Flutter 中 interactivity 的使用

Question

Example

如下例子中引入了一个 state 管理简单演示了如何管理 state，并非直接可跑的例子，可以嵌套在 Layout Tutorial 的示例代码中。

class FavoriteWidget extends StatefulWidget {
  @override
  _FavoriteWidgetState createState() => _FavoriteWidgetState();
}

class _FavoriteWidgetState extends State<FavoriteWidget> {
  bool _isFavorited = true;
  int _favoriteCount = 41;

  void _toggleFavorite() {
    setState(() {
      if (_isFavorited) {
        _favoriteCount -= 1;
        _isFavorited = false;
      } else {
        _favoriteCount += 1;
        _isFavorited = true;
      }
    });
  }
  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisSize: MainAxisSize.min,
      children: [
        Container(
          padding: EdgeInsets.all(0),
          child: IconButton(
            icon:  (_isFavorited ? Icon(Icons.star) : Icon(Icons.star_border)),
            color: Colors.red[500],
            onPressed: _toggleFavorite,
          )
        ),
        SizedBox(
          width: 18,
          child: Container(
            child: Text('$_favoriteCount')
          )
        )
      ]
    );
  }
}

Managing state

用一个简单的功能来演示了管理状态的三种不同的方法。

The widget manages its own state

import 'package:flutter/material.dart';
// TapboxA manages its own state.

//------------------------- 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],
        ),
      ),
    );
  }
}

//------------------------- MyApp ----------------------------------

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        appBar: AppBar(
          title: Text('Flutter Demo'),
        ),
        body: Center(
          child: TapboxA(),
        ),
      ),
    );
  }
}

void main() => runApp(MyApp());

The parent widget manages the widget’s state

import 'package:flutter/material.dart';

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],
        ),
      ),
    );
  }
}

//------------------------- MyApp ----------------------------------

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo B',
      home: Scaffold(
        appBar: AppBar(
          title: Text('Flutter Demo B'),
        ),
        body: Center(
          child: ParentWidget(),
        ),
      ),
    );
  }
}
void main() => runApp(MyApp());

A mix-and-match approach

//---------------------------- 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,
        ),
      ),
    );
  }
}

//------------------------- MyApp ----------------------------------

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo C',
      home: Scaffold(
        appBar: AppBar(
          title: Text('Flutter Demo C'),
        ),
        body: Center(
          child: ParentWidget(),
        ),
      ),
    );
  }
}
void main() => runApp(MyApp());