Repository: flutter
Issue: Selection indicator for disabled ToggleButtons
Labels: c: new feature,framework,f: material design,c: proposal,team-design,triaged-design

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## Use case

I need to show indicators of selected options when a `ToggleButtons` is disabled.

<img width="211" alt="Screen Shot 2020-09-18 at 14 08 59" src="https://user-images.githubusercontent.com/77626/93567461-e0db7980-f9b8-11ea-99ee-6a55d5117813.png">

The top ones should display the selection indicators similar to the bottom ones even though they are disabled (top ones have `isSelected` properly set; bottom ones are enabled).

Minimum code ([DartPad](https://dartpad.dev/b6409e10de32b280b8938aa75364fa7b)):

```dart
// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;
  final isSelected = <bool>[true, false, true];

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            ToggleButtons(
              children: <Widget>[
                Icon(Icons.ac_unit),
                Icon(Icons.call),
                Icon(Icons.cake),
              ],
              onPressed: (_counter % 2) == 0
                  ? (int index) {
                      setState(() {
                        isSelected[index] = !isSelected[index];
                      });
                    }
                  : null,
              isSelected: isSelected,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton.extended(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        label: Text('disable/enable'),
      ),
    );
  }
}
```

## Proposal

- Support for highlighting selected options using theme color when the whole `ToggleButtons` is disabled.
- Add support for additional arguments for `ToggleButtons` to customize the disabled-selection indicators (text color, fill color, border).

This might require confirmation with the Material Design team as, although there's a guideline about the disabled individual button, it doesn't have a specification for a disabled toggle button group.