Create slider with editable numerical value in gallery (#23506)

* Update doc header in Opacity class to fix issue #23311

* Added slider with editable numerical value to gallery. Fixes flutter#1542

* Revert "Update doc header in Opacity class to fix issue #23311"

This reverts commit 2d3642bbda6e00389e78631360f8274e9bb7d675.

* Fix typo in slider description

* Increase TextField size to pass accessibility test

* Added Semantics widget to pass accessibility test

* Made description start with caps to match other examples

* Removed unnecessary spacing Container widget

* Update authors file

* Fix indent

* Removed decimal and replaced boundaries with .clamp
Signed-off-by: Martin Staadecker <machstg@gmail.com>

* Undo line wrap from previous commit
Signed-off-by: Martin Staadecker <machstg@gmail.com>

* Update onSubmitted to only call setState when value has changed
Signed-off-by: Martin Staadecker <machstg@gmail.com>

AUTHORS

@@ -36,3 +36,4 @@ TruongSinh Tran-Nguyen <i@truongsinh.pro>
 Sander Dalby Larsen <srdlarsen@gmail.com>
 Marco Scannadinari <m@scannadinari.co.uk>
 Frederik Schweiger <mail@flschweiger.net>
+Martin Staadecker <machstg@gmail.com>

examples/flutter_gallery/lib/demo/material/slider_demo.dart

@@ -159,6 +159,49 @@ class _SliderDemoState extends State<SliderDemo> {
                 const Text('Continuous'),
               ],
             ),
+            Column(
+              mainAxisSize: MainAxisSize.min,
+              children: <Widget>[
+                Row(
+                  children: <Widget>[
+                    Expanded(
+                      child: Slider(
+                        value: _value,
+                        min: 0.0,
+                        max: 100.0,
+                        onChanged: (double value) {
+                          setState(() {
+                            _value = value;
+                          });
+                        },
+                      ),
+                    ),
+                    Semantics(
+                      label: 'Editable numerical value',
+                      child: Container(
+                        width: 48,
+                        height: 48,
+                        child: TextField(
+                          onSubmitted: (String value) {
+                            final double newValue = double.tryParse(value);
+                            if (newValue != null && newValue != _value) {
+                              setState(() {
+                                _value = newValue.clamp(0, 100);
+                              });
+                            }
+                          },
+                          keyboardType: TextInputType.number,
+                          controller: TextEditingController(
+                            text: _value.toStringAsFixed(0),
+                          ),
+                        ),
+                      ),
+                    ),
+                  ],
+                ),
+                const Text('Continuous with Editable Numerical Value'),
+              ],
+            ),
             Column(
               mainAxisSize: MainAxisSize.min,
               children: const <Widget>[