最近项目需要，想做一个渐变色的刻度盘，圆形按钮滑动控制，所以 用oc写了一下，代码没附上，想看代码可以私信联系，效果如下图。

部分代码

self.drawCenter = CGPointMake(self.frame.size.width / 2.0, self.frame.size.height / 2.0);
    self.radius = self.circleRadius;
    self.circleStartPoint = CGPointMake(self.drawCenter.x, self.drawCenter.y - self.circleRadius);

    CGContextRef ctx = UIGraphicsGetCurrentContext();
    
    //圆形的背景颜色
    CGContextSetStrokeColorWithColor(ctx, self.backgroundTintColor.CGColor);
    CGContextSetLineWidth(ctx, self.circleBorderWidth);
    CGContextAddArc(ctx, self.drawCenter.x, self.drawCenter.y, self.radius, 0, 2 * M_PI, 0);
    CGContextDrawPath(ctx, kCGPathStroke);
  
    //加载的进度
    UIBezierPath *loadPath = [UIBezierPath bezierPath];
    CGFloat loadStart = -M_PI_2;
    CGFloat loadCurre = loadStart + 2 * M_PI * self.loadProgress;
    
    [loadPath addArcWithCenter:self.drawCenter
                        radius:self.radius
                    startAngle:loadStart
                      endAngle:loadCurre
                     clockwise:YES];
    CGContextSaveGState(ctx);
    CGContextSetShouldAntialias(ctx, YES);
    CGContextSetLineWidth(ctx, self.circleBorderWidth);
    CGContextSetStrokeColorWithColor(ctx, self.maximumTrackTintColor.CGColor);
    CGContextAddPath(ctx, loadPath.CGPath);
    CGContextDrawPath(ctx, kCGPathStroke);
    CGContextRestoreGState(ctx);
    
    //起始位置做圆滑处理
    CGContextSaveGState(ctx);
    CGContextSetShouldAntialias(ctx, YES);
    CGContextSetFillColorWithColor(ctx, self.minimumTrackTintColor.CGColor);
    CGContextAddArc(ctx, self.circleStartPoint.x, self.circleStartPoint.y, self.circleBorderWidth / 2.0, 0, M_PI * 2, 0);
    CGContextDrawPath(ctx, kCGPathFill);
    CGContextRestoreGState(ctx);
    
    //value
    UIBezierPath *circlePath = [UIBezierPath bezierPath];
    CGFloat originstart = -M_PI_2;
    CGFloat currentOrigin = originstart + 2 * M_PI * self.value;
    [circlePath addArcWithCenter:self.drawCenter
                          radius:self.radius
                      startAngle:originstart
                        endAngle:currentOrigin
                       clockwise:YES];
    CGContextSaveGState(ctx);
    CGContextSetShouldAntialias(ctx, YES);
    CGContextSetLineWidth(ctx, self.circleBorderWidth);
    CGContextSetStrokeColorWithColor(ctx, self.minimumTrackTintColor.CGColor);
    CGContextAddPath(ctx, circlePath.CGPath);
    CGContextDrawPath(ctx, kCGPathStroke);
    CGContextRestoreGState(ctx);
    
    /*
     * 计算移动点的位置
     * alpha = 移动点相对于起始点顺时针扫过的角度(弧度)
     * x = r * sin(alpha) + 圆心的x坐标, sin在0-PI之间为正，PI-2*PI之间为负
     * y 可以通过r * cos(alpha) + 圆心的y坐标来计算。
     * 不过我这里用了另外一个比较投机的方法，先算出亮点连线在y轴上投影的长度，然后根据移动点在y轴上相对于圆心的位置将这个绝对长度a和圆心y坐标相加减。
     */
    double alpha = self.value * 2 * M_PI;
    double x = self.radius * sin(alpha) + self.drawCenter.x;
    double y = sqrt(self.radius * self.radius - pow((self.drawCenter.x - x), 2)) + self.drawCenter.y;
    double a = y - self.drawCenter.y;
    if (self.value <= 0.25 || self.value > 0.75) {
        y = self.drawCenter.y - a;
    }
    self.lastPoint = CGPointMake(x, y);
    self.thumbView.center = self.lastPoint;