CGContextFillPath不填写交叉子路径

Question

使用QuartzCore使用用户触摸在视图上绘制时，我遇到了一个奇怪的问题。我的想法是在touchesBegan中绘制椭圆，并在touchesMoved和touchesEnd中绘制一些子路径。我的代码在这里：

- (void)drawRect:(CGRect)rect
{
    // Drawing code
    CGContextRef context = UIGraphicsGetCurrentContext();

    CGContextSetLineJoin (context, kCGLineJoinRound);
    CGContextSetLineCap (context, kCGLineCapRound);

    CGContextSetStrokeColorWithColor(context, [UIColor redColor].CGColor);
    CGContextSetFillColorWithColor(context, [UIColor redColor].CGColor);

    CGContextSetLineWidth(context, 1.0);
    CGContextAddPath(context, path);
    //CGContextStrokePath(context);
    CGContextFillPath(context);
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    if (path == nil) {
        CGPathRelease(path);
        path = CGPathCreateMutable();
    }
    strokeWidth = 10;
    CGFloat halfWidth = strokeWidth / 2;
    CGPoint p = [[touches anyObject] locationInView:self];
    CGPathAddEllipseInRect(path, NULL, CGRectMake(p.x - halfWidth, p.y - halfWidth, strokeWidth, strokeWidth));

    [self setNeedsDisplay];
}

static CGPoint prevLeft, prevRight;
CGPoint midPoint(CGPoint p1, CGPoint p2) {
    return CGPointMake((p1.x + p2.x) / 2, (p1.y + p2.y) / 2);
}

CGPoint triPoint(CGPoint p1, CGPoint p2, CGPoint p3) {
    return CGPointMake((p1.x + p2.x + p3.x) / 3, (p1.y + p2.y + p3.y) / 3);
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];

    CGFloat halfWidth = strokeWidth / 2;
    CGPoint p1 = [touch previousLocationInView:self];
    CGPoint p2 = [touch locationInView:self];
    CGPoint lineVector = CGPointMake(p2.x - p1.x, p2.y - p1.y);
    CGFloat len = hypotf(lineVector.x, lineVector.y);
    CGPoint normalVector = CGPointMake(- lineVector.y / len, lineVector.x / len);

    CGFloat valx = normalVector.x * halfWidth;
    CGFloat valy = normalVector.y * halfWidth;

    CGPoint left1 = CGPointMake(p1.x + valx, p1.y + valy);
    CGPoint right1 = CGPointMake(p1.x - valx, p1.y - valy);

    CGPoint left = CGPointMake(p2.x + valx, p2.y + valy);
    CGPoint right = CGPointMake(p2.x - valx, p2.y - valy);

    prevLeft = midPoint(left1, left);
    prevRight = midPoint(right1, right);

    CGPathMoveToPoint(path, NULL, left1.x , left1.y);
    CGPathAddLineToPoint(path, NULL, left.x, left.y);
    CGPathAddLineToPoint(path, NULL, right.x, right.y);
    CGPathAddLineToPoint(path, NULL, right1.x, right1.y);
    CGPathCloseSubpath(path);
    [self setNeedsDisplay];
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];

    CGFloat halfWidth = strokeWidth / 2;
    CGPoint p1 = [touch previousLocationInView:self];
    CGPoint p2 = [touch locationInView:self];
    CGPoint lineVector = CGPointMake(p2.x - p1.x, p2.y - p1.y);
    CGFloat len = hypotf(lineVector.x, lineVector.y);
    CGPoint normalVector = CGPointMake(- lineVector.y / len, lineVector.x / len);

    CGFloat valx = normalVector.x * halfWidth;
    CGFloat valy = normalVector.y * halfWidth;

    CGPoint left1 = CGPointMake(p1.x + valx, p1.y + valy);
    CGPoint right1 = CGPointMake(p1.x - valx, p1.y - valy);

    CGPoint left = CGPointMake(p2.x + valx, p2.y + valy);
    CGPoint right = CGPointMake(p2.x - valx, p2.y - valy);

    prevLeft = midPoint(left1, left);
    prevRight = midPoint(right1, right);

    CGPathMoveToPoint(path, NULL, left1.x , left1.y);
    CGPathAddLineToPoint(path, NULL, left.x, left.y);
    CGPathAddLineToPoint(path, NULL, right.x, right.y);
    CGPathAddLineToPoint(path, NULL, right1.x, right1.y);
    CGPathCloseSubpath(path);
    [self setNeedsDisplay];
}

但是绘制touchesBegan和touchesMoved时的结果是：

它看起来像椭圆和子路径之间的交叉点没有用所选颜色填充。以前有人有这个问题吗？我很感激你的帮助。

谢谢，