[Suggestion] Provide a way to assert that a given Widget is a Sliver

[navaronbracke]

Use case

Currently a CustomScrollView accepts a list of slivers.
This list is currently a List<Widget>.

However this creates a bit of confusion.
If I put a Container or any other regular widget in that list,
I will get an exception since I used a Widget that is not a Sliver implementation.

Secondly, there is no easy option to assert that a given Widget is actually a Sliver implementation
(aside from checking against all known Sliver types, i.e. assert(widget is SliverList || widget is SliverGrid || ...)).
The current slivers are divided into implementations of SingleChildRenderObjectWidget and SliverMultiBoxAdaptorWidget (and others?).

There is no way to easily assert that a Widget is a Sliver, regardless of its actual runtime type.

Proposal

Make it possible to use an assert to check if a Widget is actually a Sliver.
I.e. assert(myWidget is SliverWidget, "This widget must be a Sliver.")

Perhaps the different Sliver implementations implement a common (but empty) interface?

/// All the Sliver widgets implement this interface.
/// This makes it easier to check if a given [Widget] is a Sliver.
abstract class SliverWidget {}

[goderbauer]

What is the exception that we throw if it is not a sliver? Would it be sufficient to just make that one better?

[navaronbracke]

@goderbauer My intention was to make this kind of code a little more type safe at compile-time:

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(primarySwatch: Colors.blue),
      // Enforce that the list only contains SliverXYZ widgets here.  
      home: MyWidgetWithSlivers(slivers: [Container()]),
    );
  }
}

class MyWidgetWithSlivers extends StatefulWidget {
  const MyWidgetWithSlivers({
    Key? key,
    required this.slivers,
  }) : super(key: key);

  final List<Widget> slivers;

  @override
  State<MyWidgetWithSlivers> createState() => _MyWidgetWithSliversState();
}

class _MyWidgetWithSliversState extends State<MyWidgetWithSlivers> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(slivers: widget.slivers),
    );
  }
}

The exception that we get for this code sample looks like this:

══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following assertion was thrown building Container:
A RenderViewport expected a child of type RenderSliver but received a child of type
RenderLimitedBox.
RenderObjects expect specific types of children because they coordinate with their children during
layout and paint. For example, a RenderSliver cannot be the child of a RenderBox because a
RenderSliver does not understand the RenderBox layout protocol.

The RenderViewport that expected a RenderSliver child was created by:
  Viewport ← IgnorePointer-[GlobalKey#b260d] ← Semantics ← Listener ← _GestureSemantics ←
  RawGestureDetector-[LabeledGlobalKey<RawGestureDetectorState>#cf98a] ← Listener ← _ScrollableScope
  ← _ScrollSemantics-[GlobalKey#b1531] ← RepaintBoundary ← CustomPaint ← RepaintBoundary ← ⋯

The RenderLimitedBox that did not match the expected child type was created by:
  LimitedBox ← Container ← Viewport ← IgnorePointer-[GlobalKey#b260d] ← Semantics ← Listener ←
  _GestureSemantics ← RawGestureDetector-[LabeledGlobalKey<RawGestureDetectorState>#cf98a] ←
  Listener ← _ScrollableScope ← _ScrollSemantics-[GlobalKey#b1531] ← RepaintBoundary ← ⋯

The relevant error-causing widget was:
  Container Container:file:///Users/navaronbracke/Desktop/myapp/lib/main.dart:15:34

When the exception was thrown, this was the stack:
#0      ContainerRenderObjectMixin.debugValidateChild.<anonymous closure> (package:flutter/src/rendering/object.dart:3227:9)
#1      ContainerRenderObjectMixin.debugValidateChild (package:flutter/src/rendering/object.dart:3254:6)
#2      MultiChildRenderObjectElement.insertRenderObjectChild (package:flutter/src/widgets/framework.dart:6272:25)
#3      RenderObjectElement.attachRenderObject (package:flutter/src/widgets/framework.dart:5870:35)
#4      RenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5544:5)
#5      SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:6194:11)
...     Normal element mounting (7 frames)
#12     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3673:14)
#13     MultiChildRenderObjectElement.inflateWidget (package:flutter/src/widgets/framework.dart:6333:36)
#14     MultiChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:6344:32)
#15     _ViewportElement.mount (package:flutter/src/widgets/viewport.dart:222:11)
...     Normal element mounting (94 frames)
#109    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3673:14)
#110    MultiChildRenderObjectElement.inflateWidget (package:flutter/src/widgets/framework.dart:6333:36)
#111    MultiChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:6344:32)
...     Normal element mounting (261 frames)
#372    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3673:14)
#373    MultiChildRenderObjectElement.inflateWidget (package:flutter/src/widgets/framework.dart:6333:36)
#374    MultiChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:6344:32)
...     Normal element mounting (377 frames)
#751    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3673:14)
#752    Element.updateChild (package:flutter/src/widgets/framework.dart:3425:18)
#753    RenderObjectToWidgetElement._rebuild (package:flutter/src/widgets/binding.dart:1198:16)
#754    RenderObjectToWidgetElement.mount (package:flutter/src/widgets/binding.dart:1167:5)
#755    RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> (package:flutter/src/widgets/binding.dart:1112:18)
#756    BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2573:19)
#757    RenderObjectToWidgetAdapter.attachToRenderTree (package:flutter/src/widgets/binding.dart:1111:13)
#758    WidgetsBinding.attachRootWidget (package:flutter/src/widgets/binding.dart:944:7)
#759    WidgetsBinding.scheduleAttachRootWidget.<anonymous closure> (package:flutter/src/widgets/binding.dart:924:7)
(elided 11 frames from class _RawReceivePortImpl, class _Timer, dart:async, and dart:async-patch)

════════════════════════════════════════════════════════════════════════════════════════════════════

Another exception was thrown: A RenderViewport expected a child of type RenderSliver but received a child of type RenderErrorBox.

Another exception was thrown: 'package:flutter/src/widgets/framework.dart': Failed assertion: line 4357 pos 14: 'owner!._debugCurrentBuildTarget
== this': is not true.

Another exception was thrown: 'package:flutter/src/widgets/framework.dart': Failed assertion: line 4357 pos 14: 'owner!._debugCurrentBuildTarget
== this': is not true.

Another exception was thrown: 'package:flutter/src/widgets/framework.dart': Failed assertion: line 4357 pos 14: 'owner!._debugCurrentBuildTarget
== this': is not true.

Another exception was thrown: 'package:flutter/src/widgets/framework.dart': Failed assertion: line 4357 pos 14: 'owner!._debugCurrentBuildTarget
== this': is not true.

// The assertion failure keeps repeating itself.

The message itself is clear to me, since I understand enough of RenderObjects. Perhaps we could include a fix in the exception message, similar to how other exceptions provide us with an alternate option.

[jamesblasco]

The problem is that any StatelessWidget/StatefulWidget can be used inside a CustomScrollView. So you could not only allow a list of List.

I added a proposal here to support a ScrollView that accepts any type of widget
https://twitter.com/JamesBlasco/status/1484564101598695428?s=20

[andynewman10]

Up until recently, I wasn't aware of the two "parallel worlds", slivers vs widgets, and it's been a shock, I must say.

The problem is that any StatelessWidget/StatefulWidget can be used inside a CustomScrollView. So you could not only allow a list of List.

@jamesblasco what do you mean, here? CustomScrollView does not accept box widgets, only slivers. The ScrollView widget that you propose in your twitter link is also what I have done in my code.

Converting between slivers and regular "box" widgets is easy IMHO:

• use SliverToBoxAdapter(child: <widget>) to do box -> sliver conversions
• use SliverList(delegate: SliverChildBuilderDelegate((context, index) { return <widget>; }, childCount: 1)) to do sliver -> box conversions

I think Flutter has a problem with redundancies between sliver and non-sliver widgets, eg. ListView vs SliverList (and even maybe some redundancies between box widgets (Column vs ListView), but this is another story)

What is the deal-breaker here, justifying the very existence of slivers? I see there is one potential issue with FutureBuilder, as these widgets somewhat behave as pass-throughs: a FutureBuilder error widget may be a box widget, whereas its content widget may be a sliver.