presentViewController 透明跳转

• (void)presentViewController:(UIViewController *)viewControllerToPresent animated: (BOOL)flag completion:(void (^ __nullable)(void))completion

这个支持透明跳转，但是有版本限制

modalPresentationStyle
UIModalPresentationCurrentContext NS_ENUM_AVAILABLE_IOS(3_2),
UIModalPresentationOverCurrentContext NS_ENUM_AVAILABLE_IOS(8_0),

判断系统版本方法：

if (NSFoundationVersionNumber < NSFoundationVersionNumber_iOS_8_0) {
    self.modalPresentationStyle = UIModalPresentationCurrentContext;
} else {
    presentVC.modalPresentationStyle = UIModalPresentationOverCurrentContext;
}

好啦， 自定义 alert sheet 什么的也不在话下啦