Fixes complex input on iOS in RN 0.55.x.

The fix is described here: facebook#19809

Author: rasom

Libraries/Text/TextInput/Multiline/RCTUITextView.m

@@ -110,7 +110,21 @@ - (void)setTextAlignment:(NSTextAlignment)textAlignment
 
 - (void)setAttributedText:(NSAttributedString *)attributedText
 {
-  [super setAttributedText:attributedText];
+  // Using `setAttributedString:` while user is typing breaks some internal mechanics
+  // when entering complex input languages such as Chinese, Korean or Japanese.
+  // see: https://github.com/facebook/react-native/issues/19339
+
+  // We try to avoid calling this method as much as we can.
+  // If the text has changed, there is nothing we can do.
+  if (![super.attributedText.string isEqualToString:attributedText.string]) {
+    [super setAttributedText:attributedText];
+  } else {
+  // But if the text is preserved, we just copying the attributes from the source string.
+    if (![super.attributedText isEqualToAttributedString:attributedText]) {
+      [self copyTextAttributesFrom:attributedText];
+    }
+  }
+
   [self textDidChange];
 }
 
@@ -241,4 +255,20 @@ - (void)invalidatePlaceholderVisibility
   _placeholderView.hidden = !isVisible;
 }
 
+#pragma mark - Utility Methods
+
+- (void)copyTextAttributesFrom:(NSAttributedString *)sourceString
+{
+  [self.textStorage beginEditing];
+
+  NSTextStorage *textStorage = self.textStorage;
+  [sourceString enumerateAttributesInRange:NSMakeRange(0, sourceString.length)
+                                   options:NSAttributedStringEnumerationReverse
+                                usingBlock:^(NSDictionary<NSAttributedStringKey,id> * _Nonnull attrs, NSRange range, BOOL * _Nonnull stop) {
+                                  [textStorage setAttributes:attrs range:range];
+                                }];
+
+  [self.textStorage endEditing];
+}
+
 @end