Create MarkerHologram.java, modify Hologram.java to support subclasses overriding metadata easily

src/main/java/net/minestom/server/entity/hologram/Hologram.java

@@ -81,19 +81,39 @@ public class Hologram implements Viewable {
 
         armorStandMeta.setNotifyAboutChanges(false);
 
+        updateDefaultMeta(armorStandMeta);
+
+        armorStandMeta.setNotifyAboutChanges(true);
+
+        this.entity.setInstance(instance, spawnPosition.clone().add(0, getOffsetY(), 0));
+        this.entity.setAutoViewable(autoViewable);
+
+        this.position = spawnPosition;
+        setText(text);
+    }
+
+    /**
+     * Sets the default {@link ArmorStandMeta} flags for this Hologram,
+     * subclasses may override this method to modify the metadata.
+     *
+     * @param armorStandMeta the meta to update
+     */
+    protected void updateDefaultMeta(ArmorStandMeta armorStandMeta) {
         armorStandMeta.setSmall(true);
         armorStandMeta.setHasNoGravity(true);
         armorStandMeta.setCustomName(Component.empty());
         armorStandMeta.setCustomNameVisible(true);
         armorStandMeta.setInvisible(true);
+    }
 
-        armorStandMeta.setNotifyAboutChanges(true);
-
-        this.entity.setInstance(instance, spawnPosition.clone().add(0, OFFSET_Y, 0));
-        this.entity.setAutoViewable(autoViewable);
-
-        this.position = spawnPosition;
-        setText(text);
+    /**
+     * Vertical offset used to center the nametag,
+     * subclasses may override this method to modify the position
+     *
+     * @return the vertical offset used to center the nametag
+     */
+    protected float getOffsetY() {
+        return OFFSET_Y;
     }
 
     /**
@@ -112,7 +132,7 @@ public class Hologram implements Viewable {
      */
     public void setPosition(Position position) {
         checkRemoved();
-        position.add(0, OFFSET_Y, 0);
+        position.add(0, getOffsetY(), 0);
         this.position = position;
         this.entity.teleport(position);
     }

src/main/java/net/minestom/server/entity/hologram/MarkerHologram.java

@@ -0,0 +1,99 @@
+package net.minestom.server.entity.hologram;
+
+import net.kyori.adventure.text.Component;
+import net.minestom.server.Viewable;
+import net.minestom.server.chat.JsonMessage;
+import net.minestom.server.entity.Entity;
+import net.minestom.server.entity.EntityType;
+import net.minestom.server.entity.Player;
+import net.minestom.server.entity.metadata.other.ArmorStandMeta;
+import net.minestom.server.instance.Instance;
+import net.minestom.server.utils.Position;
+import net.minestom.server.utils.validate.Check;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * A hologram that sets the marker flag of the Armor Stand, allowing players to place blocks inside of it
+ */
+public class MarkerHologram extends Hologram {
+
+    //Y Offset such that the spawnPosition represents the center of the rendered nametag
+    private static final float OFFSET_Y = -0.40625f;
+
+    /**
+     * Constructs a new {@link MarkerHologram} with the given parameters.
+     *
+     * @param instance      The instance where the hologram should be spawned.
+     * @param spawnPosition The spawn position of this hologram.
+     * @param text          The text of this hologram.
+     * @param autoViewable  {@code true}if the hologram should be visible automatically, otherwise {@code false}.
+     * @deprecated Use {@link #MarkerHologram(Instance, Position, Component, boolean)}
+     */
+    public MarkerHologram(Instance instance, Position spawnPosition, JsonMessage text, boolean autoViewable) {
+        super(instance, spawnPosition, text, autoViewable);
+    }
+
+    /**
+     * Constructs a new {@link MarkerHologram} with the given parameters.
+     *
+     * @param instance      The instance where the hologram should be spawned.
+     * @param spawnPosition The spawn position of this hologram.
+     * @param text          The text of this hologram.
+     * @deprecated Use {@link #MarkerHologram(Instance, Position, Component)}
+     */
+    public MarkerHologram(Instance instance, Position spawnPosition, JsonMessage text) {
+        super(instance, spawnPosition, text);
+    }
+
+    /**
+     * Constructs a new {@link MarkerHologram} with the given parameters.
+     *
+     * @param instance      The instance where the hologram should be spawned.
+     * @param spawnPosition The spawn position of this hologram.
+     * @param text          The text of this hologram.
+     */
+    public MarkerHologram(Instance instance, Position spawnPosition, Component text) {
+        super(instance, spawnPosition, text);
+    }
+
+    /**
+     * Constructs a new {@link MarkerHologram} with the given parameters.
+     *
+     * @param instance      The instance where the hologram should be spawned.
+     * @param spawnPosition The spawn position of this hologram.
+     * @param text          The text of this hologram.
+     * @param autoViewable  {@code true}if the hologram should be visible automatically, otherwise {@code false}.
+     */
+    public MarkerHologram(Instance instance, Position spawnPosition, Component text, boolean autoViewable) {
+        super(instance, spawnPosition, text, autoViewable);
+    }
+
+    /**
+     * Sets the default {@link ArmorStandMeta} flags for this Hologram,
+     * subclasses may override this method to modify the metadata.
+     *
+     * {@link MarkerHologram}: Set the marker flag to true
+     *
+     * @param armorStandMeta the meta to update
+     */
+    @Override
+    protected void updateDefaultMeta(ArmorStandMeta armorStandMeta) {
+        super.updateDefaultMeta(armorStandMeta);
+
+        armorStandMeta.setMarker(true);
+    }
+
+    /**
+     * Vertical offset used to center the nametag,
+     * subclasses may override this method to modify the position
+     *
+     * {@link MarkerHologram}: Correct the Y offset for marker Armor Stands
+     *
+     * @return
+     */
+    @Override
+    protected float getOffsetY() {
+        return OFFSET_Y;
+    }
+}
+