Fixes selection of single line text input (#24714)

Summary:
`UITextField` don't have delegate to observe the selection changes of users(multiline text input `UITextView` have it), so we can add a KVO to observe selection changes.

cpojer shergin

[iOS] [Fixed] - Fixes selection of single line text input
Pull Request resolved: https://github.com/facebook/react-native/pull/24714

Differential Revision: D15238379

Pulled By: cpojer

fbshipit-source-id: f149721d6b4df28e90f5a9405c74e01fde7c7d10

Libraries/Text/TextInput/RCTBackedTextInputDelegateAdapter.m

@@ -28,15 +28,27 @@ static void *TextFieldSelectionObservingContext = &TextFieldSelectionObservingCo
 
     [_backedTextInputView addTarget:self action:@selector(textFieldDidChange) forControlEvents:UIControlEventEditingChanged];
     [_backedTextInputView addTarget:self action:@selector(textFieldDidEndEditingOnExit) forControlEvents:UIControlEventEditingDidEndOnExit];
+    [_backedTextInputView addObserver:self forKeyPath:@"selectedTextRange" options:NSKeyValueObservingOptionNew context:NULL];
   }
 
   return self;
 }
 
+- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
+{
+  // UITextField doesn't have a delegate like UITextView to get notified on selection. Use KVO to observe changes.
+  if ([keyPath isEqualToString:@"selectedTextRange"]) {
+    [self textFieldProbablyDidChangeSelection];
+  } else {
+    [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
+  }
+}
+
 - (void)dealloc
 {
   [_backedTextInputView removeTarget:self action:nil forControlEvents:UIControlEventEditingChanged];
   [_backedTextInputView removeTarget:self action:nil forControlEvents:UIControlEventEditingDidEndOnExit];
+  [_backedTextInputView removeObserver:self forKeyPath:@"selectedTextRange"];
 }
 
 #pragma mark - UITextFieldDelegate