??????CheckBox 復選框對于所有的開發朋友并不陌生，Flutter 提供了簡單便捷的使用方法，但針對不同的業務場景，可能會有些許的不同，例如圓角矩形替換為圓形，復選框尺寸調整等；
??????小菜今天通過對 CheckBox 進行研究擴展實現如下功能的 自定義 ACECheckBox 復選框；
- 復選框可變更未選中狀態顏色；
- 復選框支持圓形樣式；
- 復選框支持自定義尺寸；
CheckBox
源碼分析
const Checkbox({
Key key,
@required this.value, // 復選框狀態 true/false/null
this.tristate = false, // 是否為三態
@required this.onChanged, // 狀態變更回調
this.activeColor, // 選中狀態填充顏色
this.checkColor, // 選中狀態對號顏色
this.materialTapTargetSize, // 點擊范圍
})
??????分析源碼可知，tristate 為 true 時復選框有三種狀態；為 false 時 value 不可為 null；
案例嘗試
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
擴展一：變更未選中顏色
源碼分析
// CheckBox
inactiveColor: widget.onChanged != null ? themeData.unselectedWidgetColor : themeData.disabledColor,
// ACECheckBox
inactiveColor: widget.onChanged != null
? widget.unCheckColor ?? themeData.unselectedWidgetColor
: themeData.disabledColor,
??????分析 CheckBox 源碼，其中復選框未選中顏色通過 ThemeData.unselectedWidgetColor 設置，修改顏色成本較大，小菜添加了 unCheckColor 屬性，可自由設置未選中狀態顏色，未設置時默認為 ThemeData.unselectedWidgetColor；
案例嘗試
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;
}
});
});
擴展二：添加圓形樣式
源碼分析
// 繪制邊框
_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);
}
}
// 繪制填充
_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);
}
}
??????分析源碼可知，CheckBox 邊框和內部填充以及對號全是通過 Canvas 進行繪制，其中繪制邊框時，采用雙層圓角矩形方式 drawDRRect，默認兩層圓角矩形之間是填充方式；小菜添加 ACECheckBoxType 屬性，允許用戶設置圓角樣式；
??????繪制邊框時畫筆屬性要與 drawDRRect 進行區分；其中復選框邊框和內部填充兩部分需要進行樣式判斷；
案例嘗試
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;
}
});
});
擴展三：自定義尺寸
源碼分析
@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);
}
}
}
??????分析源碼 CheckBox 尺寸是固定的 Checkbox.width = 18.0，無法調整尺寸，小菜添加一個 width 參數，默認為 18.0 允許用戶按需調整尺寸；如上是繪制復選框的三態情況；
案例嘗試
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 源碼
??????小菜在擴展過程中，學習 CheckBox 源碼，還有很多有意思的地方，包括對 true/false/null 三態的處理方式，以及 .lerp 動畫效果的應用，在實際應用中都很有幫助；
??????小菜自定義 ACECheckBox 的擴展還不夠完善，目前暫未添加圖片或 Icon 的樣式，以后有機會一同擴展；如有錯誤請多多指導！
來源： 阿策小和尚
