Flutter Widget不刷新问题

import 'package:flutter/material.dart';

/// 温度显示 -- ok
class TemperatureIndication extends StatefulWidget {
  final ValueChanged<int> valueChanged;
  final int choosedItem;

  TemperatureIndication({this.valueChanged, this.choosedItem = 0});

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

class _TemperatureIndicationState extends State<TemperatureIndication> {
  int _choosedItem = 0;

  @override
  void initState() {
    super.initState();
　　 // 错误点在这里
    _choosedItem = widget.choosedItem;
    print('_TemperatureIndicationState initState:$_choosedItem');
  }

  @override
  Widget build(BuildContext context) {
//    _choosedItem = widget.choosedItem;
//    print('_TemperatureIndicationState build:$_choosedItem');
    return _buildItem();
  }

  Widget _buildItem() {
    return Padding(
      padding: const EdgeInsets.symmetric(horizontal: 32.0),
      child: Column(
        children: <Widget>[
          _buildTextItem(),
          Padding(padding: const EdgeInsets.only(top: 4.0)),
          _buildButtonItem(),
        ],
      ),
    );
  }

  Widget _buildTextItem() {
    return Row(
      children: <Widget>[
        Expanded(
          child: Center(child: Text('切')),
        ),
        Expanded(
          child: Center(child: Text('低')),
        ),
        Expanded(
          child: Center(child: Text('中')),
        ),
        Expanded(
          child: Center(child: Text('高')),
        ),
      ],
    );
  }

  Widget _buildButtonItem() {
    return DecoratedBox(
      decoration: BoxDecoration(border: Border.all( 1.5, color: Color(0xffd6def1))),
      child: Row(
        children: <Widget>[
          Expanded(
            child: GestureDetector(
              behavior: HitTestBehavior.opaque,
              onTap: () {
                _choosedItem = 0;
                setState(() {});
                if (widget.valueChanged != null) {
                  widget.valueChanged(0);
                }
              },
              child: DecoratedBox(
                decoration: _choosedItem == 0
                    ? BoxDecoration(
                        color: Colors.grey,
                      )
                    : BoxDecoration(),
                child: Container(
                  height: 28.0,
                ),
              ),
            ),
          ),
          Expanded(
            child: GestureDetector(
              behavior: HitTestBehavior.opaque,
              onTap: () {
                _choosedItem = 1;
                setState(() {});
                if (widget.valueChanged != null) {
                  widget.valueChanged(1);
                }
              },
              child: DecoratedBox(
                decoration: _choosedItem == 1
                    ? BoxDecoration(
                        color: Color(0xfffe0007),
                      )
                    : BoxDecoration(),
                child: Container(
                  height: 28.0,
                ),
              ),
            ),
          ),
          Expanded(
            child: GestureDetector(
              behavior: HitTestBehavior.opaque,
              onTap: () {
                _choosedItem = 2;
                setState(() {});
                if (widget.valueChanged != null) {
                  widget.valueChanged(2);
                }
              },
              child: DecoratedBox(
                decoration: _choosedItem == 2
                    ? BoxDecoration(
                        color: Color(0xfffe0007),
                      )
                    : BoxDecoration(),
                child: Container(
                  height: 28.0,
                ),
              ),
            ),
          ),
          Expanded(
            child: GestureDetector(
              behavior: HitTestBehavior.opaque,
              onTap: () {
                _choosedItem = 3;
                setState(() {});
                if (widget.valueChanged != null) {
                  widget.valueChanged(3);
                }
              },
              child: DecoratedBox(
                decoration: _choosedItem == 3
                    ? BoxDecoration(
                        color: Color(0xfffe0007),
                      )
                    : BoxDecoration(),
                child: Container(
                  height: 28.0,
                ),
              ),
            ),
          ),
        ],
      ),
    );
  }
}

测试代码：

@override
  Widget build(BuildContext context) {
    print('父类build问题:$chooseItem');
    return Scaffold(
      appBar: AppBar(),
      body: TemperatureIndication(
        valueChanged: (i) {
          chooseItem = i;
          print('点击选中的值：$chooseItem');
          chooseItem = 0;
          print('强制更改的值：$chooseItem');
          setState(() {});
        },
        choosedItem: chooseItem,
      ),
    );
  }

简单功能：就是把选中的值返回给调用者

问题描述：在返回值函数里，强制把选中的值改为 0，发现没有效果？

结论：按照官网的文档说法，widget会由架构来判断是否会刷新，判断会刷新了才刷新。刚好，我这种写法是被判断为不刷新。

@override
  void initState() {
    super.initState();
    _choosedItem = widget.choosedItem;
    print('_TemperatureIndicationState initState:$_choosedItem');
  }

问题出在以上代码中。我在State类重新定义了一个属性在initState函数中接收StatefulWidget的属性，导致的出现这样子的问题。

正确的做法是在State的build方法中直接使用StatefulWidget的属性。即是widget.choosedItem在build方法中直接使用。

原因：State的initState只执行了一次，后面StatefulWidget的choosedItem更改了，但是没有再次执行initState方法，导致_choosedItem没有跟着更改。所以界面就不会跟着更改。

结论：StatefulWidget是State的配置文件，在State中直接使用StatefulWidget的属性。

以上代码改为：

import 'package:flutter/material.dart';

/// 温度显示 -- ok
class TemperatureIndication extends StatefulWidget {
  final ValueChanged<int> valueChanged;
  final int choosedItem;

  TemperatureIndication({this.valueChanged, this.choosedItem = 0});

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

class _TemperatureIndicationState extends State<TemperatureIndication> {
  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return _buildItem();
  }

  Widget _buildItem() {
    return Padding(
      padding: const EdgeInsets.symmetric(horizontal: 32.0),
      child: Column(
        children: <Widget>[
          _buildTextItem(),
          Padding(padding: const EdgeInsets.only(top: 4.0)),
          _buildButtonItem(),
        ],
      ),
    );
  }

  Widget _buildTextItem() {
    return Row(
      children: <Widget>[
        Expanded(
          child: Center(child: Text('切')),
        ),
        Expanded(
          child: Center(child: Text('低')),
        ),
        Expanded(
          child: Center(child: Text('中')),
        ),
        Expanded(
          child: Center(child: Text('高')),
        ),
      ],
    );
  }

  Widget _buildButtonItem() {
    return DecoratedBox(
      decoration: BoxDecoration(border: Border.all( 1.5, color: Color(0xffd6def1))),
      child: Row(
        children: <Widget>[
          Expanded(
            child: GestureDetector(
              behavior: HitTestBehavior.opaque,
              onTap: () {
                setState(() {});
                if (widget.valueChanged != null) {
                  widget.valueChanged(0);
                }
              },
              child: DecoratedBox(
                decoration: widget.choosedItem == 0
                    ? BoxDecoration(
                        color: Colors.grey,
                      )
                    : BoxDecoration(),
                child: Container(
                  height: 28.0,
                ),
              ),
            ),
          ),
          Expanded(
            child: GestureDetector(
              behavior: HitTestBehavior.opaque,
              onTap: () {
                setState(() {});
                if (widget.valueChanged != null) {
                  widget.valueChanged(1);
                }
              },
              child: DecoratedBox(
                decoration: widget.choosedItem == 1
                    ? BoxDecoration(
                        color: Color(0xfffe0007),
                      )
                    : BoxDecoration(),
                child: Container(
                  height: 28.0,
                ),
              ),
            ),
          ),
          Expanded(
            child: GestureDetector(
              behavior: HitTestBehavior.opaque,
              onTap: () {
                setState(() {});
                if (widget.valueChanged != null) {
                  widget.valueChanged(2);
                }
              },
              child: DecoratedBox(
                decoration: widget.choosedItem == 2
                    ? BoxDecoration(
                        color: Color(0xfffe0007),
                      )
                    : BoxDecoration(),
                child: Container(
                  height: 28.0,
                ),
              ),
            ),
          ),
          Expanded(
            child: GestureDetector(
              behavior: HitTestBehavior.opaque,
              onTap: () {
                setState(() {});
                if (widget.valueChanged != null) {
                  widget.valueChanged(3);
                }
              },
              child: DecoratedBox(
                decoration: widget.choosedItem == 3
                    ? BoxDecoration(
                        color: Color(0xfffe0007),
                      )
                    : BoxDecoration(),
                child: Container(
                  height: 28.0,
                ),
              ),
            ),
          ),
        ],
      ),
    );
  }
}