Change focus example to be more canonical (and correct), listening to the focus node for changes. (flutter#35913)

This changes the example for FocusNode to be more correct, listening to the focus node for changes, instead of assuming that it is the only one doing the changing.

Author: gspencergoog

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

@@ -236,16 +236,26 @@ class FocusAttachment {
 ///
 /// class _ColorfulButtonState extends State<ColorfulButton> {
 ///   FocusNode _node;
+///   bool _focused = false;
 ///   FocusAttachment _nodeAttachment;
 ///   Color _color = Colors.white;
 ///
 ///   @override
 ///   void initState() {
 ///     super.initState();
 ///     _node = FocusNode(debugLabel: 'Button');
+///     _node.addListener(_handleFocusChange);
 ///     _nodeAttachment = _node.attach(context, onKey: _handleKeyPress);
 ///   }
 ///
+///   void _handleFocusChange() {
+///     if (_node.hasFocus != _focused) {
+///       setState(() {
+///         _focused = _node.hasFocus;
+///       });
+///     }
+///   }
+///
 ///   bool _handleKeyPress(FocusNode node, RawKeyEvent event) {
 ///     if (event is RawKeyDownEvent) {
 ///       print('Focus node ${node.debugLabel} got key event: ${event.logicalKey}');
@@ -274,6 +284,7 @@ class FocusAttachment {
 ///
 ///   @override
 ///   void dispose() {
+///     _node.removeListener(_handleFocusChange);
 ///     // The attachment will automatically be detached in dispose().
 ///     _node.dispose();
 ///     super.dispose();
@@ -284,24 +295,20 @@ class FocusAttachment {
 ///     _nodeAttachment.reparent();
 ///     return GestureDetector(
 ///       onTap: () {
-///         if (_node.hasFocus) {
-///           setState(() {
+///         if (_focused) {
 ///             _node.unfocus();
-///           });
 ///         } else {
-///           setState(() {
-///             _node.requestFocus();
-///           });
+///            _node.requestFocus();
 ///         }
 ///       },
 ///       child: Center(
 ///         child: Container(
 ///           width: 400,
 ///           height: 100,
-///           color: _node.hasFocus ? _color : Colors.white,
+///           color: _focused ? _color : Colors.white,
 ///           alignment: Alignment.center,
 ///           child: Text(
-///               _node.hasFocus ? "I'm in color! Press R,G,B!" : 'Press to focus'),
+///               _focused ? "I'm in color! Press R,G,B!" : 'Press to focus'),
 ///         ),
 ///       ),
 ///     );