Avoid using -[UITextView setAttributedString:] while user is typing (#19809)

Summary:
iOS-specific.
For languages with complex input (such as Japanese or Chinese), a user has to type multiple characters that are then merged into a single one.
If `-[UITextView setAttributedString:]` is used while the user is still typing, it resets the input and characters are not being treated as typed together.

This PR avoids calling this method if possible, replacing it by just copying the attributes if the string has not been changed. That preserves the state and user can continue to type Korean or Chinese characters.

Fixes #19339

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Essentially, the steps to reproduce are described in [the issue](#19339):

1. Type some Korean characters in TextInput, such as "하늘" (buttons `ㅎ`,`ㅏ`,`ㄴ`,`ㅡ`,`ㄹ`).
2. Then move the cursor to the beginning of the text, type "파란" (buttons `ㅍ`,`ㅏ`,`ㄹ`,`ㅏ`,`ㄴ`) this time.

**Behaviour before this fix (broken)**
Actual text: `ㅍㅏㄹㅏㄴ하늘`.
Expected text: `파란하늘`.
Characters aren't combined properly.

![ezgif com-resize](https://user-images.githubusercontent.com/466427/41613572-4256dda8-73f6-11e8-99a9-0ab833202b95.gif)

**Behaviour after this fix (correct)**
Actual text: `파란하늘`.
Expected text: `파란하늘`.
Characters are combined, the same behaviour is in vanilla iOS `UITextView`.

![input-with-fix](https://user-images.githubusercontent.com/466427/41613526-1aae2284-73f6-11e8-87f2-c1cef51cd83a.gif)

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[IOS] [BUGFIX] [TextView] - Fix Korean/Chinese/Japanese input for multiline TextView on iOS.

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  [ GENERAL  ] [ BUGFIX      ] [ {Component} ]
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 EXAMPLES:

 [IOS] [BREAKING] [FlatList] - Change a thing that breaks other things
 [ANDROID] [BUGFIX] [TextInput] - Did a thing to TextInput
 [CLI] [FEATURE] [local-cli/info/info.js] - CLI easier to do things with
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Pull Request resolved: #19809

Differential Revision: D13326614

Pulled By: shergin

fbshipit-source-id: 6a5cab3f7290f0f623a6f4c29353a573eb321b0b

Author: Igor Mandrigin

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