why only viewWillAppear called on navigation back

Question

I have doubt need to be clear.. I have stack and a navigation controller.now when the stack loads the viewDidLoad viewWillAppear viewDidAppear will be called. when i click some button then this button push me to the new stack , now new stack gives me the option of the back..now when i click on the back of the navigation controller..why only viewWillAppear will be called ..why not viewDidLoad and not viewDidAppear

viewDidAppear must be called. Try putting NSLog instead if you are checking with breakpoint — Rahul Wakade, Mar 11 '13 at 05:48
`viewDidAppear` always gets called in my apps. Make sure that in your `viewWillAppear:` method that you call `[super viewWillAppear:animated]`. Call the proper `[super xxx]` in your other methods too. — rmaddy, Mar 11 '13 at 05:53

Krishnabhadra · Accepted Answer · 2013-03-11T05:38:23.217

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Stack is Last In First Out (LIFO), so when you push new view controllers to the stack, previous viewcontroller will not get destroyed( and they remain in memory). When you pop back, there is no need to recreate the Viewcontroller since it is already in memory. So only viewWillAppear gets called.

As to why viewDidAppear doesn't get called in this case, I cant remember where I have read this, but viewDidAppear gets called after your UIViewController's view was added to the application's UIWindow heirarchy. And this process is done before the UIViewController is shown for the first time.

viewDidLoad only called when viewControllers views are loaded into the memory. It will be done when

the first time the view is needed to be shown
sometimes when viewController needed to be reloaded again, because it is purged from memory for some low memory reason.

In your case, when you pop back, the viewController is already loaded, so no need to call viewDidLoad again.

edited Mar 11 '13 at 05:38

answered Mar 11 '13 at 05:33

Krishnabhadra

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yaa ok I got it ...but why not `viewDidAppear` and `viewDidLoad` – Christien Mar 11 '13 at 05:37
ok then in ur case ..`viewWillAppear` and `ViewDidAppear ` should be called ryt...but `viewWillAppear` called only not `ViewDidAppear ` – Christien Mar 11 '13 at 05:51
When a ViewController's view gets added to UIWindow, `viewDidAppear` gets called. When you push new viewController on top of it, the previous one doesn't get removed from UIWindow. So when you pop back, the controller is still in UIWindow hierarchy, so no need to re add it to UIWindow again. Therefore `viewDidAppear` won't get called – Krishnabhadra Mar 11 '13 at 05:54

score 13 · Answer 2 · edited May 23 '17 at 11:45

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Full life cycle of ios ui explain here. enter image description here

http://www.verydemo.com/demo_c134_i4568.html

Note By Abizern from comment: this is true for iOS5 and earlier. iOS6 does not unload views anymore.

edited May 23 '17 at 11:45

Community

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answered Mar 11 '13 at 05:29

Hasintha Janka

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Add reference from where you got this screenshot :) – Rajan Balana Mar 11 '13 at 05:31
ya but didnt get the logic y only `viewWillAppear` will get called on navigation back ...not other – Christien Mar 11 '13 at 05:35
Because when you back that view controller already created. viewDidLoad called in initializing moment only if that already created it not fire. ViewWillAppear call every time it come to screen. – Hasintha Janka Mar 11 '13 at 05:40
Difference between didappear and willappear is willappear called befor the event and didappear called after event(when it done). – Hasintha Janka Mar 11 '13 at 05:42
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Sorry, but how did this answers the question? – Krishnabhadra Mar 11 '13 at 05:45
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Note - this is true for iOS5 and earlier. iOS6 does not unload views anymore. – Abizern Mar 12 '13 at 10:43

score 3 · Answer 3 · answered Mar 11 '13 at 05:38

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First of all, nice question @user2102546. Checkout here the perfect reason for your query.

viewDidLoad only gets called if view controller's views were unloaded, and need to be reloaded.

Normally, if you use a navigation controller, and go back to a previous view with one of the pop methods, viewDidLoad does not get called again, because the view are not unloaded yet.

However, the system can unload the views of any view controller when it is not frontmost in order to free up memory, so viewDidLoad can get called any time a view controller is about to be presented. You need to write your code to respond correctly to the different events.

Enjoy Programming!!

answered Mar 11 '13 at 05:38

Niru Mukund Shah

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if we consider the above image by Hasintha and ur answer ...`viewDidLoad does not get called again, because the view are not unloaded yet.` fine the ..`viewdidload will not be called as not unloaded but viewWillAppear and ViewDidAppear both should be called ` – Christien Mar 11 '13 at 05:46
@Christien, Yes, Both viewWillAppear & ViewDidAppear, will be called on navigation back. – Niru Mukund Shah Mar 11 '13 at 05:53

score 2 · Answer 4 · answered Mar 11 '13 at 05:28

I don't have a complete answer for you but I hope this helps.

viewDidLoad is a callback for modifying a view after the load event has happened. In your case, the view has already loaded. The fact that it is not in view doesn't mean it has been unloaded from memory.

viewDidAppear: While I don't know why this event isn't firing and would be happy if someone else would fill in the gap.

why only viewWillAppear called on navigation back

4 Answers4