UIViewMask.m

-(void)makeMaskWithFrame:(CGRect)frame
{
    // Create a mask layer and the frame to determine what will be visible in the view.
    CAShapeLayer *maskLayer = [[CAShapeLayer alloc] init];
    
//    CGContextFillEllipseInRect();
    CGRect maskRect = CGRectMake(0, 0, frame.size.width, frame.size.height);
    
    // Create a path with the rectangle in it.
    CGPathRef path = CGPathCreateWithEllipseInRect(maskRect, NULL);
    
    // Set the path to the mask layer.
    maskLayer.path = path;
    
    // Release the path since it's not covered by ARC.
    CGPathRelease(path);
    
    // Set the mask of the view.
    _mainView.layer.mask = maskLayer;
    _mainView.layer.borderWidth = 2.0f;
    _mainView.layer.borderColor = [UIColor blueColor].CGColor;
    // 方向
    // 尝试用去 圆角矩形去实现 一个圆形  borderRadius 而不是 mask了
}