Xcode的两个标签之间的共享变量 [英] Shared variable between two tabs for Xcode

问题描述

我有两个视图控制器，我正在使用它们都从基本视图控制器继承

1. 类A_ViewController：BaseViewController


2. 类B_ViewController：BaseViewController
   

这两种VC都与我的firebase数据库有很大的交互作用。所以我想要一个变量来跟踪所有下载的项目，以便这两个VC可以访问它，而无需再次重新下载文件。

我试图把在BaseViewController中为两个A，B类访问的变量名称

  var allPostsDownloaded：[Post]！因此，在A_VC下载任何数据之前，它会检查此allPostsDownloaded变量，并在数据存在的情况下从中加载该变量。如果它不存在，我追加到这个变量。所以当切换到B_VC时，可以做同样的事情，不需要时不重新下载数据。 
 
 
 
原因我没有使用segue或protocal来传递数据，这是因为两个VC与我的数据库相互影响很大。因此，尝试使用一个可变的数据来记录事物的位置是很清晰的。 
 
 
 
但是，问题是我
 
 
  var allPostsDownloaded：[邮政]！ 
  
每次在A和B之间切换时都会被调用（这是标签）。这将导致变量为空并且被解除初始化。  
 
 
我想我可以使用全局变量，但那不是好习惯吗？任何人都可以解释为什么新标签出现时会重新调用它？这是最好的解决方案。
解决方案
 
由于@avi提到了创建新的单例类，那么您可以轻松地传递和阅读。下面是一个例子
  class PersistentData 
 {
 static let sharedInstance = PersistentData（）
 
 //您的全局持久变量
 var allPostsDownloaded = [Post]（）
} 
  
因此，在你的控制器中，你可以简单地读取和设置如下
  // read 
 print （PersistentData.sharedInstance.allPostsDownloaded）
 
 //设置新数据。这个例子，因此取决于你的情况
 PersistentData.sharedInstance.allPostsDownloaded.append（newPost）
 PersistentData.sharedInstance.allPostsDownloaded = posts 
  
也要记住，如果你想在标签之间切换时读取新值，你可以在 viewDidAppear 或 viewWillAppear  
 
I have two view controllers that I am working on which both inherits from a Base view controller

class A_ViewController: BaseViewController
class B_ViewController: BaseViewController
Both of those VC interacts heavily with my firebase database. So I want a variable to keep track of all the downloaded items so those two VC can access it without the need to re-download the file again.

I tried to put a variable name in BaseViewController for the two A,B class to access
    var allPostsDownloaded:  [Post]!
So before A_VC downloads any data, it checks for this allPostsDownloaded variable and loads from it if the data exists. If it doesnt exist, I append to this variable. So when switching to B_VC, the same can be done and no data is re-downloaded when not required. 

Reason I am not using segue or protocal to pass data around is that the two VC interacts quite heavly with my database. So it was alot cleaner to try and have a mutural data varaible to keep track of where things are. 

However, the problem is that i 
var allPostsDownloaded:  [Post]!
gets called whenever I switch between A and B VC (Which are tabs). This will cause the variable to be empty and de-initialised. 

I guess I could use a global variable instead but that is not good practice? Could anyone please explain why it gets re-called when new tab shows up? And the best solution for this.
解决方案
as @avi mentioned create new singleton class, then you can pass and read easily. Below is an example
class PersistentData
{
    static let sharedInstance = PersistentData()

    // your global persistent variable
    var allPostsDownloaded = [Post]()
}
So in your controllers you can simple read and set as below
// read
print(PersistentData.sharedInstance.allPostsDownloaded)

// set new data. this just example, hence depends on your case
PersistentData.sharedInstance.allPostsDownloaded.append(newPost)
PersistentData.sharedInstance.allPostsDownloaded = posts
also keep in mind that if you want to read new value when switching between tabs, you can get the updated in viewDidAppear or viewWillAppear

                        
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