Revamp tag api

src/main/java/net/minestom/server/tag/Tag.java

@@ -21,6 +21,8 @@ import java.util.function.Supplier;
 @ApiStatus.NonExtendable
 public class Tag<T> {
 
+    private static final String EMPTY_KEY = "";
+
     private final String key;
     private final Function<NBTCompound, T> readFunction;
     private final BiConsumer<NBTCompound, T> writeConsumer;
@@ -85,16 +87,16 @@ public class Tag<T> {
     }
 
     public @Nullable T read(@NotNull NBTCompound nbtCompound) {
-        if (nbtCompound.containsKey(key)) {
-            return readFunction.apply(nbtCompound);
-        } else {
+        T result = readFunction.apply(nbtCompound);
+        if (result == null) {
             final var supplier = defaultValue;
-            return supplier != null ? supplier.get() : null;
+            result = supplier != null ? supplier.get() : null;
         }
+        return result;
     }
 
     public void write(@NotNull NBTCompound nbtCompound, @Nullable T value) {
-        if (value != null) {
+        if (value != null || key.equals(EMPTY_KEY)) {
             this.writeConsumer.accept(nbtCompound, value);
         } else {
             nbtCompound.removeTag(key);
@@ -180,8 +182,35 @@ public class Tag<T> {
                 (nbtCompound, value) -> nbtCompound.setLongArray(key, value));
     }
 
-    public static <T> @NotNull Tag<T> Custom(@NotNull String key, @NotNull TagSerializer<T> serializer) {
+    /**
+     * Create a wrapper around a compound.
+     *
+     * @param key        the tag key
+     * @param serializer the tag serializer
+     * @param <T>        the tag type
+     * @return the created tag
+     */
+    public static <T> @NotNull Tag<T> Structure(@NotNull String key, @NotNull TagSerializer<T> serializer) {
         return new Tag<>(key,
+                nbtCompound -> {
+                    final var compound = nbtCompound.getCompound(key);
+                    if (compound == null) {
+                        return null;
+                    }
+                    return serializer.read(TagReadable.fromCompound(compound));
+                },
+                (nbtCompound, value) -> {
+                    var compound = nbtCompound.getCompound(key);
+                    if (compound == null) {
+                        compound = new NBTCompound();
+                        nbtCompound.set(key, compound);
+                    }
+                    serializer.write(TagWritable.fromCompound(compound), value);
+                });
+    }
+
+    public static <T> @NotNull Tag<T> View(@NotNull TagSerializer<T> serializer) {
+        return new Tag<>(EMPTY_KEY,
                 nbtCompound -> serializer.read(TagReadable.fromCompound(nbtCompound)),
                 (nbtCompound, value) -> serializer.write(TagWritable.fromCompound(nbtCompound), value));
     }

src/main/java/net/minestom/server/tag/TagSerializer.java

@@ -4,7 +4,7 @@ import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
 /**
- * Interface used to create custom types compatible with {@link Tag#Custom(String, TagSerializer)}.
+ * Interface used to create custom {@link Tag tags}.
  *
  * @param <T> the type to serialize
  */
@@ -14,7 +14,7 @@ public interface TagSerializer<T> {
      * Reads the custom tag from a {@link TagReadable}.
      *
      * @param reader the reader
-     * @return the deserialized value
+     * @return the deserialized value, null if invalid
      */
     @Nullable T read(@NotNull TagReadable reader);
 
@@ -22,7 +22,7 @@ public interface TagSerializer<T> {
      * Writes the custom tag to a {@link TagWritable}.
      *
      * @param writer the writer
-     * @param value  the value to serialize
+     * @param value  the value to serialize, null to remove
      */
-    void write(@NotNull TagWritable writer, @NotNull T value);
+    void write(@NotNull TagWritable writer, @Nullable T value);
 }

src/main/java/net/minestom/server/tag/TagWritable.java

@@ -18,6 +18,10 @@ public interface TagWritable {
      */
     <T> void setTag(@NotNull Tag<T> tag, @Nullable T value);
 
+    default void removeTag(@NotNull Tag<?> tag) {
+        setTag(tag, null);
+    }
+
     /**
      * Converts an nbt compound to a tag writer.
      *

src/test/java/demo/block/CampfireHandler.java

@@ -25,8 +25,7 @@ import java.util.List;
 
 public class CampfireHandler implements BlockHandler {
 
-    public static final Tag<List<ItemStack>> ITEMS = Tag.Custom("Items", new TagSerializer<>() {
-
+    public static final Tag<List<ItemStack>> ITEMS = Tag.View(new TagSerializer<>() {
         private final Tag<NBT> internal = Tag.NBT("Items");
 
         @Override
@@ -44,7 +43,11 @@ public class CampfireHandler implements BlockHandler {
         }
 
         @Override
-        public void write(@NotNull TagWritable writer, @NotNull List<ItemStack> value) {
+        public void write(@NotNull TagWritable writer, @Nullable List<ItemStack> value) {
+            if (value == null) {
+                writer.removeTag(internal);
+                return;
+            }
             NBTList<NBTCompound> items = new NBTList<>(NBTTypes.TAG_Compound);
             for (var item : value) {
                 NBTCompound compound = new NBTCompound()