绘制矩形

 CAShapeLayer *mask = [[CAShapeLayer alloc] init];

mask.frame = CGRectMake(50, 50, 100, 100);

CGFloat width = 100;

CGFloat height = 100;

CGMutablePathRef path = CGPathCreateMutable();

CGPathMoveToPoint(path, nil, 30, 30);

CGPathAddLineToPoint(path, nil, width, 30);

CGPathAddLineToPoint(path, nil, width, height);

CGPathAddLineToPoint(path, nil,30, height);

CGPathAddLineToPoint(path, nil, 30, 30);

CGPathCloseSubpath(path);

mask.path = path;

CGPathRelease(path);

self.view.layer.mask = mask;