Flutter: AutomaticKeepAliveClientMixin is not working with BottomNavigationBar

Question

I have page called AddPatientView with BottomNavigationBar containing AddPatientInfo and AddPatientImages pages. All of these three are Stateful widgets.

By default

Answer 1

So I wrapped the tab pages in Stack widget and now it keeps the state of the form intact. I am not sure if this is the right way to go but it does the job.

Here is the code:

AddPatientView (parent)

class AddPatientView extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _AddPatientViewState();
  }
}

class _AddPatientViewState extends State<AddPatientView> {
  int _currentIndex = 0;
  List<File> _imageFileList = new List<File>();

  @override
  void initState() {
    super.initState();
  }


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("New Patient Record"),
      ),
      body: new Stack(
        children: <Widget>[
          new Offstage(
            offstage: _currentIndex != 0,
            child: AddPatientInfo(savePatient),
          ),
          new Offstage(
            offstage: _currentIndex != 1,
            child: AddPatientImages(_imageFileList),
          )
        ],
      ),
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        items: [
          BottomNavigationBarItem(icon: new Icon(Icons.create), title: new Text('Info')),
          BottomNavigationBarItem(icon: new Icon(Icons.camera_alt), title: new Text('Images')),
        ],
        onTap: (int index) {
          setState(() {
            _currentIndex = index;
          });
        },
      ),
    );
  }
}

Answer 2

From the documentation on AutomaticKeepAliveClientMixin:

/// A mixin with convenience methods for clients of [AutomaticKeepAlive]. Used
/// with [State] subclasses.
///
/// Subclasses must implement [wantKeepAlive], and their [build] methods must
/// call `super.build` (the return value will always return null, and should be
/// ignored).

So in your example, before you return the Scaffold just call super.build:

  Widget build(BuildContext context) {
    super.build(context);
    return Scaffold(...);
  }

Answer 3

If you want AutomaticKeepAliveClientMixin to continue working, please use PageView to wrap the body, the code should like this

class AddPatientView extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _AddPatientViewState();
  }
}

class _AddPatientViewState extends State<AddPatientView> {
  int _currentIndex = 0;
  List<Widget> _children;
  List<File> _imageFileList = new List<File>();

  // add
  final pageController = PageController();
  void onPageChanged(int index) {
    setState(() {
      _currentIndex = index;
    });
  }

  @override
  void initState() {
    super.initState();
    _children = [AddPatientInfo(savePatient), AddPatientImages(_imageFileList)];
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("New Patient Record"),
      ),
      // here!!!
      body: PageView(
        children: _children,
        controller: pageController,
        onPageChanged: onPageChanged,
      ),
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        items: [
          BottomNavigationBarItem(
              icon: new Icon(Icons.create), title: new Text('Info')),
          BottomNavigationBarItem(
              icon: new Icon(Icons.camera_alt), title: new Text('Images')),
        ],
        onTap: (int index) {
          // setState(() {
          //   _currentIndex = index;
          // });
          // update
          pageController.jumpToPage(index);
        },
      ),
    );
  }
}

but if you just want to keep page state, I suggest you use IndexedStack, it's very simple，no need AutomaticKeepAliveClientMixin, the code should like this

class AddPatientView extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _AddPatientViewState();
  }
}

class _AddPatientViewState extends State<AddPatientView> {
  int _currentIndex = 0;
  List<Widget> _children;
  List<File> _imageFileList = new List<File>();

  @override
  void initState() {
    super.initState();
    _children = [
      AddPatientInfo(savePatient),
      AddPatientImages(_imageFileList)
    ];
  }


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("New Patient Record"),
      ),
      body: IndexedStack(
        index:_currentIndex,
        children:_children
      ),
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        items: [
          BottomNavigationBarItem(icon: new Icon(Icons.create), title: new Text('Info')),
          BottomNavigationBarItem(icon: new Icon(Icons.camera_alt), title: new Text('Images')),
        ],
        onTap: (int index) {
          setState(() {
            _currentIndex = index;
          });
        },
      ),
    );
  }
}