Indent attribute is consider block but may have null value

lib/models/documents/attribute.dart

@@ -1,3 +1,5 @@
+import 'dart:collection';
+
 import 'package:quiver/core.dart';
 
 enum AttributeScope {
@@ -14,7 +16,7 @@ class Attribute<T> {
   final AttributeScope scope;
   final T value;
 
-  static final Map<String, Attribute> _registry = {
+  static final Map<String, Attribute> _registry = LinkedHashMap.of({
     Attribute.bold.key: Attribute.bold,
     Attribute.italic.key: Attribute.italic,
     Attribute.underline.key: Attribute.underline,
@@ -26,16 +28,16 @@ class Attribute<T> {
     Attribute.background.key: Attribute.background,
     Attribute.placeholder.key: Attribute.placeholder,
     Attribute.header.key: Attribute.header,
-    Attribute.indent.key: Attribute.indent,
     Attribute.align.key: Attribute.align,
     Attribute.list.key: Attribute.list,
     Attribute.codeBlock.key: Attribute.codeBlock,
     Attribute.blockQuote.key: Attribute.blockQuote,
+    Attribute.indent.key: Attribute.indent,
     Attribute.width.key: Attribute.width,
     Attribute.height.key: Attribute.height,
     Attribute.style.key: Attribute.style,
     Attribute.token.key: Attribute.token,
-  };
+  });
 
   static final BoldAttribute bold = BoldAttribute();
 
@@ -88,22 +90,22 @@ class Attribute<T> {
     Attribute.placeholder.key,
   };
 
-  static final Set<String> blockKeys = {
+  static final Set<String> blockKeys = LinkedHashSet.of({
     Attribute.header.key,
-    Attribute.indent.key,
     Attribute.align.key,
     Attribute.list.key,
     Attribute.codeBlock.key,
     Attribute.blockQuote.key,
-  };
+    Attribute.indent.key,
+  });
 
-  static final Set<String> blockKeysExceptHeader = {
+  static final Set<String> blockKeysExceptHeader = LinkedHashSet.of({
     Attribute.list.key,
-    Attribute.indent.key,
     Attribute.align.key,
     Attribute.codeBlock.key,
     Attribute.blockQuote.key,
-  };
+    Attribute.indent.key,
+  });
 
   static Attribute<int?> get h1 => HeaderAttribute(level: 1);
 
@@ -172,6 +174,18 @@ class Attribute<T> {
     return attribute;
   }
 
+  static int getRegistryOrder(Attribute attribute) {
+    var order = 0;
+    for (final attr in _registry.values) {
+      if (attr.key == attribute.key) {
+        break;
+      }
+      order++;
+    }
+
+    return order;
+  }
+
   static Attribute clone(Attribute origin, dynamic value) {
     return Attribute(origin.key, origin.scope, value);
   }

lib/models/documents/style.dart

@@ -30,7 +30,8 @@ class Style {
 
   Iterable<String> get keys => _attributes.keys;
 
-  Iterable<Attribute> get values => _attributes.values;
+  Iterable<Attribute> get values => _attributes.values.sorted(
+      (a, b) => Attribute.getRegistryOrder(a) - Attribute.getRegistryOrder(b));
 
   Map<String, Attribute> get attributes => _attributes;