- (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
