Make FocusManager a ChangeNotifier that notifies when the prim… (#50392)

This makes FocusManager a ChangeNotifier that notifies when the primaryFocus changes so that an app can track the current focus.

Fixes #50501

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

@@ -1166,6 +1166,15 @@ enum FocusHighlightStrategy {
 /// current focus in that [FocusScopeNode] is null, it will stop there, and no
 /// [FocusNode] will have focus.
 ///
+/// If you would like notification whenever the [primaryFocus] changes, register
+/// a listener with [addListener]. When you no longer want to receive events, as
+/// when your object is about to be disposed, you must unregister with
+/// [removeListener] to avoid memory leaks. Removing listeners is typically done
+/// in [State.dispose] on stateful widgets.
+///
+/// The [highlightMode] changes are notified separately via
+/// [addHighlightModeListener] and removed with [removeHighlightModeListener].
+///
 /// See also:
 ///
 ///  * [FocusNode], which is a node in the focus tree that can receive focus.
@@ -1177,7 +1186,7 @@ enum FocusHighlightStrategy {
 ///    a given [BuildContext].
 ///  * The [focusManager] and [primaryFocus] global accessors, for convenient
 ///    access from anywhere to the current focus manager state.
-class FocusManager with DiagnosticableTreeMixin {
+class FocusManager with DiagnosticableTreeMixin, ChangeNotifier implements Diagnosticable {
   /// Creates an object that manages the focus tree.
   ///
   /// This constructor is rarely called directly. To access the [FocusManager],
@@ -1443,6 +1452,9 @@ class FocusManager with DiagnosticableTreeMixin {
       node._notify();
     }
     _dirtyNodes.clear();
+    if (previousFocus != _primaryFocus) {
+      notifyListeners();
+    }
     assert(() {
       if (_kDebugFocus) {
         debugDumpFocusTree();

packages/flutter/test/widgets/focus_manager_test.dart

@@ -745,4 +745,92 @@ void main() {
     await tester.pump();
     expect(parent1.focusedChild, equals(child2));
   });
+  testWidgets('Focus changes notify listeners.', (WidgetTester tester) async {
+    final BuildContext context = await setupWidget(tester);
+    final FocusScopeNode parent1 = FocusScopeNode(debugLabel: 'parent1');
+    final FocusAttachment parent1Attachment = parent1.attach(context);
+    final FocusNode child1 = FocusNode(debugLabel: 'child1');
+    final FocusAttachment child1Attachment = child1.attach(context);
+    final FocusNode child2 = FocusNode(debugLabel: 'child2');
+    final FocusAttachment child2Attachment = child2.attach(context);
+    parent1Attachment.reparent(parent: tester.binding.focusManager.rootScope);
+    child1Attachment.reparent(parent: parent1);
+    child2Attachment.reparent(parent: child1);
+
+    int notifyCount = 0;
+    void handleFocusChange() {
+      notifyCount++;
+    }
+    tester.binding.focusManager.addListener(handleFocusChange);
+
+    parent1.autofocus(child2);
+    expect(notifyCount, equals(0));
+    await tester.pump();
+    expect(notifyCount, equals(1));
+    notifyCount = 0;
+
+    child1.requestFocus();
+    child2.requestFocus();
+    child1.requestFocus();
+    await tester.pump();
+    expect(notifyCount, equals(1));
+    notifyCount = 0;
+
+    child2.requestFocus();
+    await tester.pump();
+    expect(notifyCount, equals(1));
+    notifyCount = 0;
+
+    child2.unfocus();
+    await tester.pump();
+    expect(notifyCount, equals(1));
+    notifyCount = 0;
+
+    tester.binding.focusManager.removeListener(handleFocusChange);
+  });
+  testWidgets('FocusManager notifies listeners when a widget loses focus because it was removed.', (WidgetTester tester) async {
+    final FocusNode nodeA = FocusNode(debugLabel: 'a');
+    final FocusNode nodeB = FocusNode(debugLabel: 'b');
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.rtl,
+        child: Column(
+          children: <Widget>[
+            Focus(focusNode: nodeA , child: const Text('a')),
+            Focus(focusNode: nodeB, child: const Text('b')),
+          ],
+        ),
+      ),
+    );
+    int notifyCount = 0;
+    void handleFocusChange() {
+      notifyCount++;
+    }
+    tester.binding.focusManager.addListener(handleFocusChange);
+
+    nodeA.requestFocus();
+    await tester.pump();
+    expect(nodeA.hasPrimaryFocus, isTrue);
+    expect(notifyCount, equals(1));
+    notifyCount = 0;
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.rtl,
+        child: Column(
+          children: <Widget>[
+            Focus(focusNode: nodeB, child: const Text('b')),
+          ],
+        ),
+      ),
+    );
+
+    await tester.pump();
+    expect(nodeA.hasPrimaryFocus, isFalse);
+    expect(nodeB.hasPrimaryFocus, isFalse);
+    expect(notifyCount, equals(1));
+    notifyCount = 0;
+
+    tester.binding.focusManager.removeListener(handleFocusChange);
+  });
 }