利用transform的帮助，使绘制过程和坐标相互独立。

draw-with-transform.mm

void drawStripes(void *info, CGContextRef con)
{
    // assume 4x4 cell
    CGContextSetFillColorWithColor(con, [[UIColor redColor] CGColor]);
    CGContextFillRect(con, CGRectMake(0, 2, 4, 2));
    CGContextSetFillColorWithColor(con, [[UIColor blueColor] CGColor]);
    CGContextFillRect(con, CGRectMake(0, 0, 4, 2));
}

void drawArrow(CGContextRef con)
{
    // 这句话避免去改变下面的坐标就可以在(0,0)开始绘制了。
    CGContextTranslateCTM(con, -80, 0);
    
    // assume the background color is white
    CGContextSaveGState(con);
    
    // punch triangle hole in context clipping region
    CGContextMoveToPoint(con, 90, 100);
    CGContextAddLineToPoint(con, 100, 90);
    CGContextAddLineToPoint(con, 110, 100);
    CGContextClosePath(con);
    CGContextAddRect(con, CGContextGetClipBoundingBox(con));
    CGContextEOClip(con);
    
    // draw the vertical line, add its shape to the clipping region
    CGContextMoveToPoint(con, 100, 100);
    CGContextAddLineToPoint(con, 100, 19);
    CGContextSetLineWidth(con, 20);
    CGContextReplacePathWithStrokedPath(con);
    CGContextClip(con);
    
    // draw the gradient
    CGFloat locs[3] = {0., .5, 1.};
    CGFloat colors[12] = {
        .3, .3, .3, .8, // starting color
        .0,.0,.0,1.0,   // intermediate color
        .3, .3, .3, .8  // ending color
    };
    CGColorSpaceRef sp = CGColorSpaceCreateDeviceGray();
    CGGradientRef grad = CGGradientCreateWithColorComponents(sp, colors, locs, 3);
    CGContextDrawLinearGradient(con, grad, CGPointMake(89, 0), CGPointMake(111, 0), 0);
    CGColorSpaceRelease(sp);
    CGGradientRelease(grad);
    
    CGContextRestoreGState(con);
    
    // draw the triangle with pattern, the point of the arrow
    CGColorSpaceRef sp2 = CGColorSpaceCreatePattern(NULL);
    CGContextSetFillColorSpace(con, sp2);
    CGColorSpaceRelease(sp2);
    CGPatternCallbacks callback = {
        0, &drawStripes, NULL
    };
    CGAffineTransform tr = CGAffineTransformIdentity;
    CGPatternRef patt = CGPatternCreate(NULL, CGRectMake(0, 0, 4, 4), tr, 4, 4, kCGPatternTilingConstantSpacingMinimalDistortion, true, &callback);
    CGFloat alph = 1.0;
    CGContextSetFillPattern(con, patt, &alph);
    CGPatternRelease(patt);
    CGContextMoveToPoint(con, 80, 25);
    CGContextAddLineToPoint(con, 100, 0);
    CGContextAddLineToPoint(con, 120, 25);
    CGContextFillPath(con);
}


// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect
{
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(40, 100), NO, 0.0);
    CGContextRef con = UIGraphicsGetCurrentContext();
    
    drawArrow(con);
    
    UIImage* im = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    con = UIGraphicsGetCurrentContext();
    [im drawAtPoint:CGPointMake(0, 0)];
    
    for (int i = 0; i < 3; i++) {
        CGContextTranslateCTM(con, 20, 100);
        CGContextRotateCTM(con, 30 * M_PI/180.);
        CGContextTranslateCTM(con, -20, -100);
        [im drawAtPoint:CGPointMake(0, 0)];
    }
}