Prevent WhitelistingTextInputFormatter to return a empty string if the current...

Prevent WhitelistingTextInputFormatter to return a empty string if the current value does not satisfy the formatter (#57264)

packages/flutter/lib/src/services/text_formatter.dart

@@ -107,10 +107,11 @@ class BlacklistingTextInputFormatter extends TextInputFormatter {
 
   @override
   TextEditingValue formatEditUpdate(
-    TextEditingValue oldValue, // unused.
+    TextEditingValue oldValue,
     TextEditingValue newValue,
   ) {
     return _selectionAwareTextManipulation(
+      oldValue,
       newValue,
       (String substring) {
         return substring.replaceAll(blacklistedPattern, replacementString);
@@ -239,10 +240,11 @@ class WhitelistingTextInputFormatter extends TextInputFormatter {
 
   @override
   TextEditingValue formatEditUpdate(
-    TextEditingValue oldValue, // unused.
+    TextEditingValue oldValue,
     TextEditingValue newValue,
   ) {
     return _selectionAwareTextManipulation(
+      oldValue,
       newValue,
       (String substring) {
         return whitelistedPattern
@@ -259,6 +261,7 @@ class WhitelistingTextInputFormatter extends TextInputFormatter {
 }
 
 TextEditingValue _selectionAwareTextManipulation(
+  TextEditingValue oldValue,
   TextEditingValue value,
   String substringManipulation(String substring),
 ) {
@@ -268,6 +271,19 @@ TextEditingValue _selectionAwareTextManipulation(
   TextSelection manipulatedSelection;
   if (selectionStartIndex < 0 || selectionEndIndex < 0) {
     manipulatedText = substringManipulation(value.text);
+  } else if (value.selection.isCollapsed) { // Non-selection text manipulation
+    int cursorPosition = value.selection.baseOffset;
+    manipulatedText = substringManipulation(value.text);
+    // We only return the old valid value if the current value is not empty and
+    // if manipulation fails.
+    if (value.text.isNotEmpty && manipulatedText.isEmpty) {
+      manipulatedText = oldValue.text;
+      // We decrease cursorPosition by one because
+      // user entered a invalid character.
+      cursorPosition -= 1;
+    }
+    // Move the caret to its previous position.
+    manipulatedSelection = TextSelection.collapsed(offset: cursorPosition);
   } else {
     final String beforeSelection = substringManipulation(
       value.text.substring(0, selectionStartIndex)

packages/flutter/test/services/text_formatter_test.dart

@@ -84,4 +84,55 @@ void main() {
       });
     });
   });
+
+  group('WhitelistingTextInputFormatter', () {
+    test('should return the old value if new value contains non-white-listed character', () {
+      const TextEditingValue oldValue = TextEditingValue(text: '12345');
+      const TextEditingValue newValue = TextEditingValue(text: '12345@');
+
+      final WhitelistingTextInputFormatter formatter = WhitelistingTextInputFormatter.digitsOnly;
+      final TextEditingValue formatted = formatter.formatEditUpdate(oldValue, newValue);
+
+      // assert that we are passing digits only at the first time
+      expect(oldValue.text, equals('12345'));
+      // The new value is always the oldValue plus a non-digit character (user press @)
+      expect(newValue.text, equals('12345@'));
+      // we expect that the formatted value returns the oldValue only since the newValue does not
+      // satisfy the formatter condition (wich is, in this case digitsOnly)
+      expect(formatted.text, equals('12345'));
+    });
+
+    test('should move the cursor to the right position', () {
+      TextEditingValue collapsedValue(String text, int offset) =>
+          TextEditingValue(
+            text: text,
+            selection: TextSelection.collapsed(offset: offset),
+          );
+
+      TextEditingValue oldValue = collapsedValue('123', 0);
+      TextEditingValue newValue = collapsedValue('123456', 6);
+
+      final WhitelistingTextInputFormatter formatter =
+          WhitelistingTextInputFormatter.digitsOnly;
+      TextEditingValue formatted = formatter.formatEditUpdate(oldValue,
+          newValue);
+
+      // assert that we are passing digits only at the first time
+      expect(oldValue.text, equals('123'));
+      // assert that we are passing digits only at the second time
+      expect(newValue.text, equals('123456'));
+      // assert that cursor is at the end of the text
+      expect(formatted.selection.baseOffset, equals(6));
+
+      // move cursor at the middle of the text and then add the number 9.
+      oldValue = newValue.copyWith(
+          selection: const TextSelection.collapsed(offset: 4));
+      newValue = oldValue.copyWith(text: '1239456');
+
+      formatted = formatter.formatEditUpdate(oldValue, newValue);
+
+      // cursor must be now at fourth position (right after the number 9)
+      expect(formatted.selection.baseOffset, equals(4));
+    });
+  });
 }