Fix bug when tapping ListTitle with CheckboxListTile tristate enable (#63925)

* fix bug when tap ListTitle when tristate enable #63846

* fix bug when tap ListTitle when tristate enable #63846

* code style

* improve the unit test case

* bow to convention

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

@@ -382,6 +382,21 @@ class CheckboxListTile extends StatelessWidget {
   /// If tristate is false (the default), [value] must not be null.
   final bool tristate;
 
+  void _handleValueChange() {
+    assert(onChanged != null);
+    switch (value) {
+      case false:
+        onChanged(true);
+        break;
+      case true:
+        onChanged(tristate ? null : false);
+        break;
+      default: // case null:
+        onChanged(false);
+        break;
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     final Widget control = Checkbox(
@@ -416,7 +431,7 @@ class CheckboxListTile extends StatelessWidget {
           isThreeLine: isThreeLine,
           dense: dense,
           enabled: onChanged != null,
-          onTap: onChanged != null ? () { onChanged(!value); } : null,
+          onTap: onChanged != null ? _handleValueChange : null,
           selected: selected,
           autofocus: autofocus,
           contentPadding: contentPadding,

packages/flutter/test/material/checkbox_list_tile_test.dart

@@ -148,7 +148,8 @@ void main() {
   });
 
   testWidgets('CheckboxListTile tristate test', (WidgetTester tester) async {
-    bool _value;
+    bool _value = false;
+    bool _tristate = false;
 
     await tester.pumpWidget(
       Material(
@@ -157,7 +158,7 @@ void main() {
             return wrap(
               child: CheckboxListTile(
                 title: const Text('Title'),
-                tristate: true,
+                tristate: _tristate,
                 value: _value,
                 onChanged: (bool value) {
                   setState(() {
@@ -171,12 +172,33 @@ void main() {
       ),
     );
 
-    expect(tester.widget<Checkbox>(find.byType(Checkbox)).value, null);
+    expect(tester.widget<Checkbox>(find.byType(Checkbox)).value, false);
+
+    // Tap the checkbox when tristate is disabled.
+    await tester.tap(find.byType(Checkbox));
+    await tester.pumpAndSettle();
+    expect(_value, true);
 
     await tester.tap(find.byType(Checkbox));
     await tester.pumpAndSettle();
     expect(_value, false);
 
+    // Tap the listTile when tristate is disabled.
+    await tester.tap(find.byType(ListTile));
+    await tester.pumpAndSettle();
+    expect(_value, true);
+
+    await tester.tap(find.byType(ListTile));
+    await tester.pumpAndSettle();
+    expect(_value, false);
+
+    // Enable tristate
+    _tristate = true;
+    await tester.pumpAndSettle();
+
+    expect(tester.widget<Checkbox>(find.byType(Checkbox)).value, false);
+
+    // Tap the checkbox when tristate is enabled.
     await tester.tap(find.byType(Checkbox));
     await tester.pumpAndSettle();
     expect(_value, true);
@@ -184,5 +206,22 @@ void main() {
     await tester.tap(find.byType(Checkbox));
     await tester.pumpAndSettle();
     expect(_value, null);
+
+    await tester.tap(find.byType(Checkbox));
+    await tester.pumpAndSettle();
+    expect(_value, false);
+
+    // Tap the listTile when tristate is enabled.
+    await tester.tap(find.byType(ListTile));
+    await tester.pumpAndSettle();
+    expect(_value, true);
+
+    await tester.tap(find.byType(ListTile));
+    await tester.pumpAndSettle();
+    expect(_value, null);
+
+    await tester.tap(find.byType(ListTile));
+    await tester.pumpAndSettle();
+    expect(_value, false);
   });
 }