ios - 如果对可选的UIViewController数组进行操作，如何避免嵌套过多？

Question

我有一个UIPageViewController，它具有一个可选的UIViewController数组。

pageViewController.viewControllers

我注意到，每当我需要对其执行一些操作时，都需要编写 if检查的许多级别(4)。

  // TODO: How can we improve this code?
  //
  // Logic: We will call setViewControllers, if viewControllers is null, or empty, or 1st element of
  // viewControllers.pageIndex is not equal to self.selectedTabIndex
  if let viewControllers = self.pageViewController.viewControllers {
      if viewControllers.count > 0 {
          if let pageIndexable = viewControllers[0] as? PageIndexable {
              if pageIndexable.pageIndex != self.selectedTabIndex {
                  self.pageViewController.setViewControllers([viewController(At: self.selectedTabIndex)!], direction: direction, animated: true, completion: nil)
                  self.tabCollectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)
              }
          }
      } else {
          self.pageViewController.setViewControllers([viewController(At: self.selectedTabIndex)!], direction: direction, animated: true, completion: nil)
          self.tabCollectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)
      }
  } else {
      self.pageViewController.setViewControllers([viewController(At: self.selectedTabIndex)!], direction: direction, animated: true, completion: nil)
      self.tabCollectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)
  }

我觉得由于 if语句级别过多，我可能以错误的方式使用了 可选。
您能为我提出一种改进代码的方法吗？

Answer 1

考虑一下您何时不想调用setViewControllers。那是当以下是假的:
viewControllers为nil，或者为空，或者viewControllers[0].pageIndex != self.selectedTabIndex
使用德摩根定律，我们可以看到它的取反是:
viewControllers不为nil，也不为空，并且viewControllers[0].pageIndex == self.selectedTabIndex
在if语句中进行连接，我们得到:

if let vcs = self.pageViewController.viewControllers,
   let firstVC = vcs.first as? PageIndexable, // this checks for non-empty
   firstVC.pageIndex == self.selectedTabIndex {
   // do stuff
} else {
  self.pageViewController.setViewControllers([viewController(At: self.selectedTabIndex)!], direction: direction, animated: true, completion: nil)
  self.tabCollectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)
}
// do other stuff

如果该方法将在执行 setViewControllers和 scrollToItem之后立即返回(即，您在 // do other stuff位置没有任何代码)，则可以改用 guard语句:

guard let vcs = self.pageViewController.viewControllers,
   let firstVC = vcs.first as? PageIndexable,
   firstVC.pageIndex == self.selectedTabIndex else {
   self.pageViewController.setViewControllers([viewController(At: self.selectedTabIndex)!], direction: direction, animated: true, completion: nil)
   self.tabCollectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)
   return
}
// do stuff

正如亚历山大在 comments中所说的，您也可以在一个表达式中执行此操作:

if !((self.pageViewController.viewControllers?.first as? PageIndexable)?.pageIndex == self.selectedTabIndex) {

   self.pageViewController.setViewControllers([viewController(At: self.selectedTabIndex)!], direction: direction, animated: true, completion: nil)
   self.tabCollectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)

}

但是我认为这种方式的意图不太明确。