[SuperTextField] - Create a controller that supports undo/redo (Resolves #189)

super_editor/lib/src/infrastructure/super_textfield/infrastructure/attributed_text_editing_value.dart

@@ -0,0 +1,37 @@
+import 'package:attributed_text/attributed_text.dart';
+import 'package:flutter/services.dart';
+
+/// The logical value of a text editable that displays attributed
+/// text.
+class AttributedTextEditingValue {
+  factory AttributedTextEditingValue.empty() => AttributedTextEditingValue(
+        text: AttributedText(text: ""),
+      );
+
+  factory AttributedTextEditingValue.emptyWithCaret() => AttributedTextEditingValue(
+        text: AttributedText(text: ""),
+        selection: const TextSelection.collapsed(offset: 0),
+      );
+
+  const AttributedTextEditingValue({
+    required this.text,
+    this.selection = const TextSelection.collapsed(offset: -1),
+    this.composingRegion = TextRange.empty,
+  });
+
+  /// The text displayed in the editable.
+  final AttributedText text;
+
+  /// The text selection in the editable.
+  ///
+  /// A [selection] of `TextSelection.collapsed(offset: -1)` represents
+  /// the absence of a selection.
+  final TextSelection selection;
+
+  /// The region of [text] that's currently being composed.
+  ///
+  /// Composing regions are used by operating system input method
+  /// engines (IMEs) to create compound characters, check spelling,
+  /// and apply other effects on the platform side.
+  final TextRange composingRegion;
+}