Maintain cursor position when text changes in text input

Summary:
Changelog: [internal]

Cursor position needs to be calculated when attributed string changes.

Reviewed By: JoshuaGross

Differential Revision: D29786190

fbshipit-source-id: 99a42dc4d7c84e77c40f75bf4a9108d010bb1792

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

@@ -528,16 +528,21 @@ - (void)_updateState
 
 - (void)_setAttributedString:(NSAttributedString *)attributedString
 {
- UITextRange *selectedRange = [_backedTextInputView selectedTextRange];
+ UITextRange *selectedRange = _backedTextInputView.selectedTextRange;
+ NSInteger oldTextLength = _backedTextInputView.attributedText.string.length;
  if (![self _textOf:attributedString equals:_backedTextInputView.attributedText]) {
  _backedTextInputView.attributedText = attributedString;
  }
- if (_lastStringStateWasUpdatedWith.length == attributedString.length) {
- // Calling `[_backedTextInputView setAttributedText]` moves caret
- // to the end of text input field. This cancels any selection as well
- // as position in the text input field. In case the length of string
- // doesn't change, selection and caret position is maintained.
- [_backedTextInputView setSelectedTextRange:selectedRange notifyDelegate:NO];
+ if (selectedRange.empty) {
+ // Maintaining a cursor position relative to the end of the old text.
+ NSInteger offsetStart = [_backedTextInputView offsetFromPosition:_backedTextInputView.beginningOfDocument
+ toPosition:selectedRange.start];
+ NSInteger offsetFromEnd = oldTextLength - offsetStart;
+ NSInteger newOffset = attributedString.string.length - offsetFromEnd;
+ UITextPosition *position = [_backedTextInputView positionFromPosition:_backedTextInputView.beginningOfDocument
+ offset:newOffset];
+ [_backedTextInputView setSelectedTextRange:[_backedTextInputView textRangeFromPosition:position toPosition:position]
+ notifyDelegate:YES];
  }
  _lastStringStateWasUpdatedWith = attributedString;
 }