Bug fix: <TextInput> content is reset when emoji is entered at the max length

Summary:
When maxLength is defined in <TextInput>, if the last character at max length is an emoji, the content of the input is cleared:

{F620865178} {F620865237}

Related Github issues:

#10929
#10964

## Root cause:

When NSString was created, unicode characters were 16-bit long, so Objective-C considers every unicode character as 16-bit. However, unicode was later extended to more than 16bit, for example, emojis, which causes NSString substring method cuts off at the wrong position.

Example:

```
NSString *s = @"abc{emoji:1f601}";
NSInteger len = s.length; //length is 5 (as {emoji:1f601} occupies two 16-bit characters)
NSString *s3 = [s substringToIndex: 3]; //s3 is "abc"
NSString *s4 = [s substringToIndex: 4]; //s4 is null!
```

If string s, "abc{emoji:1f601}", is entered in <TextInput>, which has max length 4, it will truncate the string to the first 4 characters, "cutting" the emoji in half which causes encoding error and returns null. The text input is cleared.

## Solution:

If the character at max length is longer than 16-bit, truncate the character BEFORE it instead. In the previous example, truncate till index 3 instead of 4. The end result will be "abc" and the emoji is dropped.

## Changelog:

[iOS] [Fixed] - <TextInput> content is reset when emoji is entered at the max length

Reviewed By: p-sun

Differential Revision: D28821909

fbshipit-source-id: 4720d864970b554160ed5388f65b352ce95a6199

Author: Shuhao Zhang

Libraries/Text/TextInput/RCTBaseTextInputView.m

@@ -397,6 +397,14 @@ - (NSString *)textInputShouldChangeText:(NSString *)text inRange:(NSRange)range
     if (text.length > allowedLength) {
       // If we typed/pasted more than one character, limit the text inputted.
       if (text.length > 1) {
+        if (allowedLength > 0) {
+          //make sure unicode characters that are longer than 16 bits (such as emojis) are not cut off
+          NSRange cutOffCharacterRange = [text rangeOfComposedCharacterSequenceAtIndex: allowedLength - 1];
+          if (cutOffCharacterRange.location + cutOffCharacterRange.length > allowedLength) {
+            //the character at the length limit takes more than 16bits, truncation should end at the character before
+            allowedLength = cutOffCharacterRange.location;
+          }
+        }
         // Truncate the input string so the result is exactly maxLength
         NSString *limitedString = [text substringToIndex:allowedLength];
         NSMutableAttributedString *newAttributedText = [backedTextInputView.attributedText mutableCopy];

React/Fabric/Mounting/ComponentViews/TextInput/RCTTextInputComponentView.mm

@@ -335,6 +335,15 @@ - (NSString *)textInputShouldChangeText:(NSString *)text inRange:(NSRange)range
   if (props.maxLength) {
     NSInteger allowedLength = props.maxLength - _backedTextInputView.attributedText.string.length + range.length;
 
+    if (allowedLength > 0 && text.length > allowedLength) {
+      // make sure unicode characters that are longer than 16 bits (such as emojis) are not cut off
+      NSRange cutOffCharacterRange = [text rangeOfComposedCharacterSequenceAtIndex:allowedLength - 1];
+      if (cutOffCharacterRange.location + cutOffCharacterRange.length > allowedLength) {
+        // the character at the length limit takes more than 16bits, truncation should end at the character before
+        allowedLength = cutOffCharacterRange.location;
+      }
+    }
+
     if (allowedLength <= 0) {
       return nil;
     }