add delete handler (#501)

lib/src/widgets/controller.dart

@@ -11,6 +11,7 @@ import '../models/quill_delta.dart';
 import '../utils/diff_delta.dart';
 
 typedef ReplaceTextCallback = bool Function(int index, int len, Object? data);
+typedef DeleteCallback = void Function(int cursorPosition, bool forward);
 
 class QuillController extends ChangeNotifier {
   QuillController({
@@ -18,6 +19,7 @@ class QuillController extends ChangeNotifier {
     required TextSelection selection,
     bool keepStyleOnNewLine = false,
     this.onReplaceText,
+    this.onDelete,
   })  : _selection = selection,
         _keepStyleOnNewLine = keepStyleOnNewLine;
 
@@ -39,10 +41,13 @@ class QuillController extends ChangeNotifier {
   TextSelection get selection => _selection;
   TextSelection _selection;
 
-  /// Manual [replaceText] handler
+  /// Custom [replaceText] handler
   /// Return false to ignore the event
   ReplaceTextCallback? onReplaceText;
 
+  /// Custom delete handler
+  DeleteCallback? onDelete;
+
   /// Store any styles attribute that got toggled by the tap of a button
   /// and that has not been applied yet.
   /// It gets reset after each format action within the [document].
@@ -186,6 +191,12 @@ class QuillController extends ChangeNotifier {
     ignoreFocusOnTextChange = false;
   }
 
+  /// Called in two cases:
+  /// forward == false && textBefore.isEmpty
+  /// forward == true && textAfter.isEmpty
+  void handleDelete(int cursorPosition, bool forward) =>
+      onDelete?.call(cursorPosition, forward);
+
   void formatText(int index, int len, Attribute? attribute) {
     if (len == 0 &&
         attribute!.isInline &&

lib/src/widgets/raw_editor/raw_editor_state_keyboard_mixin.dart

@@ -153,12 +153,16 @@ mixin RawEditorStateKeyboardMixin on EditorState {
     final newSelection = TextSelection.collapsed(offset: cursorPosition);
     final newText = textBefore + textAfter;
     final size = plainText.length - newText.length;
-    widget.controller.replaceText(
-      cursorPosition,
-      size,
-      '',
-      newSelection,
-    );
+    if (size == 0) {
+      widget.controller.handleDelete(cursorPosition, forward);
+    } else {
+      widget.controller.replaceText(
+        cursorPosition,
+        size,
+        '',
+        newSelection,
+      );
+    }
   }
 
   TextSelection _jumpToBeginOrEndOfWord(