Add kept items expression and effect

src/main/java/ch/njol/skript/effects/EffKeepItems.java

@@ -18,6 +18,7 @@
  */
 package ch.njol.skript.effects;
 
+import java.util.Iterator;
 import java.util.List;
 
 import org.bukkit.event.Event;
@@ -40,21 +41,29 @@
 import ch.njol.util.Kleenean;
 
 @Name("Keep Items")
-@Description("Keeps the given items from dropping in a 'death of player' event.")
-@Examples({"on death of player:",
-		"\tkeep the drops"})
+@Description({
+	"Keeps the given items from dropping in a 'death of player' event.",
+	"Note: this effect will only keep items that have been in the drops.",
+	"Use the <a href='expressions.html#ExprKeptItems'>kept items</a> expression for items outside of the drops."
+})
+@Examples({
+	"on death of player:",
+		"\tkeep the drops"
+})
 @Events("death")
-@RequiredPlugins("Paper 1.13+ (keep specific items)")
+@RequiredPlugins("Paper (keep specific items)")
 @Since("INSERT VERSION")
 public class EffKeepItems extends Effect {
 
 	static {
-		Skript.registerEffect(EffKeepItems.class, "keep %item types% [from ([the] drops|dropping)]");
+		Skript.registerEffect(EffKeepItems.class, "keep %itemtypes% [from ([the] drops|dropping)]");
 	}
 
-	private static final boolean SPIGOT = Skript.methodExists(PlayerDeathEvent.class, "getItemsToKeep");
+	private static final boolean SPIGOT = !Skript.methodExists(PlayerDeathEvent.class, "getItemsToKeep");
 	private boolean keepAll;
+
 	private Expression<ItemType> items;
+
 	@Override
 	public boolean init(Expression<?>[] exprs, int matchedPattern, Kleenean isDelayed, ParseResult parseResult) {
 		if (!getParser().isCurrentEvent(PlayerDeathEvent.class)) {
@@ -74,6 +83,9 @@ public boolean init(Expression<?>[] exprs, int matchedPattern, Kleenean isDelaye
 
 	@Override
 	protected void execute(Event event) {
+		if (!(event instanceof PlayerDeathEvent))
+			return;
+
 		PlayerDeathEvent e = (PlayerDeathEvent) event;
 		List<ItemStack> drops = e.getDrops();
 
@@ -84,14 +96,19 @@ protected void execute(Event event) {
 		}
 
 		List<ItemStack> keptItems = e.getItemsToKeep();
-		for (ItemType item : items.getArray(e)) {
-			item.addTo(keptItems);
-			item.removeFrom(drops);
-		}
+		// We make sure the items we're keeping are part of the drops this way
+		List<ItemStack> itemsToKeep = items.stream(event)
+			.filter(item -> item.isContainedIn(drops))
+			.map(ItemType::getRandom)
+			.toList();
+
+		keptItems.addAll(itemsToKeep);
+		drops.removeAll(itemsToKeep);
 	}
 
 	@Override
 	public String toString(@Nullable Event event, boolean debug) {
 		return "keep " + items.toString(event, debug) + " from the drops";
 	}
+
 }

src/main/java/ch/njol/skript/expressions/ExprKeptItems.java

@@ -42,14 +42,17 @@
 import ch.njol.util.coll.CollectionUtils;
 
 @Name("Kept Items")
-@Description({"The kept items in a 'death of player' event.",
-	"",
-	"Note: this expression won't remove the items from the drops when changed."})
+@Description({
+	"The kept items in a 'death of player' event.",
+	"Note: this expression won't remove the items from the drops when changed.",
+	"Use the <a href='effects.html#EffKeepItems'>keep items</a> effect for that."
+})
 @Examples({
 	"on death of player:",
-	"\tset kept items to the drops where [input isn't victim's tool]"})
+		"\tset kept items to the drops where [input isn't victim's tool]"
+})
 @Events("death")
-@RequiredPlugins("Paper 1.13+")
+@RequiredPlugins("Paper")
 @Since("INSERT VERSION")
 public class ExprKeptItems extends SimpleExpression<ItemType> {
 
@@ -69,15 +72,20 @@ public boolean init(Expression<?>[] exprs, int matchedPattern, Kleenean isDelaye
 	}
 
 	@Override
-	protected @Nullable ItemType[] get(Event event) {
+	@Nullable 
+	protected ItemType[] get(Event event) {
+		if (!(event instanceof PlayerDeathEvent))
+			return null;
+
 		return ((PlayerDeathEvent) event).getItemsToKeep()
 			.stream()
 			.map(ItemType::new)
 			.toArray(ItemType[]::new);
 	}
 
 	@Override
-	public @Nullable Class<?>[] acceptChange(ChangeMode mode) {
+	@Nullable 
+	public Class<?>[] acceptChange(ChangeMode mode) {
 		switch (mode) {
 			case ADD:
 			case REMOVE:
@@ -96,11 +104,14 @@ public void change(Event event, @Nullable Object[] delta, ChangeMode mode) {
 			return;
 
 		List<ItemStack> keptItems = ((PlayerDeathEvent) event).getItemsToKeep();
-
+		boolean canChange = delta != null;
 		// Clear items before adding the new values
-		if (mode == ChangeMode.SET)
+		if ((mode == ChangeMode.SET && canChange) || mode == ChangeMode.DELETE || mode == ChangeMode.RESET)
 			keptItems.clear();
 
+		if (!canChange)
+			return;
+
 		for (Object o : delta) {
 			ItemType item = (ItemType) o;
 			switch (mode) {
@@ -113,9 +124,6 @@ public void change(Event event, @Nullable Object[] delta, ChangeMode mode) {
 					break;
 				case REMOVE_ALL:
 					item.removeAll(keptItems);
-				case DELETE:
-				case RESET:
-					keptItems.clear();
 					break;
 			}
 		}
@@ -132,9 +140,8 @@ public Class<? extends ItemType> getReturnType() {
 	}
 
 	@Override
-	public String toString(@Nullable Event e, boolean debug) {
+	public String toString(@Nullable Event event, boolean debug) {
 		return "the kept items";
 	}
 
 }
-