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:
https://github.com/facebook/react-native/issues/10929https://github.com/facebook/react-native/issues/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
f3b8d4976f