Check if MultiChildRenderObjectElement has an associated RO

[xu-baolin]

Fixes #78159

If we mount a non-RenderObjectElement to MultiChildRenderObjectElement will cause ContainerRenderObjectMixin works abnormal. This is because the MultiChildRenderObjectElement.insertRenderObjectChild depend on the RO to decide where the new RO to insert. If some of the elements's RO is null, the new RO child will insert at the first position by mistake.

This change tries to add a checker for the situation.

[xu-baolin]

If we check after updateChildren, some scenarios where GlobalKey are reused will throw more exceptions. See the test cases for details.
If we want to avoid this happens, we need to extend the updateChildren parameter and pass the check function to it. However, if we do this, the modification is very invasive. What is your opinion?

[xu-baolin]

@chunhtai

[xu-baolin]

It seems like that we can override the inflateWidge and make the check instead. I will try it.

[xu-baolin]

@chunhtai Hi,
If we make checking after updateChildren like this commit, it will break some tests because retaken the globalKey will re-parent the RO and trigger the new assert first. Also, need to traverse all the children extra.

[xu-baolin]

And we are not easy to override the Element.updateChildren , because it is too complicated.

[chunhtai]

The original issue seems to be an indexedStack specific issue, where it assume the first renderobject corresponds to index 0, second to index 1, ... and so on. We had the similar issue in RenderParagraph, where we tag the child renderobject with the index.

I am not confident whether we should force the same thing to other widget. For example, column widget does not really care about this

[xu-baolin]

@chunhtai Hey
I think this is a common bug of MultiChildRenderObjectElement, for example

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}
final Key key = GlobalKey(debugLabel: 'problematic');
class MyApp extends StatelessWidget {
  const MyApp({
    Key key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        appBar: AppBar(title: const Text('Oops')),
        body: Center(
          child: Column(
            // index: 0,
            children: [
              // Container(key: const ValueKey<int>(1)),
              // Container(key: const ValueKey<int>(1), child: Container(key: key)),
              // // Container(key: const ValueKey<int>(2),),
              // Container(key: const ValueKey<int>(2), child: Container(key: key)),
              Text('1'),
              Text('11'),
              const AnotherWidget(),
              Text('2'),
              // Container(),
              // Text('1'),
            ],
          ),
        ),
      ),
    );
  }
}

class AnotherWidget extends Widget {
  const AnotherWidget({Key key}) : super(key: key);

  @override
  Element createElement() => _AnotherElement(this);
}

class _AnotherElement extends Element {
  _AnotherElement(AnotherWidget widget) : super(widget);

  @override
  bool get debugDoingBuild => false;

  @override
  void performRebuild() {}
}

[xu-baolin]

Empty widgets will cause the order of ContainerRenderObjectMixin children to be disordered, resulting in misplaced rendering.

[chunhtai]

Sounds good, thanks for fixing this

[chunhtai]

Can you use the FlutterErrorDetails?

flutter/packages/flutter/lib/src/rendering/object.dart

Line 1314 in 515ef91

FlutterError.reportError(FlutterErrorDetails(

and also provides an debugCreateor to the error details. Doing this will print out the where the problematic widget is created

[xu-baolin]

done

[chunhtai]

LGTM

[chunhtai]

nit: new line at the end of file