I have an app where I am saving data but when the app is killed in the background I have to delete all the saved data.
I tried using this method:
- (void)applicationWillTerminate:(UIApplication *)application
But its not working. Can someone please suggest me how to do that?
Not sure why you need to wipe the data on terminate specifically, but here is something to consider:
Another way to handle this type of situation when apps write data that may be incomplete when they are forced to quit, is that they write out a flag when the data is known to be good.
That way, if the app exits normally, the data will be written and the flag will be written.
If the app is forced to quit, the flag will not get written by nature of the forced quit.
THEN, when the app starts it can look for the flag. If the flag isn't there, the app knows that any data is incomplete and can discard it (delete it) and start over.
Hope this helps.
applicationWillTerminate: is basically never called and you should no longer rely on it.
The app is not given any callback when it is killed so you can't do anything. You need to decide on a different approach based on the actual user requirement. This may involve encryption / not holding certain data on disk (only ever in memory) / etc...
I know, I'm super late to the party but, if you can live with data "sticking around" until the next app launch you can delete/clear data in application(_:didFinishLaunchingWithOptions).
Related
So I've had a look at the apple documentation on the NSUserDefaults's synchronize() method. See below for reference:
https://developer.apple.com/reference/foundation/userdefaults/1414005-synchronize
The page currently reads:
Because this method is automatically invoked at periodic intervals, use this method only if you cannot wait for the automatic synchronization (for example, if your application is about to exit) or if you want to update the user defaults to what is on disk even though you have not made any changes.
However, what I still don't understand is when should this method be called? For example, should it be called every time the user changes the app's settings? Or should I just trust that the background api is going to handle that? And does the leaving of the view immediately after a settings change in memory result in that change being lost?
Also, when might a failure to call synchronize() result in user settings not getting changed correctly?
Furthermore, what is the cost (performance, memory or otherwise) of calling this method? I know it involves reading and writing from/to the disk but does that really take that much effort on phones?
There seems to be so much confusion about user defaults. Think of it this way. It's essentially the same as you having a global dictionary available throughout your app. If you add/edit/remove a key/value to the global dictionary, that change is immediately visible anywhere in your code. Since this dictionary is in memory, all would be lost when your app terminates if it wasn't persisted to a file. NSUserDefaults automatically persists the dictionary to a file every once in a while.
The only reason there is a synchronize method is so your app can tell NSUserDefaults to persist the dictionary "now" instead of waiting for the automatic saving that will eventually happen.
And the only reason you ever need to do that is because your app might be terminated (or crash) before the next automatic save.
In my own apps, the only place I call synchronize is in the applicationDidEnterBackground delegate method. This is to ensure the latest unsaved changes are persisted in case the app is terminated while in the background.
I think much of the confusion comes from debugging an app during development. It's not uncommon during development that you kill the app with the "stop" button in the debugger. And many times this happens before the most recent NSUserDefaults changes have been persisted. So I've developed the habit of putting my app in the background by pressing the Home button before killing the app in the debugger whenever I want to make sure the latest updates are persisted.
Given the above summary, let's review your questions:
should it be called every time the user changes the app's settings?
No. As described above, any change is automatically available immediately.
Or should I just trust that the background api is going to handle that?
Yes, trust the automatic persistence with the exception of calling synchronize when your app enters the background.
And does the leaving of the view immediately after a settings change in memory result in that change being lost?
This has no effect. Once you add/edit/delete a key/value in NSUserDefaults, the change is made.
Also, when might a failure to call synchronize() result in user settings not getting changed correctly?
The only time a change can be lost is if your app is terminated before the latest changes have been persisted. Calling synchronize when your app enters the background solves most of these issues. The only remaining possible problem is if your app crashes. Any unsaved changes that have not yet been persisted will be lost. Fix your app so it doesn't crash.
Furthermore, what is the cost (performance, memory or otherwise) of calling this method? I know it involves reading and writing from/to the disk but does that really take that much effort on phones?
The automatic persistence is done in the background and it simply writes a dictionary to a plist file. It's very fast unless you are not following recommendations. It will be slower if you are misusing NSUserDefaults to store large amounts of data.
Apple's documentation for synchronize() has been updated and now reads:
Waits for any pending asynchronous updates to the defaults database and returns; this method is unnecessary and shouldn't be used.
UPDATE
As anticipated, it has been deprecated as mentioned in Apple Doc
synchronize()
Waits for any pending asynchronous updates to the defaults database and returns; this method is unnecessary and shouldn't be used.
Original Answer
-synchronize is deprecated and will be marked with the NS_DEPRECATED macro in a future release.
-synchronize blocks the calling thread until all in-progress set operations have completed. This is no longer necessary. Replacements for previous uses of -synchronize depend on what the intent of calling synchronize was. If you synchronized…
— …before reading in order to fetch updated values: remove the synchronize call
— …after writing in order to notify another program to read: the other program can use KVO to observe the default without needing to notify
— …before exiting in a non-app (command line tool, agent, or daemon) process: call CFPreferencesAppSynchronize(kCFPreferencesCurrentApplication)
— …for any other reason: remove the synchronize call
As far i know, synchronize is used to sync the data immediately but iOS can handle that in smart way. So you dont need to call it everytime. If you call it everytime then it will turn to performance issue.
Check apple documentation:
Official Link
While developing with Firebase, I manually added a data record in the console but forgot one entry, which caused the app to crash. I've corrected the problem in the console but, because I was using Firebase's data persistence, the original data error persists, causing the crash again. If I switch the persistence off, everything is fine but the cached store isn't being updated. Has anyone had this problem and found a way to solve it?
I too have faced this issue, and have lost sleep over the thought of users being trapped in an endless crash-on-start-up loop.
As suggested, the opportunity for the issue to arise is created in the time window between the start up of an app and the subsequent arrival of cache updates from the Firebase server. If data is read from the cache during this time window, and then, if the data happens to be missing an expected value, and then, if the app uses the data in a way that assumes the data will not be nil, the app crashes. If the app crashes before the cache updates, the cache will never have the chance to update, and the user will be trapped in an endless loop (absent wiping the app's data from the device's memory).
Thus far, I've dealt with the issue by more strenuously guarding against the possibility of nil values in code that is called during start up. If nil is checked and inconveniently found, then, depending upon the situation, either (1) if possible and if it will not lead to further data corruption, the app substitutes an appropriate value in place of nil, or otherwise (2) the app goes into a wait mode for a couple of seconds, and then initiates a fresh read from the problem node, and then reattempts the start-up routine.
Perhaps the moral of the story is never assume that a value is non-nil or otherwise within an expected range. Either validate the value upon receipt or check the value at the time of intended use or both, and then handle errors accordingly.
The cache should update automatically. Since this happens in a background thread without, while it invokes your code on the main thread, I'd expect it to update the disk cache even if you application code crashes.
But if that doesn't happen, the fastest way to get back in a good state is to clear the app data for you application or to completely uninstall/reinstall.
Please, anyone, help me: Is calling NSUserDefaults's synchronize() method mandatory?. If I don't call it, what will happen? My application is working fine without it.
No.
Since iOS12, it isn't mandatory anymore.
Apple says:
This method is unnecessary and shouldn't be used.
You can find more information on iOS12 release note:
UserDefaults
NSUserDefaults has several bug fixes and improvements:
Removed synchronization requirements. It's no longer necessary to use synchronize, CFPreferencesAppSynchronize, or CFPreferencesSynchronize. These methods will be deprecated in a future version of the OS.
Now that you don't need to call these synchronization methods, the performance characteristics of NSUserDefaults and Preferences Utilities are slightly different: The time taken for enqueueing write operations is now paid by the writing thread, rather than by the next thread to call synchronize or do a read operation.
From the docs:
Because this method is automatically invoked at periodic intervals, use this method only if you cannot wait for the automatic synchronization (for example, if your application is about to exit) or if you want to update the user defaults to what is on disk even though you have not made any changes.
Meaning that if you kill the app right after something is written to the defaults without the periodic interval catching it, it will get lost. You probably did not kill the app right after a write event yet which is why your app seems to work fine so far.
Normally it works perfectly fine and you only have to use it in special cases, for example when the app will close directly after writing to NSUserDefaults. So you can simply add the synchronize method to the corresponding AppDelegate-method.
As others have said, you don't normally have to call synchronize at all.
Normally the system calls it for you so your defaults changes get written.
However, when working with Xcode it's pretty common to terminate your app by pressing command period or clicking the stop button. In that case it terminates the app without warning and your user defaults changes will quite likely not be written out, and will be lost.
This can be a good thing or a bad thing, depending on what you want. It's certainly confusing.
You can "fix" it by calling synchronize each time you make a change, or on some time interval. However that does slow your app down and increase it's power requirements (both by very small amounts.) If you are in a loop, writing changes to 10,000 server or Core Data records, and you change user defaults after each pass, then calling synchronize after each one might have a measurable effect on app speed and on battery life. In most cases you're not likely to notice the difference however.
I understand how to use CoreData, but I'm confused when it's best to save the data. When they press the home button? On every interaction in case the app crashes?
The reason why saving data is a separate call is so that you can batch multiple smaller changes that comprise a larger operation and save all at once, rather than saving at each step along the way.
You should save the data after each atomic operation, and never have committed data sitting only in memory for any significant period of time.
Each time the user commits a change to the data, they will expect the data to be there the next time they run the app, so it's your job to make sure it's there.
After your user submits a change to the data, your app is likely going to be waiting for the user do something else anyway, so save the data while the user decides what to do next.
If you wait to save data in the applicationDidEnterBackground, there is no guarantee that it will ever be called.
Obviously, not all data is critical, for example, data that a user has entered on a form, but hasn't submitted, is not critical. However, any submitted data is critical.
I don't think its a good idea to save on every interaction(Honestly your App shouldn't be able to crash on "every" Interaction ;) ).
I only save in my app in
- (void)applicationDidEnterBackground:(UIApplication *)application
In fact you are right regarding crashes. But what if an invalid Data causes the crash? So you will reload in the worst case the data so that it crashes every time.
But to be honest that is just an educated guess - I think it depends on how sensitive your Data/App is
edit: This answer provides a similar opinion Saving Core Data Context before Crashing
But there's is a really good point I missed:
you should save whenever the user performs critical operations
If you save in the background, you can do it very often without harming UX much. Remember though that you probably need to update you UI and that will have its impact (merge after save to main will be done on the main thread).
Keep you saves small (small number of objects) so not to stall the main thread.
it is very dependant on your CoreData stack architecture.
You will want to save on critical moments like entering background, or user important data/demand.
This question io3->ios4 upgrade said to support applicationWillResignActive. While implementing this call, I also implemented applicationDidEnterBackground and applicationWillEnterForeground. However, I found that my app would crash. After some debugging on the simulator I determined that I needed to reinitialize a key data structure in applicationWillEnterForeground. So my question is how would I have known that from reading the documentation? (In fact, I may be doing the wrong thing and just so happened to get it working again.) Is there an exact description of what to do when these methods are called?
Thanks.
The only things you should do when supporting multitasking, is saving the state of your app when it enters the background, and reload it when it becomes active. (If you generate a new template in Xcode, you'll see this.)
Saving state means writing any user preferences or data to disk. Reloading the state involves reading saved preferences and data, recreating any in memory data structures that might need it (like in your example you gave).
In most circumstances, there's little else you need to do. The only thing that would crash your app that is unique to multitasking would be trying to run code in the background for longer than the allotted amount of time, (which is 10 minutes.) Otherwise, it sounds like you have got other problems with your code.