformer;
 import com.earth2me.essentials.textreader.BookInput;
 import com.earth2me.essentials.textreader.BookPager;
 import com.earth2me.essentials.textreader.IText;
@@ -53,6 +54,7 @@
 public class MetaItemStack {
     private static final Map<String, DyeColor> colorMap = new HashMap<>();
     private static final Map<String, FireworkEffect.Type> fireworkShape = new HashMap<>();
+    private static final transient Map<String, PluginItemTransformer> itemTransformers = new HashMap<>();
     private static boolean useNewSkullMethod = true;
 
     static {
@@ -84,6 +86,32 @@ public MetaItemStack(final ItemStack stack) {
         this.stack = stack.clone();
     }
 
+    /**
+     * Registers an item transformer, belonging to a plugin, that can manipulate certain item metadata.
+     * @param key the key for the transformer.
+     * @param itemTransformer the actual transformer.
+     */
+    public static void registerItemTransformer(final String key, final PluginItemTransformer itemTransformer) {
+        //Warn people if they're trying to register over top of someone else.
+        if (itemTransformers.containsKey(key)) {
+            Essentials.getWrappedLogger().log(Level.WARNING, String.format("Plugin transformer registered to \"%s\" attempted to register already existing item transformer \"%s\" belonging to \"%s\"!",
+                    itemTransformer.getPlugin().getName(),
+                    key,
+                    itemTransformers.get(key).getPlugin().getName()));
+            return;
+        }
+
+        itemTransformers.put(key, itemTransformer);
+    }
+
+    /**
+     * Unregisters a certain item transformer under key "key".
+     * @param key the transformer key.
+     */
+    public static void unregisterItemTransformer(final String key) {
+        itemTransformers.remove(key);
+    }
+
     private static void setSkullOwner(final IEssentials ess, final ItemStack stack, final String owner) {
         if (!(stack.getItemMeta() instanceof SkullMeta)) return;
 
@@ -358,11 +386,28 @@ public void addStringMeta(final CommandSource sender, final boolean allowUnsafe,
             armorMeta.setTrim(new ArmorTrim(material, pattern));
 
             stack.setItemMeta(armorMeta);
+        } else if (split.length > 1 && itemTransformers.containsKey(split[0])) {
+            transformItem(split[0], split[1]);
         } else {
             parseEnchantmentStrings(sender, allowUnsafe, split, ess);
         }
     }
 
+    private void transformItem(final String key, final String data){
+        final PluginItemTransformer transformer = itemTransformers.get(key);
+
+        //Ignore, the plugin is disabled.
+        if (!transformer.getPlugin().isEnabled()) {
+            return;
+        }
+
+        try {
+            stack = transformer.apply(data, stack);
+        } catch(final Throwable thr) {
+            Essentials.getWrappedLogger().log(Level.SEVERE, String.format("Error applying data \"%s\" to itemstack! Plugin: %s, Key: %s", data, transformer.getPlugin().getName(), key), thr);
+        }
+    }
+
     public void addItemFlags(final String string) throws Exception {
         final String[] separate = splitPattern.split(string, 2);
         if (separate.length != 2) {

Essentials/src/main/java/com/earth2me/essentials/items/transform/PluginItemTransformer.java

@@ -0,0 +1,22 @@
+package com.earth2me.essentials.items.transform;
+
+import org.bukkit.inventory.ItemStack;
+import org.bukkit.plugin.Plugin;
+
+public abstract class PluginItemTransformer {
+    private final Plugin plugin;
+
+    public PluginItemTransformer(final Plugin thePlugin) {
+        if (thePlugin == null) {
+            throw new IllegalArgumentException("Plugin cannot be null!");
+        }
+
+        this.plugin = thePlugin;
+    }
+
+    public abstract ItemStack apply(String data, ItemStack original);
+
+    public Plugin getPlugin() {
+        return plugin;
+    }
+}