TextInput: Simplified `selectTextOnFocus` logic

Summary: Previous implementation was not quite correct (because it was framed by hacky event handling) and caused some issues with new ScrollView improvements (autoscroll to focused TextInput).

Reviewed By: javache

Differential Revision: D5414439

fbshipit-source-id: 72d1f23170340c453b939dca8b72422822acc1d7

Libraries/Text/RCTTextInput.m

@@ -95,12 +95,6 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithFrame:(CGRect)frame)
 
 - (BOOL)textInputShouldBeginEditing
 {
-  if (_selectTextOnFocus) {
-    dispatch_async(dispatch_get_main_queue(), ^{
-      [self.backedTextInputView selectAll:nil];
-    });
-  }
-
   return YES;
 }
 
@@ -110,6 +104,10 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithFrame:(CGRect)frame)
     self.backedTextInputView.text = @"";
   }
 
+  if (_selectTextOnFocus) {
+    [self.backedTextInputView selectAll:nil];
+  }
+
   [_eventDispatcher sendTextEventWithType:RCTTextEventTypeFocus
                                  reactTag:self.reactTag
                                      text:self.backedTextInputView.text