On status management of flutter

Time:2021-4-21

preface

Recently, I learned about the cross platform solution based on the app that wants to realize a collection of personal common functions. And then began to flutter learning, through the article to record the learning harvest.

The main content of this article is to introduce the mode of state management in flutter and the selection of different modes.

This article first appeared in my personal blogTechnology residence( wcc.im )On status management of flutter

catalog

The way of state management

There are three common ways of state management, which are the management by the widget itself, the management by the parent widget, and the mixed management by the widget itself and the parent widget.

In the analysis of three common state management methods, we define three tapboxes, each of which can represent activated or not activated. So tapbox is a stateful widget.

It is managed by the widget itself

//------------------------- TapboxA ----------------------------------

class TapboxA extends StatefulWidget {
  TapboxA({Key key}) : super(key: key);

  @override
  _TapboxAState createState() => _TapboxAState();
}

class _TapboxAState extends State<TapboxA> {
  bool _active = false;

  void _handleTap() {
    setState(() {
      _active = !_active;
    });
  }

  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: _handleTap,
      child: Container(
        child: Center(
          child: Text(
            _active ? 'Active' : 'Inactive',
            style: TextStyle(fontSize: 32.0, color: Colors.white),
          ),
        ),
        width: 200.0,
        height: 200.0,
        decoration: BoxDecoration(
          color: _active ? Colors.lightGreen[700] : Colors.grey[600],
        ),
      ),
    );
  }
}

It’s very simple to manage the state by the widget itself, which is managed by the state class of tapboxa. use_activeTo determine the current value and color and use_handleTapTo change the value.

Managed by parent widget

//------------------------ ParentWidget --------------------------------

class ParentWidget extends StatefulWidget {
  @override
  _ParentWidgetState createState() => _ParentWidgetState();
}

class _ParentWidgetState extends State<ParentWidget> {
  bool _active = false;

  void _handleTapboxChanged(bool newValue) {
    setState(() {
      _active = newValue;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: TapboxB(
        active: _active,
        onChanged: _handleTapboxChanged,
      ),
    );
  }
}

//------------------------- TapboxB ----------------------------------

class TapboxB extends StatelessWidget {
  TapboxB({Key key, this.active: false, @required this.onChanged})
      : super(key: key);

  final bool active;
  final ValueChanged<bool> onChanged;

  void _handleTap() {
    onChanged(!active);
  }

  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: _handleTap,
      child: Container(
        child: Center(
          child: Text(
            active ? 'Active' : 'Inactive',
            style: TextStyle(fontSize: 32.0, color: Colors.white),
          ),
        ),
        width: 200.0,
        height: 200.0,
        decoration: BoxDecoration(
          color: active ? Colors.lightGreen[700] : Colors.grey[600],
        ),
      ),
    );
  }
}

The management by the parent widget mainly includes:

  • How does the parent widget manage tapbox B_activeStatus.
  • In the state class of the parent widget_handleTapboxChanged()

The main features of subwidget are as follows:

  • inheritStatelessWidgetClass because its state is managed by the parent widget.
  • When it is clicked, the parent widget will be informed instead of managing its own state.

It is managed by the widget itself and the parent widget

//---------------------------- ParentWidget ----------------------------

class ParentWidget extends StatefulWidget {
  @override
  _ParentWidgetState createState() => _ParentWidgetState();
}

class _ParentWidgetState extends State<ParentWidget> {
  bool _active = false;

  void _handleTapboxChanged(bool newValue) {
    setState(() {
      _active = newValue;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      child: TapboxC(
        active: _active,
        onChanged: _handleTapboxChanged,
      ),
    );
  }
}

//----------------------------- TapboxC ------------------------------

class TapboxC extends StatefulWidget {
  TapboxC({Key key, this.active: false, @required this.onChanged})
      : super(key: key);

  final bool active;
  final ValueChanged<bool> onChanged;

  _TapboxCState createState() => _TapboxCState();
}

class _TapboxCState extends State<TapboxC> {
  bool _highlight = false;

  void _handleTapDown(TapDownDetails details) {
    setState(() {
      _highlight = true;
    });
  }

  void _handleTapUp(TapUpDetails details) {
    setState(() {
      _highlight = false;
    });
  }

  void _handleTapCancel() {
    setState(() {
      _highlight = false;
    });
  }

  void _handleTap() {
    widget.onChanged(!widget.active);
  }

  Widget build(BuildContext context) {
    // This example adds a green border on tap down.
    // On tap up, the square changes to the opposite state.
    return GestureDetector(
      onTapDown: _handleTapDown, // Handle the tap events in the order that
      onTapUp: _handleTapUp, // they occur: down, up, tap, cancel
      onTap: _handleTap,
      onTapCancel: _handleTapCancel,
      child: Container(
        child: Center(
          child: Text(widget.active ? 'Active' : 'Inactive',
              style: TextStyle(fontSize: 32.0, color: Colors.white)),
        ),
        width: 200.0,
        height: 200.0,
        decoration: BoxDecoration(
          color:
              widget.active ? Colors.lightGreen[700] : Colors.grey[600],
          border: _highlight
              ? Border.all(
                  color: Colors.teal[700],
                  width: 10.0,
                )
              : null,
        ),
      ),
    );
  }
}

It can be seen that this mode only combines the first two modes. In the example, by_highlightAdd the state of adding border on the edge of the tapbox, which is managed by the widget itself, and_activeThe state is managed by the parent widget.

Selection of state management mode

Generally speaking, the state management mode is selected through the following ways:

  • If the state in question is user data, such as the checked or unselected mode of a check box, or the position of a slider, it is best for the parent widget to manage the state.
  • If the state in question is animation, it is better for the widget itself to manage the state.

This article first appeared in my personal blogTechnology residence( wcc.im )
Link to the original text:https://wcc.im/zh/post/flutter-state-management/
The articles in this blog are all original except for special announcement, which are written byCC by-nc-sa 4.0 license agreementPermission. go beyondCC by-nc-sa 4.0 license agreementPlease contact the author for authorization.

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