效果：

QQ20190929-105959@2x.png

代码：

-(UIImage *)createGradientRingImage:(CGFloat)radius width:(CGFloat)width gradient:(NSArray <UIColor *>*)gradient{
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(2*radius, 2*radius),NO,[UIScreen mainScreen].scale);  //开始画线
    //生成圆环上部分
    UIImage *upperImage = [self createGradientSemiCircleImageRadius:radius width:width gradient:gradient upper:YES];
    //生成圆环下部分
    UIImage *image = [self createGradientSemiCircleImageRadius:radius width:width gradient:gradient upper:NO];
    //合并成一个圆环
    [upperImage drawInRect:CGRectMake(0, 0, radius*2, radius)];
    [image drawInRect:CGRectMake(0, radius, radius*2, radius)];
    UIImage *theImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return theImage;
}


-(UIImage *)createGradientSemiCircleImageRadius:(CGFloat)radius width:(CGFloat)width gradient:(NSArray <UIColor *>*)gradient upper:(BOOL)upper{
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(2*radius, radius),NO,[UIScreen mainScreen].scale);  //开始画线
    //获取上下文
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSaveGState(context);
    CGContextSetLineWidth(context, width);
    //绘制半圆环
    if (upper) {
        CGContextAddArc(context, radius, radius, radius-width*0.5, M_PI, 2*M_PI, 0);
    }else{
        CGContextAddArc(context, radius, 0, radius-width*0.5, 0, M_PI, 0);
    }
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    NSMutableArray *colorArray = [NSMutableArray array];
    for (UIColor *color in gradient) {
        [colorArray addObject:(id)color.CGColor];
    }
    //null标识渐变色均匀分布
    CGGradientRef gradientRef = CGGradientCreateWithColors(colorSpace, (__bridge CFArrayRef)colorArray, NULL);
    // 释放色彩空间
    CGColorSpaceRelease(colorSpace);
    CGContextReplacePathWithStrokedPath(context);
    CGContextClip(context);
    // 4. 用渐变色填充
    //圆环是逆时针分布，所以上半部分圆环渐变色从-2*radius到28radius
    //下半部分圆环从2*radius到-2radius
    if (upper) {
        CGContextDrawLinearGradient(context, gradientRef, CGPointMake(-radius*2, radius), CGPointMake(radius*2, radius), 0);
    }else{
        CGContextDrawLinearGradient(context, gradientRef, CGPointMake(radius*2, radius), CGPointMake(-radius*2, radius), 0);
    }
    // 释放渐变色
    CGGradientRelease(gradientRef);
    UIImage *theImage = UIGraphicsGetImageFromCurrentImageContext();
    //拉伸当前图像
    CGContextRestoreGState(context);
    UIGraphicsEndImageContext();
    return theImage;
}