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This is a question about the life cycle of UINavigationController (assuming your LMSNavigationController is its subclass) and UIViewController (hereinafter referred to as VC). . . UINavigationController（假設(shè)你們的 LMSNavigationController 是它的子類）以及 UIViewController （后文簡稱 VC ）生命周期的問題。。。

簡單版本的答案

把 VC A 的 viewDidLoad 改成 viewWillAppear 。
（下面還有個感覺上更優(yōu)的辦法。）

復(fù)雜版本（也雜不到哪去。。。）

先看看文檔對 viewDidLoad 是怎么描述的：

Called after the controller'??s view is loaded into memory.

在 VC 的視圖加載進(jìn)內(nèi)存的時候，這個方法會被調(diào)用。VC B pop 出去的時候不符合調(diào)用 VC A 的 viewDidLoad 的條件，解釋如下：

UINavigationController 維護(hù)著一個 VC 棧，你 push、pop 那些 VC 時都是在操作這個棧。

push 的時候它會創(chuàng)建一個新的 VC 壓入棧中，就是你的 VC B，這個過程中這個新的 VC B 綁定的 view 經(jīng)歷了被加入內(nèi)存的過程， viewDidLoad 被調(diào)用。 VC B 處于棧頂，成為 topViewController 。 VC A 不再處于棧頂，但仍處于棧中，與它關(guān)聯(lián)的 view 仍在內(nèi)存中。再 pop VC B 出去的時候，VC A 關(guān)聯(lián)的 view 的 viewDidLoad 不再被調(diào)用。

所以 push pop 具體調(diào)用了什么？

上圖

Figure 1-5 Messages sent during stack changes

圖片來源：Navigation Controller 文檔的 Monitoring Changes to the Navigation Stack 部分

Monitoring Changes to the Navigation Stack
監(jiān)控導(dǎo)航棧的變化

以 push VC B 出去為例：

1. VC B 作為當(dāng)前的 Topmost view controller ，其 viewWillDispear 最先被調(diào)用

2. VC A 的 viewWillAppear 被調(diào)用

3. UINavigationControllerDelegate 的 navigationController:willShowViewController:animated: 被調(diào)用

4. 轉(zhuǎn)場動畫（完成？不清楚）

5. VC B 的 viewDidDispear 被調(diào)用

6. VC A 的 viewDidAppear 被調(diào)用

7. UINavigationControllerDelegate 的 navigationController:didShowViewController:animated:

   ---

   Simple version of the answer

   Change VC A’s viewDidLoad to viewWillAppear.
   (There is a better way below.) ??

   Complicated version (not too complicated...)

   ??Let’s first take a look at how the document describes viewDidLoad: ??

   ??Called after the controller's view is loaded into memory.??

   ??This method will be called when the VC view is loaded into memory. VC B pop does not meet the conditions for calling VC A's viewDidLoad when it goes out. The explanation is as follows: ?? ??UINavigationController maintains a VC stack. When you push and pop those VCs, you are operating this stack. ?? ??When push, it will create a new VC and push it onto the stack, which is your VC B. In this process, this new VC B > The bound view undergoes the process of being added to the memory, and viewDidLoad is called. VC B is on top of the stack and becomes topViewController. VC A is no longer on top of the stack, but still on the stack, and its associated view is still in memory. When pop VC B goes out, the viewDidLoad of the view associated with VC A will no longer be called. ??

   So what exactly does push pop call?

   ??Picture above?? ??Figure 1-5 Messages sent during stack changes
   
   Image source: Monitoring Changes to the Navigation Stack section of the Navigation Controller document??

   ??Monitoring Changes to the Navigation Stack
   Monitoring changes to the Navigation Stack??

   ??Take push VC B going out as an example: ??
   1. ??VC B As the current Topmost view controller, its viewWillDispear is called first??
   2. ??VC A's viewWillAppear is called??
   3. ??UINavigationControllerDelegate's navigationController:willShowViewController:animated: is called??
   4. ??Transition animation (completed? Unclear)??
   5. ??VC B's viewDidDispear is called??
   6. ??VC A's viewDidAppear is called??
   7. ??UINavigationControllerDelegate's navigationController:didShowViewController:animated: is called??

   In summary, VC A’s viewDidLoad can be changed to viewWillAppear. viewDidLoad 改成 viewWillAppear 即可。

   然而個人感覺更好的辦法是把兩個 viewDidLoad 都拆了，邏輯重寫在 UINavigationControllerDelegate 的 navigationController:willShowViewController:animated: 中。即可在一個位置解決改背景圖這個需求。

   完。

   ---

   更新，上面說的用 UINavigationControllerDelegate 的辦法：

       func navigationController(navigationController: UINavigationController, willShowViewController viewController: UIViewController, animated: Bool) {
           if self.childViewControllers.count > 1 {
               navigationController.navigationBar.setBackgroundImage(UIImage(named: "a"), forBarMetrics: .Default)
           } else {
               navigationController.navigationBar.setBackgroundImage(UIImage(named: "b"), forBarMetrics: .Default)
           }
       }
   

   

   viewWillAppear

   However, I personally feel that a better way is to dismantle both viewDidLoad and rewrite the logic in UINavigationControllerDelegate's navigationController:willShowViewController:animated: middle. You can solve the need to change the background image in one place.
   End.

   ---

   Update, use UINavigationControllerDelegate as mentioned above:

   rrreee

   ?? ??viewWillAppear I also tried it, it is exactly the same as this one, no black box appears. As far as this question is concerned, these two places meet your requirements. Just choose what you like. ?? ??The navigation controller that comes with the system has already done what it needs to do before the animation starts, so there will be no picture loading until halfway through the animation. The worst thing is that the animation will be stuck for a few seconds before it starts, and it won't start until it is loaded. ?? ??Your navigation controller is customized. Check the customized part to see if there are any black boxes caused by problems. ?? ?? (Also... I still don’t see where the background image has been changed. It’s not the button image that was changed on the first controller) ??

All VCs under nav use the same navBar
You modified the navBar when entering B
but did not change it back when exiting B

You have two navigation bar controllers (pop). The navigation bar returned is the same as the current navigation bar. You should create a proxy and reassign the value. (But it’s more troublesome)

You must have modified the color or image of the navigation bar in B

Try setting the background image of vc again in the viewWillAppear method of A