Apply the fix for CJK languages on single-line text fields. (#22546)

Summary:
Follow-up to https://github.com/facebook/react-native/pull/19809

This fix generalizes the `setAttributedString:` fix to single-line text fields.

Fixes #19339

_Pull requests that expand test coverage are more likely to get reviewed. Add a test case whenever possible!_
Pull Request resolved: https://github.com/facebook/react-native/pull/22546

Differential Revision: D13948951

Pulled By: shergin

fbshipit-source-id: 67992c02b32f33f6d61fac4554e4f46b973262c1

Libraries/Text/TextInput/Singleline/RCTUITextField.m

@@ -14,6 +14,7 @@
 
 @implementation RCTUITextField {
   RCTBackedTextFieldDelegateAdapter *_textInputDelegateAdapter;
+  NSMutableAttributedString *_attributesHolder;
 }
 
 - (instancetype)initWithFrame:(CGRect)frame
@@ -25,6 +26,7 @@
                                                object:self];
 
     _textInputDelegateAdapter = [[RCTBackedTextFieldDelegateAdapter alloc] initWithTextField:self];
+    _attributesHolder = [[NSMutableAttributedString alloc] init];
   }
 
   return self;
@@ -107,6 +109,49 @@
   return [super caretRectForPosition:position];
 }
 
+#pragma mark - Fix for CJK Languages
+
+/* 
+ * The workaround to fix inputting complex locales (like CJK languages).
+ * When we use `setAttrbutedText:` while user is inputting text in a complex
+ * locale (like Chinese, Japanese or Korean), some internal state breaks and
+ * input stops working.
+ *
+ * To workaround that, we don't skip underlying attributedString in the text
+ * field if only attributes were changed. We keep track of these attributes in
+ * a local variable.
+ *
+ * There are two methods that are altered by this workaround:
+ *
+ * (1) `-setAttributedText:` 
+ *     Applies the attributed string change to a local variable `_attributesHolder` instead of calling `-[super setAttributedText:]`.
+ *     If new attributed text differs from the existing one only in attributes,
+ *     skips `-[super setAttributedText:`] completely.
+ *
+ * (2) `-attributedText` 
+ *     Return `_attributesHolder` context.
+ *     Updates `_atributesHolder` before returning if the underlying `super.attributedText.string` was changed.
+ *
+ */
+- (void)setAttributedText:(NSAttributedString *)attributedText
+{
+  BOOL textWasChanged = ![_attributesHolder.string isEqualToString:attributedText.string];
+  [_attributesHolder setAttributedString:attributedText];
+
+  if (textWasChanged) {
+    [super setAttributedText:attributedText];
+  }
+}
+
+- (NSAttributedString *)attributedText
+{
+  if (![super.attributedText.string isEqualToString:_attributesHolder.string]) {
+    [_attributesHolder setAttributedString:super.attributedText];
+  }
+
+  return _attributesHolder;
+}
+
 #pragma mark - Positioning Overrides
 
 - (CGRect)textRectForBounds:(CGRect)bounds