Refactor placeholder registration

Includes support for registering relational placeholders with PAPI

src/main/java/io/github/apickledwalrus/skriptplaceholders/SkriptPlaceholders.java

@@ -2,13 +2,22 @@
 
 import ch.njol.skript.Skript;
 import io.github.apickledwalrus.skriptplaceholders.placeholder.PlaceholderPlugin;
+import io.github.apickledwalrus.skriptplaceholders.placeholder.PlaceholderRegistry;
 import org.bukkit.plugin.java.JavaPlugin;
 
 import java.io.IOException;
 
 public class SkriptPlaceholders extends JavaPlugin {
 
 	private static SkriptPlaceholders instance;
+	private PlaceholderRegistry registry;
+
+	public static SkriptPlaceholders getInstance() {
+		if (instance == null) {
+			throw new IllegalStateException("skript-placeholders has not been initialized yet.");
+		}
+		return instance;
+	}
 
 	@Override
 	public void onEnable() {
@@ -26,10 +35,17 @@ public void onEnable() {
 			getLogger().severe(e.toString());
 			getServer().getPluginManager().disablePlugin(this);
 		}
+
+		this.registry = new PlaceholderRegistry(this);
 	}
 
-	public static SkriptPlaceholders getInstance() {
-		return instance;
+	@Override
+	public void onDisable() {
+		instance = null;
+	}
+
+	public PlaceholderRegistry getRegistry() {
+		return registry;
 	}
 
 }

src/main/java/io/github/apickledwalrus/skriptplaceholders/placeholder/MVdWPlaceholderAPIListener.java

@@ -4,18 +4,21 @@
 import org.bukkit.OfflinePlayer;
 import org.bukkit.plugin.Plugin;
 
+import java.util.HashSet;
+import java.util.Set;
+
 /**
  * A placeholder listener for placeholders created using {@link PlaceholderPlugin#MVDW_PLACEHOLDER_API}.
  */
 public class MVdWPlaceholderAPIListener implements PlaceholderListener {
 
 	private final Plugin plugin;
-	private final PlaceholderEvaluator evaluator;
 	private final String placeholder;
 
-	public MVdWPlaceholderAPIListener(Plugin plugin, PlaceholderEvaluator evaluator, String placeholder) {
+	private final Set<PlaceholderEvaluator> evaluators = new HashSet<>();
+
+	public MVdWPlaceholderAPIListener(Plugin plugin, String placeholder) {
 		this.plugin = plugin;
-		this.evaluator = evaluator;
 		this.placeholder = placeholder;
 	}
 
@@ -26,7 +29,13 @@ public void registerListener() {
 			if (player == null) { // this is for an actual offline player
 				player = event.getOfflinePlayer();
 			}
-			return evaluator.evaluate(placeholder, player);
+			for (PlaceholderEvaluator evaluator : evaluators) {
+				String result = evaluator.evaluate(placeholder, player);;
+				if (result != null) {
+					return result;
+				}
+			}
+			return null;
 		});
 	}
 
@@ -35,4 +44,19 @@ public void unregisterListener() {
 		// TODO determine if this is possible
 	}
 
+	@Override
+	public void addEvaluator(PlaceholderEvaluator evaluator) {
+		evaluators.add(evaluator);
+	}
+
+	@Override
+	public void removeEvaluator(PlaceholderEvaluator evaluator) {
+		evaluators.remove(evaluator);
+	}
+
+	@Override
+	public boolean hasEvaluators() {
+		return !evaluators.isEmpty();
+	}
+
 }

src/main/java/io/github/apickledwalrus/skriptplaceholders/placeholder/PlaceholderAPIListener.java

@@ -1,23 +1,28 @@
 package io.github.apickledwalrus.skriptplaceholders.placeholder;
 
 import me.clip.placeholderapi.expansion.PlaceholderExpansion;
+import me.clip.placeholderapi.expansion.Relational;
 import org.bukkit.OfflinePlayer;
+import org.bukkit.entity.Player;
 import org.bukkit.plugin.Plugin;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
+import java.util.HashSet;
+import java.util.Set;
+
 /**
  * A placeholder listener for placeholders created using {@link PlaceholderPlugin#PLACEHOLDER_API}.
  */
-public class PlaceholderAPIListener extends PlaceholderExpansion implements PlaceholderListener {
+public class PlaceholderAPIListener extends PlaceholderExpansion implements Relational, PlaceholderListener {
 
 	private final Plugin plugin;
-	private final PlaceholderEvaluator evaluator;
 	private final String prefix;
 
-	public PlaceholderAPIListener(Plugin plugin, PlaceholderEvaluator evaluator, String prefix) {
+	private final Set<PlaceholderEvaluator> evaluators = new HashSet<>();
+
+	public PlaceholderAPIListener(Plugin plugin, String prefix) {
 		this.plugin = plugin;
-		this.evaluator = evaluator;
 		this.prefix = prefix;
 	}
 
@@ -43,7 +48,24 @@ public boolean persist() {
 
 	@Override
 	public @Nullable String onRequest(@Nullable OfflinePlayer player, @NotNull String identifier) {
-		return evaluator.evaluate(this.prefix + "_" + identifier, player);
+		for (PlaceholderEvaluator evaluator : evaluators) {
+			String result = evaluator.evaluate(this.prefix + "_" + identifier, player);
+			if (result != null) {
+				return result;
+			}
+		}
+		return null;
+	}
+
+	@Override
+	public @Nullable String onPlaceholderRequest(Player one, Player two, String identifier) {
+		for (PlaceholderEvaluator evaluator : evaluators) {
+			String result = evaluator.evaluateRelational(this.prefix + "_" + identifier, one, two);
+			if (result != null) {
+				return result;
+			}
+		}
+		return null;
 	}
 
 	@Override
@@ -56,4 +78,19 @@ public void unregisterListener() {
 		unregister();
 	}
 
+	@Override
+	public void addEvaluator(PlaceholderEvaluator evaluator) {
+		evaluators.add(evaluator);
+	}
+
+	@Override
+	public void removeEvaluator(PlaceholderEvaluator evaluator) {
+		evaluators.remove(evaluator);
+	}
+
+	@Override
+	public boolean hasEvaluators() {
+		return !evaluators.isEmpty();
+	}
+
 }

src/main/java/io/github/apickledwalrus/skriptplaceholders/placeholder/PlaceholderEvaluator.java

@@ -1,6 +1,7 @@
 package io.github.apickledwalrus.skriptplaceholders.placeholder;
 
 import org.bukkit.OfflinePlayer;
+import org.bukkit.entity.Player;
 import org.jetbrains.annotations.Nullable;
 
 /**
@@ -13,8 +14,17 @@ public interface PlaceholderEvaluator {
 	 * @param placeholder The placeholder to evaluate.
 	 * @param player The player to use for evaluating this placeholder.
 	 *  Can be null for placeholders that do not require such context.
-	 * @return The value of the placeholder.
+	 * @return The value of the placeholder, or null if it could not be evaluated.
 	 */
 	@Nullable String evaluate(String placeholder, @Nullable OfflinePlayer player);
 
+	/**
+	 * A method to evaluate the value of a relational placeholder, which depends on two players.
+	 * @param placeholder The placeholder to evaluate.
+	 * @param one The first player to use for evaluating <code>placeholder</code>.
+	 * @param two The second player to use for evaluating <code>placeholder</code>.
+	 * @return The value of the placeholder, or null if it could not be evaluated.
+	 */
+	@Nullable String evaluateRelational(String placeholder, Player one, Player two);
+
 }

src/main/java/io/github/apickledwalrus/skriptplaceholders/placeholder/PlaceholderListener.java

@@ -3,7 +3,7 @@
 /**
  * A placeholder listener is responsible for registering, unregistering, and handling placeholder requests.
  */
-public interface PlaceholderListener {
+interface PlaceholderListener {
 
 	/**
 	 * Registers this listener with the providing plugin.
@@ -15,4 +15,21 @@ public interface PlaceholderListener {
 	 */
 	void unregisterListener();
 
+	/**
+	 * Adds an evaluator to be used when trying to evaluate a placeholder.
+	 * @param evaluator The evaluator to add.
+	 */
+	void addEvaluator(PlaceholderEvaluator evaluator);
+
+	/**
+	 * Removes an evaluator from this listener. It will no longer be used when trying to evaluate a placeholder.
+	 * @param evaluator The evaluator to remove.
+	 */
+	void removeEvaluator(PlaceholderEvaluator evaluator);
+
+	/**
+	 * @return Whether this listener has any evaluators.
+	 */
+	boolean hasEvaluators();
+
 }

src/main/java/io/github/apickledwalrus/skriptplaceholders/placeholder/PlaceholderPlugin.java

@@ -1,10 +1,10 @@
 package io.github.apickledwalrus.skriptplaceholders.placeholder;
 
 import ch.njol.skript.Skript;
-import io.github.apickledwalrus.skriptplaceholders.SkriptPlaceholders;
 import org.apache.commons.lang.StringUtils;
 import org.bukkit.OfflinePlayer;
 import org.bukkit.entity.Player;
+import org.bukkit.plugin.Plugin;
 import org.jetbrains.annotations.Nullable;
 
 import java.util.Arrays;
@@ -35,8 +35,8 @@ public enum PlaceholderPlugin {
 		}
 
 		@Override
-		public PlaceholderListener registerPlaceholder(PlaceholderEvaluator evaluator, String placeholder) {
-			PlaceholderListener listener = new PlaceholderAPIListener(SkriptPlaceholders.getInstance(), evaluator, placeholder);
+		PlaceholderListener registerPlaceholder(Plugin plugin, String placeholder) {
+			PlaceholderListener listener = new PlaceholderAPIListener(plugin, placeholder);
 			listener.registerListener();
 			return listener;
 		}
@@ -80,8 +80,8 @@ public boolean supportsRelationalPlaceholders() {
 		}
 
 		@Override
-		public PlaceholderListener registerPlaceholder(PlaceholderEvaluator evaluator, String placeholder) {
-			PlaceholderListener listener = new MVdWPlaceholderAPIListener(SkriptPlaceholders.getInstance(), evaluator, placeholder);
+		PlaceholderListener registerPlaceholder(Plugin plugin, String placeholder) {
+			PlaceholderListener listener = new MVdWPlaceholderAPIListener(plugin, placeholder);
 			listener.registerListener();
 			return listener;
 		}
@@ -145,10 +145,11 @@ public final boolean isInstalled() {
 
 	/**
 	 * Registers a new placeholder with this plugin.
+	 * @param plugin The plugin registering the placeholder.
 	 * @param placeholder The name of the placeholder to register.
 	 * @return The registered listener.
 	 */
-	public abstract PlaceholderListener registerPlaceholder(PlaceholderEvaluator evaluator, String placeholder);
+	abstract PlaceholderListener registerPlaceholder(Plugin plugin, String placeholder);
 
 	/**
 	 * @param placeholder The placeholder to obtain the value of.

src/main/java/io/github/apickledwalrus/skriptplaceholders/placeholder/PlaceholderRegistry.java

@@ -0,0 +1,59 @@
+package io.github.apickledwalrus.skriptplaceholders.placeholder;
+
+import org.bukkit.plugin.Plugin;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class PlaceholderRegistry {
+
+	private final Plugin plugin;
+	private final Map<PlaceholderPlugin, PlaceholderRegister> registers = new HashMap<>();
+
+	public PlaceholderRegistry(Plugin plugin) {
+		this.plugin = plugin;
+	}
+
+	public void registerPlaceholder(PlaceholderPlugin plugin, String placeholder, PlaceholderEvaluator evaluator) {
+		PlaceholderRegister register = registers.computeIfAbsent(plugin, PlaceholderRegister::new);
+		register.registerPlaceholder(placeholder, evaluator);
+	}
+
+	public void unregisterPlaceholder(PlaceholderPlugin plugin, String placeholder, PlaceholderEvaluator evaluator) {
+		PlaceholderRegister register = registers.get(plugin);
+		if (register != null) {
+			register.unregisterPlaceholder(placeholder, evaluator);
+		}
+	}
+
+	private final class PlaceholderRegister {
+
+		private final PlaceholderPlugin plugin;
+		private final Map<String, PlaceholderListener> listeners = new HashMap<>();
+
+		public PlaceholderRegister(PlaceholderPlugin plugin) {
+			this.plugin = plugin;
+		}
+
+		public void registerPlaceholder(String placeholder, PlaceholderEvaluator evaluator) {
+			PlaceholderListener listener = listeners.computeIfAbsent(placeholder,
+					key -> plugin.registerPlaceholder(PlaceholderRegistry.this.plugin, key)
+			);
+			listener.addEvaluator(evaluator);
+		}
+
+		public void unregisterPlaceholder(String placeholder, PlaceholderEvaluator evaluator) {
+			PlaceholderListener listener = listeners.get(placeholder);
+			if (listener == null) {
+				return;
+			}
+			listener.removeEvaluator(evaluator);
+			if (!listener.hasEvaluators()) { // if this was the last evaluator, unregister the listener
+				listener.unregisterListener();
+				listeners.remove(placeholder);
+			}
+		}
+
+	}
+
+}

src/main/java/io/github/apickledwalrus/skriptplaceholders/skript/RelationalPlaceholderEvent.java

@@ -0,0 +1,27 @@
+package io.github.apickledwalrus.skriptplaceholders.skript;
+
+import org.bukkit.entity.Player;
+
+/**
+ * An event to be used by Skript for passing context regarding relational placeholders during execution.
+ */
+public class RelationalPlaceholderEvent extends PlaceholderEvent {
+
+	private final Player other;
+
+	public RelationalPlaceholderEvent(String placeholder, Player player, Player other) {
+		super(placeholder, player);
+		this.other = other;
+	}
+
+	@Override
+	public Player getPlayer() {
+		//noinspection ConstantConditions - player is passed up as NotNull
+		return super.getPlayer().getPlayer();
+	}
+
+	public Player getOther() {
+		return other;
+	}
+
+}