I want some advice on how to properly take on implementing state preservation and restoration for an app that has been build already, having lot of view controllers and complex hierarchy.
What are the things to be kept in mind when trying to preserve state for an app that is running for a while already?
EDIT:
I want to get clarified or possibly get a solution to a problem that I am facing in terms of restoring state goes. I successfully restore app' state on launch. I restore a view controller and other view controllers in hierarchy. Now when I push another view controller and sequence of other view controllers to nav. stack, at a particular view controller, for some reason the app crashes due to an exception. So when I open the app back, it still restores the first view controller's state that was preserved when pressing home button.
Specifically I want to know if there is any way to discard app state info. when app terminates due to uncaught exception? I know that the state will be discarded if user manually force kills app, or state restoration fails, or the app terminates at launch. Is there a way to catch a termination and configure app state accordingly? Suggestions would be great. Thanks.
From my understanding, state restoration should be done as we move along building the app
No, I don't think that's right at all. The whole beauty of the built-in state saving-and-restoration mechanism is that it can be patched right onto the working app, one view controller at a time. You don't even have to complete it for the whole app before testing; on the contrary, another beauty of the mechanism is that it works for just the subset of the view controller hierarchy for which it is in fact implemented, with no harm done to the rest of the app. So just start at the top of the view controller hierarchy and start implementing it.
Related
I have a doubt:
I have an app with 10 views. I want that, if the user is on View1 and sends the app to the background, it terminates the application (exit (0)). But I wanted this to happen only on View1, on the other screens, if the app goes to the background and then returns, it will continue where it left off.
What can I do?
Apple's guidelines seem to be strictly against terminating your app programmatically (for example, with exit()); it would go against what iOS users expect of how apps work.
Instead, I recommend the following approach:
When the app is sent to the background (applicationWillResignActive(_:) is called), check which view controller is currently being displayed.
If it's such that you wish to start over next time the app is brought to the foreground, just reset the app window's root view controller to whatever the initial view controller of your app is (typically, it involves reloading the inital view controller from Main.stroyboard, but your setup could be different).
You can not choose at runtime whether your app goes to the background or is terminated when the user presses the home button ("multitasking"); that is set in stone in your Info.plist file at build time.
Also, remember that even if you are in the screen that you wish to preserve when the user resumes, your app might be terminated by the system while it is in the background (to reclaim scarce system resources), so in that case it will still start from the initial screen. To prevent this, you might want to check out the APIs for state preservation and restoration.
Here is another SO question asking how to find the identity of the current view controller. Why not query the current view when you receive applicationWillResignActive indicating that your app is going to move to the background and then choose the action you want?
As far as I understand your description Preserving and Restoring State is what you are looking for.
Excerpt from Documentation:
The preservation and restoration process is mostly automatic, but you need to tell iOS which parts of your app to preserve. The steps for preserving your app’s view controllers are as follows:
Required
Assign restoration identifiers to the view controllers whose
configuration you want to preserve; see Tagging View Controllers for
Preservation.
Tell iOS how to create or locate new view controller objects at
launch time; see Restoring View Controllers at Launch Time.
Optional
For each view controller, store any specific configuration data needed to return that view controller to its original configuration; see Encoding and Decoding Your View Controller’s State.
Here is a link to Preserving Your App’s Visual Appearance Across Launches
I want to get clarified or possibly get a solution to a problem that I am facing in terms of restoring state. I successfully restore app's state on launch. I restore a view controller and other view controllers in hierarchy that was saved to disk when I pressed home button. Now when I push another view controller and sequence of other view controllers to nav. stack, at a particular view controller, for some reason the app crashes due to an exception. So when I open the app back, it still restores the first view controller's state that was preserved when pressing home button.
Specifically I want to know if there is any way to discard app state info. when app terminates due to uncaught exception? I know that the state will be discarded if user manually force-kills app, or state restoration fails, or the app terminates at launch. Is there a way to catch a termination when it occurs, and we configure app state accordingly? Suggestions would be great. Thanks.
I have an app (iOS, Swift) which is usually started in the morning by the user. The user will press a couple of buttons, enter some text, move to other views (using a navigation controller) and so on. When everything is filled out the right way, the user will put the iPhone into standby mode. As far as the app life cycle is concerned, it will enter the background state. When the user opens the app again, he will still be on the same page, with the same parameter, ... Everything is fine.
In case of a suspended app (due to a lack of resources or other stuff), the app will start again from the beginning and not from the view where the user has navigated to before. In such a case, what's the best way to navigate to the specific view the user was when bringing the phone into the standby mode and keep respectively build up the whole navigation stack as it was before the suspension?
You can use Apple state restoration and preservation technique for this purpose. Here you can find an example provided by Apple.
Hope this will help.
What I'm looking for is a mechanism to keep my app from resetting to the root view controller every time the application gets backgrounded or goes inactive. There are many apps out there that operate this way, namely Instagram, eBay, etc.
My instincts told me initially to poke into the AppDelegate's applicationWillEnterForeground method, where I would try to present the viewcontroller I'm after, however when I instantiate the viewController, I can present it, but without the navigation controller that normally be there.
This makes me think that I need to save the "state" of the application (maybe the NavigationController's stack?) and then restore the stack somehow when it gets relaunched.
I have watched the execution timings of each event and notice that closing the application and relaunching it will start the application fresh. I assume that my NSUserDefaults are still in place and thus could be checked for a logged in user. This could help determine which view in the navigation controller to push to (either login or dashboard).
Any direction is greatly appreciated.
The most revealing answer in this post was the use of some storage (NSUserDefaults) in order to store persistent data across uses.
For my specific case, this was storing a key holding user info. Then when the application loads, the would-be first view comes up, but if that key is missing, will modally pull a login view in front of it.
To do this I would use NSUserDefaults to store the current view of the app and then switch to that view when your app finishes launching. See the answers to this question: Swift: How to store user preferences?
I'm developing a small app on top of core data. At startup, I need to launch a maintenance task - fast but important -. To run this task, the app delegate must open a UIManagedDocument, and perform various checks on it. The app views must not start until the checks are completed. Because opening a UIMD is asynchronous, the app delegate isn't done when the first UIview tries to access the doc. The application then crashes due to a race condition because the app delegate and the view are trying to open the doc at the same time, while the doc state isn't yet finalised.
I'm using a storyboard, so segues are pretty much in control of the OS... Next time, I'll do it all manually..
What would be your recommendations ?
Note:
1)I can not perform the task when the app. goes into background state, because if it is brought back up again, avoiding inconsistent states between the underlying database and what's displayed in the view will be very tedious.
2)For the same reasons, performing the maintenance task during normal execution is not easily done.
3) All views access the UIMD via a singleton, according to the code proposed here
Setting a mutex lock in the UIView isn't my preferred route, because the screen remains black - no data -, while displaying the tab bars, until it is released by the app delegate.
Is there a way to have the app delegate wait for a signal before it hands the control over to the UIViews ? In this case, are there any gotchas ? I suspect this is probably not the recommended way to do, and iOS might kill the app if the delegate stays too long waiting for the maintenance task to complete. What would be "too long" in this case?
You could do it more elegant way. The first view the user will see must be some kind of SplashView - just an image with progress indicator. It should stay on top while your task is going on. It's not too important how you're showing this view. You can set it as the first in your storyboard or just create it manually in applicationDidFinishLaunching message.
Then in your task send a NSNotification when it's about to finish and in the observer in your AppDelegate just hide your SplashView and present your first view with valuable content.
You can even add some visual effects for transition between those views and your app will look really great! :)