Flutter 73: 图解自定义 ACECheckBox 复选框-原创手记-慕课网

CheckBox 复选框对于所有的开发朋友并不陌生，Flutter 提供了简单便捷的使用方法，但针对不同的业务场景，可能会有些许的不同，例如圆角矩形替换为圆形，复选框尺寸调整等；
小菜今天通过对 CheckBox 进行研究扩展实现如下功能的自定义 ACECheckBox 复选框；

复选框可变更未选中状态颜色；
复选框支持圆形样式；
复选框支持自定义尺寸；

CheckBox

源码分析

const Checkbox({
    Key key,
    @required this.value,       // 复选框状态 true/false/null
    this.tristate = false,      // 是否为三态
    @required this.onChanged,   // 状态变更回调
    this.activeColor,           // 选中状态填充颜色
    this.checkColor,            // 选中状态对号颜色
    this.materialTapTargetSize, // 点击范围
})

分析源码可知，tristate 为 true 时复选框有三种状态；为 false 时 value 不可为 null；

案例尝试

return Checkbox( value: state, onChanged: (value) => setState(() => state = value));

return Checkbox(value: state, checkColor: Colors.purpleAccent.withOpacity(0.7),
      onChanged: (value) => setState(() => state = value));

return Checkbox(value: state, activeColor: Colors.teal.withOpacity(0.3), checkColor: Colors.purpleAccent.withOpacity(0.7),
      onChanged: (value) => setState(() => state = value));

return Checkbox(tristate: true, value: _triState == null ? _triState : state, 
      activeColor: Colors.teal.withOpacity(0.3), checkColor: Colors.purpleAccent.withOpacity(0.7),
      onChanged: (value) => setState(() {
            if (value == null) {
              _triState = value;
            } else {
              _triState = ''; state = value;
            }
          }));
}

ACECheckBox

扩展一：变更未选中颜色

源码分析

// CheckBox
inactiveColor: widget.onChanged != null ? themeData.unselectedWidgetColor : themeData.disabledColor,
// ACECheckBox
inactiveColor: widget.onChanged != null
    ? widget.unCheckColor ?? themeData.unselectedWidgetColor
    : themeData.disabledColor,

分析 CheckBox 源码，其中复选框未选中颜色通过 ThemeData.unselectedWidgetColor 设置，修改颜色成本较大，小菜添加了 unCheckColor 属性，可自由设置未选中状态颜色，未设置时默认为 ThemeData.unselectedWidgetColor；

案例尝试

return ACECheckbox(value: aceState, unCheckColor: Colors.amberAccent, onChanged: (value) => setState(() => aceState = value));

return ACECheckbox(value: aceState, checkColor: Colors.red.withOpacity(0.7),
      unCheckColor: Colors.amberAccent, onChanged: (value) => setState(() => aceState = value));

return ACECheckbox(value: aceState, activeColor: Colors.indigoAccent.withOpacity(0.3), checkColor: Colors.red.withOpacity(0.7),
      unCheckColor: Colors.amberAccent, onChanged: (value) => setState(() => aceState = value));

return ACECheckbox(tristate: true, value: _triAceState == null ? _triAceState : aceState,
      activeColor: Colors.indigoAccent.withOpacity(0.7), checkColor: Colors.red.withOpacity(0.4),
      unCheckColor: Colors.amberAccent, onChanged: (value) {
        setState(() {
          if (value == null) {
            _triAceState = value;
          } else {
            _triAceState = ''; aceState = value;
          }
        });
      });

扩展二：添加圆形样式

源码分析

// 绘制边框
_drawBorder(canvas, outer, t, offset, type, paint) {
  assert(t >= 0.0 && t <= 0.5);
  final double size = outer.width;
  if ((type ?? ACECheckBoxType.normal) == ACECheckBoxType.normal) {
    canvas.drawDRRect(
        outer, outer.deflate(math.min(size / 2.0, _kStrokeWidth + size * t)),
        paint..strokeWidth = _kStrokeWidth / 2.0..style = PaintingStyle.fill);
  } else {
    canvas.drawCircle(
        Offset(offset.dx + size / 2.0, offset.dy + size / 2.0), size / 2.0,
        paint..strokeWidth = _kStrokeWidth..style = PaintingStyle.stroke);
  }
}
// 绘制填充
_drawInner(canvas, outer, offset, type, paint) {
  if ((type ?? ACECheckBoxType.normal) == ACECheckBoxType.normal) {
    canvas.drawRRect(outer, paint);
  } else {
    canvas.drawCircle(
        Offset(offset.dx + outer.width / 2.0, offset.dy + outer.width / 2.0),
        outer.width / 2.0, paint);
  }
}

分析源码可知，CheckBox 边框和内部填充以及对号全是通过 Canvas 进行绘制，其中绘制边框时，采用双层圆角矩形方式 drawDRRect，默认两层圆角矩形之间是填充方式；小菜添加 ACECheckBoxType 属性，允许用户设置圆角样式；
绘制边框时画笔属性要与 drawDRRect 进行区分；其中复选框边框和内部填充两部分需要进行样式判断；

案例尝试

return ACECheckbox(value: aceState, unCheckColor: Colors.amberAccent, type: ACECheckBoxType.circle, onChanged: (value) => setState(() => aceState = value));

return ACECheckbox(value: aceState, checkColor: Colors.red.withOpacity(0.7),
      unCheckColor: Colors.amberAccent, type: ACECheckBoxType.circle,
      onChanged: (value) => setState(() => aceState = value));

return ACECheckbox(
      value: aceState, activeColor: Colors.indigoAccent.withOpacity(0.3), checkColor: Colors.red.withOpacity(0.7),
      unCheckColor: Colors.amberAccent, type: ACECheckBoxType.circle,
      onChanged: (value) => setState(() => aceState = value));

return ACECheckbox(tristate: true, value: _triAceState == null ? _triAceState : aceState,
      activeColor: Colors.indigoAccent.withOpacity(0.7), checkColor: Colors.red.withOpacity(0.4),
      unCheckColor: Colors.amberAccent, type: ACECheckBoxType.circle,
      onChanged: (value) {
        setState(() {
          if (value == null) {
            _triAceState = value;
          } else {
            _triAceState = ''; aceState = value;
          }
        });
      });

扩展三：自定义尺寸

源码分析

@override
void paint(PaintingContext context, Offset offset) {
  final Canvas canvas = context.canvas;
  paintRadialReaction(canvas, offset, size.center(Offset.zero));
  
  final Paint strokePaint = _createStrokePaint(checkColor);
  final Offset origin = offset + (size / 2.0 - Size.square(width) / 2.0);
  final AnimationStatus status = position.status;
  final double tNormalized = status == AnimationStatus.forward || status == AnimationStatus.completed ? position.value : 1.0 - position.value;
  if (_oldValue == false || value == false) {
    final double t = value == false ? 1.0 - tNormalized : tNormalized;
    final RRect outer = _outerRectAt(origin, t);
    final Paint paint = Paint()..color = _colorAt(t);
    if (t <= 0.5) {
      _drawBorder(canvas, outer, t, origin, type, paint);
    } else {
      _drawInner(canvas, outer, origin, type, paint);
      final double tShrink = (t - 0.5) * 2.0;
      if (_oldValue == null || value == null)
        _drawDash(canvas, origin, tShrink, width, strokePaint);
      else
        _drawCheck(canvas, origin, tShrink, width, strokePaint);
    }
  } else {
    final RRect outer = _outerRectAt(origin, 1.0);
    final Paint paint = Paint()..color = _colorAt(1.0);
    _drawInner(canvas, outer, origin, type, paint);
    if (tNormalized <= 0.5) {
      final double tShrink = 1.0 - tNormalized * 2.0;
      if (_oldValue == true)
        _drawCheck(canvas, origin, tShrink, width, strokePaint);
      else
        _drawDash(canvas, origin, tShrink, width, strokePaint);
    } else {
      final double tExpand = (tNormalized - 0.5) * 2.0;
      if (value == true)
        _drawCheck(canvas, origin, tExpand, width, strokePaint);
      else
        _drawDash(canvas, origin, tExpand, width, strokePaint);
    }
  }
}

分析源码 CheckBox 尺寸是固定的 Checkbox.width = 18.0，无法调整尺寸，小菜添加一个 width 参数，默认为 18.0 允许用户按需调整尺寸；如上是绘制复选框的三态情况；

案例尝试

return ACECheckbox(value: aceState, width: 10.0, onChanged: (value) => setState(() => aceState = value));

return ACECheckbox(value: aceState, checkColor: Colors.red.withOpacity(0.7), width: 18.0,
      onChanged: (value) => setState(() => aceState = value));

return ACECheckbox(value: aceState, activeColor: Colors.indigoAccent.withOpacity(0.3), checkColor: Colors.red.withOpacity(0.7),
      width: 28.0, onChanged: (value) => setState(() => aceState = value));

return ACECheckbox(tristate: true, value: _triAceState == null ? _triAceState : aceState,
      activeColor: Colors.indigoAccent.withOpacity(0.7), checkColor: Colors.red.withOpacity(0.4),
      type: ACECheckBoxType.normal, width: 38.0, onChanged: (value) {
        setState(() {
          if (value == null) {
            _triAceState = value;
          } else {
            _triAceState = ''; aceState = value;
          }
        });
      });

ACECheckBox 源码

小菜在扩展过程中，学习 CheckBox 源码，还有很多有意思的地方，包括对 true/false/null 三态的处理方式，以及 .lerp 动画效果的应用，在实际应用中都很有帮助；
小菜自定义 ACECheckBox 的扩展还不够完善，目前暂未添加图片或 Icon 的样式，以后有机会一同扩展；如有错误请多多指导！

来源：阿策小和尚