I want to know what happens when the app comes to foreground after being in background for long time? How basically iOS works to free up memory and how it kills the app.
Check The App Life Cycle document. Execution States for Apps in particular.
Related
If I send my app to background by clicking the home button and wait a certain time (perhaps 1 or 2 hours).
What will happen if i tap on the app icon now?
Will the app relaunch or simply be brought from background to foreground?
Quick "prologue":
Welcome to the wonderful world of stack overflow (SO), I myself am rather new here, but found it much friendlier to use welcome you anyway!
Just in case you haven't: Before you ask a question please look around a bit on SO in case someone else has asked the same thing, but if you can't get your question answered from that, then you should of course ask your own question.
Answer:
This question has no definite answer, because it depends. When you tap the home button your app enters, as you've said, the background and is still running to a certain degree. However after done so, the apps life cycle is up to iOS (the devices operating system) to determine. iOS controls and checks memory and CPU usage (etc..) of the device, and if you start another activity while your app is in the background that makes the available memory and CPU etc of the device not sufficient, iOS will terminate any apps in the background to not waste those resources (or battery etc). If so your app will relaunch next time you tap on it.
Although if you don't do anything performance heavy it is more likely that iOS keeps your device running in the background.
I'm not sure about the exact conditions and such the iOS works on, but i would say its very likely your app will have gotten terminated and is relaunched after 1 or 2 hours "in the background". Additional conditions apply if the device is locked during that period.
For proper documentation of this i would recommend reading Apples documentation for handling App State Transitions and/or the api for UIApplicationDelegate on apples developer website. Where you can see what the different methods in the AppDelegate does and how they interact.
Edit (answer to comment):
A way to relaunch the app everytime it goes into background?
Hm, yes, but also no. I'm not 100% sure about this (never encountered that "wish" before), but you can do this in your AppDelegate: (It will basically crash your app, but beware that apple does not encourage you to crash your own app anywhere). Doing this might stop the app from passing through apple store review process (i.e. your app might not be accepted to the App Store).
func applicationDidEnterBackground(_ application: UIApplication) {
exit(0)
}
Check out the answer to these post for a bit more information: call exit(0) in iphone app , objc - Proper way to exit iPhone application?.
Personally I would recommend you to work around it and don't do this. Also remember that when your app will enter the background applicationWillResignActive will be called and when the user opens it again, applicationDidBecomeActive gets called so you can do a reload or something from there if you want to refresh any data.
Whenever we press the home button in our device, the application releases the currently being used memory and moves to background stage. However, when we press this application app icon again it brings the app on top of the iPhone screen and occupies memory again. This is definitely not the relaunch because relaunch depends upon the UIApplicationDelegate method
didFinishLaunchingWithOptions. When we are switching app from background to foreground then applicationWillEnterForeground method fires.
For better understanding, the following link might be useful
iOS Application Life Cycle
I have a question about application background refresh.
In my AppDelegate.swift, I use an NSTimer to refresh my app when my app enters background mode.
On the iOS Simulator, refresh works perfectly in background mode, but when I run my app on the real device, refresh doesn't work.
Is it impossible to use a NSTimer for refresh my app in background?
Your application may not stay long in the 'Background' state. It could go to 'Suspended' or be killed by the system, if it wants to. And of course, no code from your app can run in those states.
That said, I think it's not a good practice to use NSTimer for scheduling background work, and use NSNotification instead.
I recommend to read thoroughly the Energy Efficiency Guide for iOS Apps.
I have a music signal processor app I want to publish and it needs to run in the back ground. I used to Info-plist to do so with UIBackgroundModes. It runs on IOS7 or less, and the problem I am having is once the phone locks and the app goes into the background I can't get it to come out of the background and its keeps running with the red banner at the top. Some of this I have learned as I go, but I have noticed some run on a timer, but I want mine, since its a music processor to keep going until the phone either unlocks or whatever interrupted it stops.
The app is basically a stomp box used with iRig. If that helps.
Thanks
Can you clarify about what you mean when the phone locks, the app goes into the background and yo can't get it to come out of the background? Are you saying like the app is playing music while the app is locked?
I am trying to force my app to automatically enter the "Suspended" state programmatically, so I can do testing on Core Bluetooth restoration.
I have tried calling
[[UIApplication sharedApplication] performSelector:#selector(suspend)];
but this merely sends it into the background. Am I doing it wrong? Is there a better way? Or is it impossible?
I would like to remind everyone exactly what it means to have an app be suspended, as there is always confusion on the terminology:
(source: apple.com)
An app in the background will get indefinitely suspended if the system needs memory, but Core Bluetooth's restoration can send an app back into the background state temporarily.
I don't know how to do this programmatically, but what I do is to press the home button, then launch a number of heavyweight apps like Safari and graphics-intensive games. The memory pressure causes the system to terminate your app pretty quickly. Core Bluetooth should then initiate the restore process and re-launch your app when it detects activity from a peripheral you are connected to/have asked to connect to.
I am facing the above issue and unsure why it is happening or how to fix it. When the app goes to the background and is later reopened, it always starts from the initial view.
I would like it to show the view that was shown when the app was dismissed - which is usually the default for iOS apps.
Can someone please explain why this might be happening and how I can resolve it.
Your app is probably killed while in the background to free it's resources for other tasks.
If applicable you can opt to use a background mode for your app so it keeps running in the background. This is possible if you require location updates, play audio or interact with bluetooth le devices.
By your description it's more likely you want to implement State preservation and Restoration (Programmig Guide)
Even if your app supports background execution, it cannot run forever. At some point, the system might need to terminate your app to free up memory for the current foreground app. However, the user should never have to care if an app is already running or was terminated. [...]
The state preservation system in UIKit provides a simple but flexible infrastructure for preserving and restoring the state of your app’s view controllers and views.
If you are on an iPhone older that the 3GS, this is normal. Apps are closed instead of backgrounded on these older models.
The other possible reason is that you've set
UIApplicationExitsOnSuspend=YES
in your Info.plist. In that case, your app will also always terminate when going to the background.