- (void)drawRect:(CGRect)rect {

  CGContextRef ctx = UIGraphicsGetCurrentContext();//获取上下文

  CGPoint center = CGPointMake(100, 100); //设置圆心位置
  CGFloat radius = 90; //设置半径
  CGFloat startA = - M_PI_2; //圆起点位置
  CGFloat endA = -M_PI_2 + M_PI * 2 * _progress; //圆终点位置

  UIBezierPath *path = [UIBezierPath bezierPathWithArcCenter:center radius:radius startAngle:startA endAngle:endA clockwise:YES];

  CGContextSetLineWidth(ctx, 10); //设置线条宽度
  [[UIColor blueColor] setStroke]; //设置描边颜色

  CGContextAddPath(ctx, path.CGPath); //把路径添加到上下文

  CGContextStrokePath(ctx); //渲染

}

[self setNeedsDisplay];

- (void)drawProgress:(CGFloat )progress
{
  _progress = progress;
  [self setNeedsDisplay];
}

//生成渐变色
  CALayer *gradientLayer = [CALayer layer];

  //左侧渐变色
  CAGradientLayer *leftLayer = [CAGradientLayer layer];
  leftLayer.frame = CGRectMake(0, 0, self.bounds.size.width / 2, self.bounds.size.height);  // 分段设置渐变色
  leftLayer.locations = @[@0.3, @0.9, @1];
  leftLayer.colors = @[(id)[UIColor yellowColor].CGColor, (id)[UIColor greenColor].CGColor];
  [gradientLayer addSublayer:leftLayer];

  //右侧渐变色
  CAGradientLayer *rightLayer = [CAGradientLayer layer];
  rightLayer.frame = CGRectMake(self.bounds.size.width / 2, 0, self.bounds.size.width / 2, self.bounds.size.height);
  rightLayer.locations = @[@0.3, @0.9, @1];
  rightLayer.colors = @[(id)[UIColor yellowColor].CGColor, (id)[UIColor redColor].CGColor];
  [gradientLayer addSublayer:rightLayer];

CGPoint center = CGPointMake(100, 100);
  CGFloat radius = 90;
  CGFloat startA = - M_PI_2; //设置进度条起点位置
  CGFloat endA = -M_PI_2 + M_PI * 2 * _progress; //设置进度条终点位置

  //获取环形路径（画一个圆形，填充色透明，设置线框宽度为10，这样就获得了一个环形）
  _progressLayer = [CAShapeLayer layer];//创建一个track shape layer
  _progressLayer.frame = self.bounds;
  _progressLayer.fillColor = [[UIColor clearColor] CGColor]; //填充色为无色
  _progressLayer.strokeColor = [[UIColor redColor] CGColor]; //指定path的渲染颜色,这里可以设置任意不透明颜色
  _progressLayer.opacity = 1; //背景颜色的透明度
  _progressLayer.lineCap = kCALineCapRound;//指定线的边缘是圆的
  _progressLayer.lineWidth = 10;//线的宽度
  UIBezierPath *path = [UIBezierPath bezierPathWithArcCenter:center radius:radius startAngle:startA endAngle:endA clockwise:YES];//上面说明过了用来构建圆形
  _progressLayer.path =[path CGPath]; //把path传递給layer，然后layer会处理相应的渲染，整个逻辑和CoreGraph是一致的。
  [self.layer addSublayer:_progressLayer];

[gradientLayer setMask:_progressLayer]; //用progressLayer来截取渐变层
self.layer addSublayer:gradientLayer];