Rect rect=Rect.fromLTRB(0.0, 0.0, size.width, size.height/2);

canvas.drawArc(rect, 0.0, 2*pi/4, false, paint);

}

//在实际场景中正确利用此回调可以避免重绘开销，本示例我们简单的返回true

@override

bool shouldRepaint(CustomPainter oldDelegate) => true;

}

Rect.fromCenter({ Offset center, double width, double height })

center	矩形中心相对于原点偏移量
width	矩形宽度
height	矩形高度

class MyPainter extends CustomPainter {

@override

void paint(Canvas canvas, Size size) {

var paint = Paint()

…isAntiAlias = true

…strokeWidth=0.0

…color=Colors.redAccent

…invertColors=false;

Rect rect=Rect.fromCenter(center: Offset(size.width/2,size.height/2),width:size.width/2,height:size.height/2);

canvas.drawArc(rect, 0.0, 2*pi/4, false, paint);

}

//在实际场景中正确利用此回调可以避免重绘开销，本示例我们简单的返回true

@override

bool shouldRepaint(CustomPainter oldDelegate) => true;

}

Rect.fromLTWH(double left, double top, double width, double height)

left	矩形最左边相对于原点偏移量
top	矩形最顶部相对于原点偏移量
width	矩形宽度
height	矩形高度

class MyPainter extends CustomPainter {

@override

void paint(Canvas canvas, Size size) {

var paint = Paint()

…isAntiAlias = true

…strokeWidth=0.0

…color=Colors.redAccent

…invertColors=false;

Rect rect=Rect.fromLTWH(0.0, 0.0, size.width/2, size.height/2);

canvas.drawArc(rect, 0.0, 2*pi/4, false, paint);

}

//在实际场景中正确利用此回调可以避免重绘开销，本示例我们简单的返回true

@override

bool shouldRepaint(CustomPainter oldDelegate) => true;

}

Rect.fromPoints(Offset a, Offset b) : this.fromLTRB(

math.min(a.dx, b.dx),

math.min(a.dy, b.dy),

math.max(a.dx, b.dx),

math.max(a.dy, b.dy),

);

a	相对于原点x方向上的偏移范围
b	相对于原点y方向上的偏移范围

下面三种写法效果一样（主要是取x或y方向上偏移量的范围）

class MyPainter extends CustomPainter {

@override

void paint(Canvas canvas, Size size) {

var paint = Paint()

…isAntiAlias = true

…strokeWidth=1.0

…color=Colors.redAccent

…invertColors=false;

Rect rect=Rect.fromPoints(Offset(size.width, 0.0),Offset(0,size.height));

canvas.drawArc(rect, 0.0, 2*pi/4, false, paint);

}

//在实际场景中正确利用此回调可以避免重绘开销，本示例我们简单的返回true

@override

bool shouldRepaint(CustomPainter oldDelegate) => true;

}

class MyPainter extends CustomPainter {

@override

void paint(Canvas canvas, Size size) {

var paint = Paint()

…isAntiAlias = true

…strokeWidth=1.0

…color=Colors.redAccent

…invertColors=false;

Rect rect=Rect.fromPoints(Offset(0.0, 0.0),Offset(size.width,size.height));

canvas.drawArc(rect, 0.0, 2*pi/4, false, paint);

}

//在实际场景中正确利用此回调可以避免重绘开销，本示例我们简单的返回true

@override

bool shouldRepaint(CustomPainter oldDelegate) => true;

}

class MyPainter extends CustomPainter {

@override

void paint(Canvas canvas, Size size) {

var paint = Paint()

…isAntiAlias = true

…strokeWidth=1.0

…color=Colors.redAccent

…invertColors=false;

Rect rect=Rect.fromPoints(Offset(size.width, size.height),Offset(0.0,0.0));

canvas.drawArc(rect, 0.0, 2*pi/4, false, paint);

}

//在实际场景中正确利用此回调可以避免重绘开销，本示例我们简单的返回true

@override

bool shouldRepaint(CustomPainter oldDelegate) => true;

}

useCenter=false（绘制扇形）

class MyPainter extends CustomPainter {

@override

void paint(Canvas canvas, Size size) {

var paint = Paint()

…isAntiAlias = true

…strokeWidth=0.0

…color=Colors.redAccent

…invertColors=false;

Rect rect=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: 100.0);

canvas.drawArc(rect, 0.0, 2*pi/4, true, paint);

}

//在实际场景中正确利用此回调可以避免重绘开销，本示例我们简单的返回true

@override

bool shouldRepaint(CustomPainter oldDelegate) => true;

}

class MyPainter extends CustomPainter {

@override

void paint(Canvas canvas, Size size) {

var paint = Paint()

…isAntiAlias = true

…strokeWidth=0.0

…color=Colors.redAccent

…invertColors=false;

Rect rect=Rect.fromLTRB(0.0, 0.0, size.width, size.height/2);

canvas.drawArc(rect, 0.0, 2*pi/4, true, paint);

}

//在实际场景中正确利用此回调可以避免重绘开销，本示例我们简单的返回true

@override

bool shouldRepaint(CustomPainter oldDelegate) => true;

}

Rect.lerp(在两个矩形之间进行线性插值)

源码：

static Rect lerp(Rect a, Rect b, double t) {

assert(t != null);

if (a == null && b == null)

return null;

if (a == null)

return Rect.fromLTRB(b.left * t, b.top * t, b.right * t, b.bottom * t);

if (b == null) {

final double k = 1.0 - t;

return Rect.fromLTRB(a.left * k, a.top * k, a.right * k, a.bottom * k);

}

return Rect.fromLTRB(

lerpDouble(a.left, b.left, t),

lerpDouble(a.top, b.top, t),

lerpDouble(a.right, b.right, t),

lerpDouble(a.bottom, b.bottom, t),

);

}

double lerpDouble(num a, num b, double t) {

if (a == null && b == null)

return null;

a ??= 0.0;

b ??= 0.0;

return a + (b - a) * t;

}

a	矩形
b	矩形
t	时间值（0.0~1.0之间）

class MyPainter extends CustomPainter {

@override

void paint(Canvas canvas, Size size) {

var paint = Paint()

…isAntiAlias = true

…strokeWidth=0.0

…color=Colors.redAccent

…invertColors=false;

Rect a=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: size.width/3);

Rect b=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: size.width/2);

Rect rect=Rect.lerp(a, b, 1.0);

canvas.drawArc(rect, 0.0, 2*pi/4, true, paint);

rect=Rect.lerp(a, b, 0.2);

canvas.drawArc(rect, 0.0, 2*pi/4, true, paint…color=Colors.blue);

}

//在实际场景中正确利用此回调可以避免重绘开销，本示例我们简单的返回true

@override

bool shouldRepaint(CustomPainter oldDelegate) => true;

}

class MyPainter extends CustomPainter {

@override

void paint(Canvas canvas, Size size) {

var paint = Paint()

…isAntiAlias = true

…strokeWidth=0.0

…color=Colors.redAccent

…invertColors=false;

Rect a=Rect.fromLTRB(80.0, 80.0, size.width-30.0, size.height-60.0);

Rect b=Rect.fromLTRB(50.0, 50.0, size.width-20.0, size.height-20.0);

Rect rect=Rect.lerp(a, b, 1.0);

canvas.drawArc(rect, 0.0, 2*pi/4, true, paint);

rect=Rect.lerp(a, b, 0.2);

canvas.drawArc(rect, 0.0, 2*pi/4, true, paint…color=Colors.blue);

}

//在实际场景中正确利用此回调可以避免重绘开销，本示例我们简单的返回true

@override

bool shouldRepaint(CustomPainter oldDelegate) => true;

}

class MyPainter extends CustomPainter {

@override

void paint(Canvas canvas, Size size) {

var paint = Paint()

…isAntiAlias = true

…strokeWidth=0.0

…color=Colors.redAccent

…invertColors=false;

Rect a=Rect.fromLTRB(80.0, 80.0, size.width-80.0, size.height-80.0);

Rect b=Rect.fromLTRB(50.0, 50.0, size.width-50.0, size.height-50.0);

Rect rect=Rect.lerp(a, b, 1.0);

canvas.drawArc(rect, 0.0, 2*pi/4, true, paint);

rect=Rect.lerp(a, b, 0.7);

canvas.drawArc(rect, 0.0, 2*pi/4, true, paint…color=Colors.blue);

rect=Rect.lerp(a, b, 0.5);

canvas.drawArc(rect, 0.0, 2*pi/4, true, paint…color=Colors.green);

rect=Rect.lerp(a, b, 0.3);

canvas.drawArc(rect, 0.0, 2*pi/4, true, paint…color=Colors.deepPurpleAccent);

rect=Rect.lerp(a, b, 0.1);

canvas.drawArc(rect, 0.0, 2*pi/4, true, paint…color=Colors.cyanAccent);

rect=Rect.lerp(a, b, -0.2);

canvas.drawArc(rect, 0.0, 2*pi/4, true, paint…color=Colors.lightGreenAccent);

}

//在实际场景中正确利用此回调可以避免重绘开销，本示例我们简单的返回true

@override

bool shouldRepaint(CustomPainter oldDelegate) => true;

}

画圆

class MyPainter extends CustomPainter {

@override

void paint(Canvas canvas, Size size) {

var paint = Paint()

…isAntiAlias = true

…strokeWidth=0.0

…color=Colors.redAccent

…invertColors=false;

Rect rect=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: 100.0);

canvas.drawArc(rect, 0.0, 2*pi, false, paint);

}

//在实际场景中正确利用此回调可以避免重绘开销，本示例我们简单的返回true

@override

bool shouldRepaint(CustomPainter oldDelegate) => true;

}

画同心圆

class MyPainter extends CustomPainter {

@override

void paint(Canvas canvas, Size size) {

var paint = Paint()

…isAntiAlias = true

…strokeWidth=0.0

…color=Colors.redAccent

…invertColors=false;

Rect rect=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: 100.0);

canvas.drawArc(rect, 0.0, 2*pi, false, paint);

rect=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: 80.0);

canvas.drawArc(rect, 0.0, 2*pi, false, paint…color=Colors.blue);

rect=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: 60.0);

canvas.drawArc(rect, 0.0, 2*pi, false, paint…color=Colors.deepPurple);

rect=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: 40.0);

canvas.drawArc(rect, 0.0, 2*pi, false, paint…color=Colors.lightGreenAccent);

rect=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: 20.0);

canvas.drawArc(rect, 0.0, 2*pi, false, paint…color=Colors.grey);

rect=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: 10.0);

canvas.drawArc(rect, 0.0, 2*pi, false, paint…color=Colors.pinkAccent);

rect=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: 2.0);

canvas.drawArc(rect, 0.0, 2*pi, false, paint…color=Colors.blueAccent);

}

//在实际场景中正确利用此回调可以避免重绘开销，本示例我们简单的返回true

@override

bool shouldRepaint(CustomPainter oldDelegate) => true;

}

绘制渐变圆

class MyPainter extends CustomPainter {

@override

void paint(Canvas canvas, Size size) {

var paint = Paint()

…isAntiAlias = true

…strokeWidth=0.0

…invertColors=false;

Rect rect=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: 100.0);

canvas.drawArc(rect, 0.0, 2*pi, false, paint…color=Colors.redAccent.withOpacity(0.2));

rect=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: 80.0);

canvas.drawArc(rect, 0.0, 2*pi, false, paint…color=Colors.redAccent.withOpacity(0.3));

rect=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: 60.0);

canvas.drawArc(rect, 0.0, 2*pi, false, paint…color=Colors.redAccent.withOpacity(0.4));

rect=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: 40.0);

canvas.drawArc(rect, 0.0, 2*pi, false, paint…color=Colors.redAccent.withOpacity(0.5));

rect=Rect.fromCircle(center: Offset(size.width/2,size.height/2),radius: 20.0);

canvas.drawArc(rect, 0.0, 2*pi, false, paint…color=Colors.redAccent.withOpacity(0.6));

}

//在实际场景中正确利用此回调可以避免重绘开销，本示例我们简单的返回true

@override

bool shouldRepaint(CustomPainter oldDelegate) => true;

}

同心圆段拼凑正矩形

class MyPainter extends CustomPainter {
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