在view扣除正方形

- (void)drawRect:(CGRect)rect
{
    [super drawRect:rect];
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    CGContextSetRGBFillColor(ctx, .3, .3, .3, .1);
    CGContextFillRect(ctx, self.bounds);
    
    CGContextSetStrokeColorWithColor(ctx, [UIColor whiteColor].CGColor);
    CGContextStrokeRectWithWidth(ctx, _clearRect, 2);
    
    CGContextClearRect(ctx, _clearRect);
}