Fix EditableText misplaces caret when selection is invalid

[bleroux]

Description

EditableTextState._handleFocusChanged adjusts the selection when a text field is focused. But if the selection becomes invalid (for instance after setting TextController.text) on an already focused field the position won’t be adjusted. The issue can be very visible when TextField.textAlign is set to TextAlign.center (video using the Android emulator):

120631_Before.mp4

Code sample for the repro

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  final TextEditingController textController = TextEditingController();

  MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        body: Center(
          child: Row(children: [
            SizedBox(
                width: 60,
                child: TextField(
                  controller: textController,
                  textAlign: TextAlign.center,
                )),
            TextButton(
              child: const Text("Reset text", style: TextStyle(color: Colors.red)),
              onPressed: () {
                textController.text = "0";
              },
            ),
          ]),
        ),
      ),
    );
  }
}

Implementation

AFAIK, there are several ways to fix this issue:

1. Not showing the caret when the selection is invalid. This is described in RenderEditable should not show the caret when the selection is invalid (i.e. (-1, -1)) #79495. Unfortunately a PR for it was reverted (see [RenderEditable] Dont paint caret when selection is invalid #79607) due to a Google internal test failure.

2. Changing the selection to a valid one when TextController selection is invalid and the field is focused. this is the solution implemented in this PR. It breaks several tests which expect that the selection stays invalid.

3. Adding an assert to throw a detailed error message. The impact on existing tests will probably be worth than with solution 2 because for some of these tests It might be difficult to make them complied with this new requirement (for solution 2, tests can be updated by updating some expect calls).

Related Issue

Fixes #120631

Tests

Adds 1 test.
Will need to update several existing tests if becoming a real PR.

[justinmc]

Thanks for fixing this! I say do it and go ahead with updating the failing tests. It looks like there are only 2 failing in the framework tests check?

I'll also run the Google tests on this.

[justinmc]

This comment isn't exactly what you're doing, is it old?

[bleroux]

I can make this comment more self explanatory, but it was done on purpose because TextEditingController.test setter explicitly set the selection offset to -1 (which is the root of the issue addressed in this PR):

flutter/packages/flutter/lib/src/widgets/editable_text.dart

Lines 224 to 230 in f98499f

	set text(String newText) {
	value = value.copyWith(
	text: newText,
	selection: const TextSelection.collapsed(offset: -1),
	composing: TextRange.empty,
	);
	}

Because TextEditingController.text is a public API, I did not focus on trying to change its implementation as it will probably be a breaking change and also because there might be no fix that would work in all cases. What can be done at the TextEditingController.text level is probably to enhance its documentation to discourage its use and to recommand using TextEditingController.value instead.

While writing this, I wonder if we can add a method to TextEditingController, with a String parameter, which will set the text using the given value AND set the selection collapsed at the end of the text?

[bleroux]

Oh, got it: my comment is true for the moment but it would be wrong if the PR is merged.
I updated it, thanks!

[justinmc]

Thanks 👍 . I've shied away from adding new methods to TextEditingController because it's used so widely. I think we do need a better place to put typical text editing operations, though.

[justinmc]

This seems reasonable to me. There have been some problems with invalid selection. The engine often will set the selection to -1, -1, to mean "no selection" (and elsewhere, sometimes a null selection is used to mean no selection). If the field is focused though, it makes sense to set the cursor to the end of the field as an improvement on those kinds of bugs.

[bleroux]

Thanks for fixing this! I say do it and go ahead with updating the failing tests. It looks like there are only 2 failing in the framework tests check?

I see a lot more tests failures: 22 test failures locally 😅
I will try to fix them so we can discuss in detail if it makes sense to update these tests.

[bleroux]

@justinmc
I updated failing tests. One choice was to replace most of TextController.text setter usage. With this PR, the selection won't be invalid but I think it is better to promote TextController.value instead.
I commented out one test in editable_text_shortcuts_test.dartbecause it would be obsoleted by this PR. I don't know if removing a test is ok?

[justinmc]

Nit: I find it a little strange that _adjustedSelectionWhenFocused returns a nullable value and we use ! here. Like you have to have knowledge about the if statement above and the internals of _adjustedSelectionWhenFocused in order to use that !.

I can't think of a better way to do it though. If nothing jumps out at you as a way to improve that, I say don't worry about it and leave it as-is.

[justinmc]

Good catch 🔎

[justinmc]

I think it's fine to delete the whole test! Making a bug impossible is even better than having a test to catch it.

[justinmc]

Thanks 👍 . I've shied away from adding new methods to TextEditingController because it's used so widely. I think we do need a better place to put typical text editing operations, though.

[justinmc]

LGTM after you delete that test 👍

[justinmc]

CC @Renzo-Olivares Can you run these Google tests locally and see if they pass?

[ebadta81]

Great job guys. I tested this on my test project, it is working fine.