TextField docs for getting value (#40695)

Added docs examples of getting the string from a TextField

packages/flutter/lib/src/material/text_field.dart

@@ -188,6 +188,71 @@ class _TextFieldSelectionGestureDetectorBuilder extends TextSelectionGestureDete
 /// ```
 /// {@end-tool}
 ///
+/// ## Reading values
+///
+/// A common way to read a value from a TextField is to use the [onSubmitted]
+/// callback. This callback is applied to the text field's current value when
+/// the user finishes editing.
+///
+/// {@tool dartpad --template=stateful_widget_material}
+///
+/// This sample shows how to get a value from a TextField via the [onSubmitted]
+/// callback.
+///
+/// ```dart
+/// TextEditingController _controller;
+///
+/// void initState() {
+///   super.initState();
+///   _controller = TextEditingController();
+/// }
+///
+/// void dispose() {
+///   _controller.dispose();
+///   super.dispose();
+/// }
+///
+/// Widget build(BuildContext context) {
+///   return Scaffold(
+///     body: Center(
+///       child: TextField(
+///         controller: _controller,
+///         onSubmitted: (String value) async {
+///           await showDialog<void>(
+///             context: context,
+///             builder: (BuildContext context) {
+///               return AlertDialog(
+///                 title: const Text('Thanks!'),
+///                 content: Text ('You typed "$value".'),
+///                 actions: <Widget>[
+///                   FlatButton(
+///                     onPressed: () { Navigator.pop(context); },
+///                     child: const Text('OK'),
+///                   ),
+///                 ],
+///               );
+///             },
+///           );
+///         },
+///       ),
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
+/// For most applications the [onSubmitted] callback will be sufficient for
+/// reacting to user input.
+///
+/// The [onEditingComplete] callback also runs when the user finishes editing.
+/// It's different from [onSubmitted] because it has a default value which
+/// updates the text controller and yields the keyboard focus. Applications that
+/// require different behavior can override the default [onEditingComplete]
+/// callback.
+///
+/// Keep in mind you can also always read the current string from a TextField's
+/// [TextEditingController] using [TextEditingController.text].
+///
 /// See also:
 ///
 ///  * <https://material.io/design/components/text-fields.html>

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

@@ -761,6 +761,61 @@ class EditableText extends StatefulWidget {
   /// To be notified of all changes to the TextField's text, cursor,
   /// and selection, one can add a listener to its [controller] with
   /// [TextEditingController.addListener].
+  ///
+  /// {@tool dartpad --template=stateful_widget_material}
+  ///
+  /// This example shows how onChanged could be used to check the TextField's
+  /// current value each time the user inserts or deletes a character.
+  ///
+  /// ```dart
+  /// TextEditingController _controller;
+  ///
+  /// void initState() {
+  ///   super.initState();
+  ///   _controller = TextEditingController();
+  /// }
+  ///
+  /// void dispose() {
+  ///   _controller.dispose();
+  ///   super.dispose();
+  /// }
+  ///
+  /// Widget build(BuildContext context) {
+  ///   return Scaffold(
+  ///     body: Column(
+  ///       mainAxisAlignment: MainAxisAlignment.center,
+  ///       children: <Widget>[
+  ///         const Text('What number comes next in the sequence?'),
+  ///         const Text('1, 1, 2, 3, 5, 8...?'),
+  ///         TextField(
+  ///           controller: _controller,
+  ///           onChanged: (String value) async {
+  ///             if (value != '13') {
+  ///               return;
+  ///             }
+  ///             await showDialog<void>(
+  ///               context: context,
+  ///               builder: (BuildContext context) {
+  ///                 return AlertDialog(
+  ///                   title: const Text('Thats correct!'),
+  ///                   content: Text ('13 is the right answer.'),
+  ///                   actions: <Widget>[
+  ///                     FlatButton(
+  ///                       onPressed: () { Navigator.pop(context); },
+  ///                       child: const Text('OK'),
+  ///                     ),
+  ///                   ],
+  ///                 );
+  ///               },
+  ///             );
+  ///           },
+  ///         ),
+  ///       ],
+  ///     ),
+  ///   );
+  /// }
+  /// ```
+  /// {@end-tool}
   /// {@endtemplate}
   ///
   /// See also: