What init metod is called everytime a viewcontroller is pushed to window?
-viewDidLoad and -initWithNibName work only 1st time the view is started. But I want to do some startup checking in my vc everytime I start it.
Does such method exist? or do I have to declare it somehow manually?
Try view controller's viewWillAppear: method (if you want to setup one specific controller).
Related
i initialize tables, data etc in my main ViewController. For more settings, i want to call another Viewcontroller with:
DateChangeController *theController = [self.storyboard instantiateViewControllerWithIdentifier:#"dateChangeController"];
[self presentViewController:theController animated:YES completion:^{NSLog(#"View controller presented.");}];
And some clicks later i return with a segue (custom:)
NSLog(#"Scroll to Ticket");
[self.sourceViewController presentModalViewController:self.destinationViewController animated:YES];
My Problem:
After returning to my main ViewController, viewDidLoad is called (everytime).I read, that the ARC releasing my mainView after "going" to the other ViewController and calling the viewDidUnload Method, but i want to keep all my data and tables i initialize at the beginning..
Any solution? Thank you very much!
The problem is that you are doing this:
main view controller ->
date change controller ->
a *different* main view controller
In other words, although in your verbal description you use the words "returning to my main ViewController", you are not in fact returning; you are moving forward to yet another instance of this main view controller every time, piling up all these view controllers on top of one another.
The way to return to an existing view controller is not to make a segue but to return! The return from presentViewController is dismissViewController. You do not use a segue for that; you just call dismissViewController. (Okay, in iOS 6 you can in fact use a segue, but it is a very special and rather complicated kind of segue called an Unwind or Exit segue.)
If you do that, you'll be back at your old view controller, which was sitting there all along waiting for your return, and viewDidLoad will not be called.
So, this was a good question for you to ask, because the double call of viewDidLoad was a sign to you that your architecture was all wrong.
I think you're taking the wrong approach - viewDidLoad is supposed to be called when it is called - it's a notification to you that the view is being refreshed or initially loaded. What you want to do is move that table initialization code somewhere else, or, at least, set a Boolean variable so that it is only called once. Would it work to create an object that has your table data when viewDidLoad is first called, then to check it to see if it's already been called?
I have a stack of UIViewController subclasses. Each modifies a NSManagedObject model. Many of them also present their own modal view controllers.
I need to save changes to the NSManagedObjectContext when a user either 'pops' the view controller or pushes the next view controller.
Currently, I'm hiding the default back button and setting my own UIBarButtonItem with a target of self and a custom action.
This works okay, but ideally, I want to use the default back button and run code before the pop. Is there a way I can run my own code before the pop?
(I'd prefer not to put code into viewWillDisappear as persisting to disk can be expensive and this method can also be triggered by modals being displayed by view controller.) Can it be done?
You can do it in viewDidDisappear, after checking that self is either 1) the second last element in self.navigationController.viewControllers (the case where the next VC just got pushed) or 2) self.navigationController is nil (the self VC just got popped).
Yes.. Navigation controller has a delegate which indicates when a view controller popped or pushed.. You can use that to do your task...
Add following method in your code:
- (void) viewWillDisappear:(BOOL)animated{
//your code here
}
I use viewWillDissappear to make any changes persistant.
If required i Use viewWillAppear to recoginze any changes (reload the data) that may have taken place while other puhed view controlers did their work.
For pop check isMovingFromParent in viewWillDisappear
func viewWillDisappear(_ animation:Bool){
super.viewWillDisappear(animation);
if isMovingFromParent {
// your code here
}
}
Interview question: For example, I have 3 classes A,B & C. I navigate from A -> B -> C, while pushing viewDidLoad function calls automatically and during popping viewWillAppear get called. But would it be possible to call my viewDidLoad function while popping?
No, Its not possible.Only viewDidAppear and viewWillAppear will be called by itself.You can call it manually.
All the best.
The viewDidLoad method is called when the view just loaded. Then viewWillAppear is called by the navigation controller when it's about to display the view.
If you want some code to be executed when the view is about to be displayed, be it when it's being pushed or when the top one is being popped, it makes more sense to use the appropriate method viewWillAppear for that, instead of trying to call the viewDidLoad method at a time when it isn't appropriate.
Can't you just move whatever code you have in viewDidLoad to viewWillAppear?
If the question is just "how can we call viewDidLoad while popping?", then it's simple:
- (void) viewWillDisappear:(BOOL)animated
{
[self viewDidLoad];
}
Just as a side note, if you have a view controller stack like A->B, it's possible that viewDidLoad will be called automatically on A when popping B if, while B was on top, the navigation controller unloaded A's view (if the app received a memory warning, for example)
You should not call viewDidLoad manually, it's not designed to be handled this way. Use viewWillAppear, as other users already mentioned.
As for the question whether it may happen that viewDidLoad will be called upon popping from a higher view controller: I imagine that may happen when the device got a low memory warning and unloaded the view controllers further down in the navigation hierarchy. Then the OS has to reload the view, I haven't however tested this and it's possible that this will never happen. The OS only unloads views when they don't have a superview, I didn't check whether upon pushing, views down the hierarchy actually get removed from the hierarchy.
ViewDidLoad should be for view creation.
ViewWillAppear - for Data Interaction if Server Request is Asynchronous. like calling API or any functionality.or any functionality we like to call when view going to appear.
ViewDidAppear - for Data Interaction if Server Request is synchronous.
Please let me know at what times init and loadView method gets called.
To my knowledge init method gets called only once when view is initialized and loadView is called anytime view is loaded. So, even if you are pushing a new view in the view stack and then popping it then also the loadView of the poped up view should get called. But when I am running my code in debugging mode, both of these methods are getting called once, irrespective of how many times I am loading the same screen. Please let me know if I am missing something.
you are right at some points :)
The init method is being called when the ViewController object is instantiated. The loadView method gets called every time a ViewController should load its view into memory. This can happen before the view is displayed for the first time OR when it should be displayed for a second, third,... time but had been removed from memory before. (this might happen if your app runs out of memory.)
If you want to execute some code every time the view becomes visible, you should have a look at the methods viewWillAppear/viewWillDisappear/viewDidAppear/viewDidDisappear.
loadView is called when you access the view property of your view controller and it's nil.
If the view has been unloaded (viewDidUnload has been called for memory purpose) then loadView will be called again. If not it will not be called.
What you want is viewWillAppear: or viewDidAppear:.
I shift to my view by
[[self navigationController] popToViewController:controller animated:YES];
In that ViewController, I'm not able to get a notice, that it comes back to front (e.g. by viewWillAppear). I want to reload a table, as soon as the view is visible again.
How do I get a notice, that the view comes back on the screen?
----> solved: See my last comment on Corey's answer
viewWillAppear should be called if you are using a UINavigationController.
Are you sure you have added it correctly to the view hierarchy?
Did you check if viewWillDisappear gets called when it goes offscreen?
Did you try viewDidAppear just to make sure?
Did you spell the method name correctly?
To add:
Is the instance of UINavigationController added directly to the UIWindow instance?
The delegate methods like viewWillappear are sent from UIApplication (I believe). UIApplication only "knows" about viewControllers whose views are either:
Added Directly to UIWindow.
Added to a
UINavigationController/UITabBarCOntroller
that is added directly to UIWindow
(or a chain of these that leads to UIWindow).