Abstract Leashable to a separate interface

src/main/java/org/spongepowered/api/data/Keys.java

@@ -2133,7 +2133,7 @@ public final class Keys {
     public static final Key<Value<Integer>> LAYER = Keys.key(ResourceKey.sponge("layer"), Integer.class);
 
     /**
-     * The holder of a leashed {@link Agent}
+     * The holder of a leashed {@link Entity}
      * e.g. a {@link Player} or {@link LeashKnot}.
      * <p>Usually, a {@link LeashKnot} will always exist so long as there is
      * a leashed {@link Entity} attached. If the leash is broken, the leash

src/main/java/org/spongepowered/api/entity/Leashable.java

@@ -0,0 +1,45 @@
+/*
+ * This file is part of SpongeAPI, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) SpongePowered <https://www.spongepowered.org>
+ * Copyright (c) contributors
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package org.spongepowered.api.entity;
+
+import org.spongepowered.api.data.Keys;
+import org.spongepowered.api.data.value.Value;
+
+import java.util.Optional;
+
+/**
+ * Represents an {@link Entity} which can be leashed.
+ */
+public interface Leashable extends Entity {
+
+    /**
+     * {@link Keys#LEASH_HOLDER}
+     *
+     * @return The holder of the leashed entity
+     */
+    default Optional<Value.Mutable<Entity>> leashHolder() {
+        return this.getValue(Keys.LEASH_HOLDER).map(Value::asMutable);
+    }
+}

src/main/java/org/spongepowered/api/entity/living/Agent.java

@@ -28,6 +28,7 @@
 import org.spongepowered.api.data.type.HandPreference;
 import org.spongepowered.api.data.value.Value;
 import org.spongepowered.api.entity.Entity;
+import org.spongepowered.api.entity.Leashable;
 import org.spongepowered.api.entity.ai.goal.GoalExecutor;
 import org.spongepowered.api.entity.ai.goal.GoalExecutorType;
 import org.spongepowered.api.entity.ai.goal.builtin.creature.target.TargetGoal;
@@ -39,7 +40,7 @@
  * An Agent represents a {@link Living} that has AI. In the future Sponge will
  * allow for custom AIs, but for now vanilla behavior can only be disabled.
  */
-public interface Agent extends Living, ArmorEquipable {
+public interface Agent extends Living, Leashable, ArmorEquipable {
 
     /**
      * {@link Keys#TARGET_ENTITY}
@@ -78,15 +79,6 @@ default Value.Mutable<Boolean> persistent() {
         return this.requireValue(Keys.IS_PERSISTENT).asMutable();
     }
 
-    /**
-     * {@link Keys#LEASH_HOLDER}
-     *
-     * @return The holder of the leashed agent
-     */
-    default Optional<Value.Mutable<Entity>> leashHolder() {
-        return this.getValue(Keys.LEASH_HOLDER).map(Value::asMutable);
-    }
-
     /**
      * Gets a {@link GoalExecutor} based on the {@link GoalExecutorType}.
      *

src/main/java/org/spongepowered/api/entity/vehicle/Boat.java

@@ -27,11 +27,12 @@
 import org.spongepowered.api.data.Keys;
 import org.spongepowered.api.data.type.BoatType;
 import org.spongepowered.api.data.value.Value;
+import org.spongepowered.api.entity.Leashable;
 
 /**
  * Represents a Boat entity.
  */
-public interface Boat extends Vehicle {
+public interface Boat extends Vehicle, Leashable {
 
     /**
      * {@link Keys#BOAT_TYPE}