I tried to simulate memory warning on iOS Simulator and it turned out that when the app was in the background, the function didn't get called at that moment, but after the app became active again that the applicationDidReceiveMemoryWarning was being called.
When tested with Instrument:Activity Monitor, my app didn't release any memory when in the background while the other apps memory consumption were getting less.
However, if I call applicationDidReceiveMemoryWarning via applicationDidEnterBackground, the memory get released correctly, but I don't want to release memory at this stage though.
- (void)applicationDidEnterBackground:(UIApplication *)application
{
[glView applicationDidEnterBackground:application];
// Function works if it's called from here, but not what I want.
[self applicationDidReceiveMemoryWarning:application];
}
- (void)applicationDidReceiveMemoryWarning:(UIApplication *)application
{
[glView applicationDidReceiveMemoryWarning:application];
}
How do I fix this?
Unfortunately, what you want to do is discouraged by Apple in the iOS App Programming Guide. An app briefly enters the background, allowing you to free up memory. After that, the app is suspended and cannot receive messages, except applicationWillTerminate. If a low-memory warning fires, the operating system kills suspended apps as needed.
Related
It is a very simple example. I cannot explain why these lines of code cause a crash. I only want to create multiple threads in a for loop.
In the AppDelegate:
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
for (int i = 0; i < 5; i++) {
[self performSelectorInBackground:#selector(workInBackground)
withObject:nil];
}
return YES;
}
-(void)workInBackground{
}
The app crashes only sometimes (lldb crash, iPhone Simulator 5.0 - 6.1, Xcode Version 4.6.2). I use ARC.
Can anyone explain this behavior?
Update
I "solved" the problem. When I perform a cleanup before each test, the crash no longer occurs.
I don't know what the problem is, but you should check this out:
What are the differences between didFinishLaunchingWithOption and viewDidLoad
Also you could use NSOperationQueue if you have multiple task that you want to run in background.
Ok, so if you read the apple documetation they say: "This method is called after state restoration has occurred but before your app’s window and other UI have been presented. At some point after this method returns, the system calls another of your app delegate’s methods to move the app to the active (foreground) state or the background state." this mean that actually the app in this point in not yet in the foreground state, here you should store somewhere the intention to start your threads, and after that the sistem call the method - (void)applicationDidBecomeActive:(UIApplication *)application, at this point you are absolutelly sure that your app is in a foreground state and active, now if you have recorded the intention to start your threads you can do it, and don't forget to reset the intention variable status
Is there a way to perform some last actions when the user kills the application on iPhone?
In UIApplicationDelegate there is applicationWillTerminate: but as I understand it's not guaranteed to get called when the application terminates. Is there another way?
You can't rely on applicationWillTerminate being called. From the documentation:
For apps that do not support background execution or are linked against iOS 3.x or earlier, this method is always called when the user quits the app. For apps that support background execution, this method is generally not called when the user quits the app because the app simply moves to the background in that case. However, this method may be called in situations where the app is running in the background (not suspended) and the system needs to terminate it for some reason.
The proper place to save any state is when the app enters the background. Once that happens, there is no way to know if the app will return to the foreground or if it gets killed and then started from the beginning.
All methods concerning your app state are in your AppDelegate when you use one of the project templates.
Put the code in the applicationWillResignActive: method. It will get called if your app goes to an inactive state (terminating or no).
- (void)applicationWillResignActive:(UIApplication *)application
{
// Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application and it begins the transition to the background state.
// Use this method to pause ongoing tasks, disable timers, and throttle down OpenGL ES frame rates. Games should use this method to pause the game.
}
The "correct" place to save state is in both -applicationDidEnterBackground: and -applicationWillTerminate:. Don't worry about double-saving; generally only one of them is called (IME, -applicationWillTerminate: is not called when your app is killed in the background).
Caveat: -applicationDidEnterBackground: is not guaranteed to be called, since it is called after your app enters the background (and thus becomes eligible for killing without notice!). If the device is low on memory when your app is backgrounded, it might be killed. The best way to avoid this is to not use too much memory in the first place.
You could use
-applicationWillResignActive:, but I do not recommend this: apps become inactive quite frequently. An obvious is system dialogs (location/privacy prompts, Wi-Fi, notifications that show as alerts, alarms), TWTweetSheet, and I suspect MFMailComposeViewController, MFMessageComposeViewController, Notification Center, the app-switcher bar (e.g. to change tracks/enable orientation lock).
you can use applicationWillResignActive method in the appdelegate, or you can do the following, if you want to save stuff, but for some reason, you dont want to do it in the app delegate:
- (void) viewDidLoad/init {
[[NSNotificationCenter defaultCenter]
addObserver:self
selector:#selector(myApplicationWillResign)
name:UIApplicationWillResignActiveNotification
object:NULL];
}
- (void) myApplicationWillResign {
NSLog(#"About to die, perform last actions");
}
What delegate is called in this situation if I have an app running.
Over time, if iPad is set to lock after 2 mins, what is its state? Does the app still run? or does it suspend?
I'd like to know thanks
If the iOS device automatically locks after a period of inactivity, the same thing happens as when it's locked by pressing the lock button: the app briefly goes into the background before being suspended. (Unless of course it is an audio-playing app or other app that requests extra execution time, in which case it may stay in the background longer, or indefinitely, without being suspended).
In terms of UIApplicationDelegate methods, applicationWillResignActive: will be called first, followed by applicationDidEnterBackground:.
I have an app that needs to do something when it’s sent to background using the Home button and something else when the device is locked using the top hardware button. The standard way of solving these requirements are the notifications and delegate methods sent out by UIApplication. On iOS 4 they look like this:
// Pressing the home button
Will resign active.
Did enter background.
// Tapping app icon on Springboard
Will enter foreground.
Did become active.
// Pressing the lock button
Will resign active.
// Unlocking the device
Did become active.
In other words, it’s quite easy to tell between locking and backgrounding. On iOS 5 the behaviour changed:
// Pressing the home button
Will resign active.
Did enter background.
// Tapping app icon on Springboard
Will enter foreground.
Did become active.
// Pressing the lock button
Will resign active.
Did enter background.
// Unlocking the device
Will enter foreground.
Did become active.
Notice that the didEnterBackground and willEnterForeground notifications are now sent out even when (un)locking the device, making it impossible to tell between locking and backgrounding. Is this change documented somewhere? Is it a regression? Do you know another way to distinguish the two cases?
iOS 6
In my preliminary testing via the simulator, checking the application state with
[[UIApplication sharedApplication] applicationState]
in either
- (void)applicationWillEnterForeground:(UIApplication *)application
- (void)applicationDidEnterBackground:(UIApplication *)application
allows you to differentiate between a call to lock the device and just switching back to the homescreen. A lock screen will return 1 (UIApplicationStateInactive), whereas a home button press will register as a 2 (UIApplicationStateBackground).
It seems consistent and should work on an iOS device just as reliably as it does in the simulator.
iOS 7
The iOS 6 method no longer works in iOS 7. In order to do this now, you have to utilize CFNotificationCenter and listen for a darwin notification (labeled: com.apple.springboard.lockcomplete). You can find the github repo with the sample project here: https://github.com/binarydev/ios-home-vs-lock-button
Credit for the iOS 7 fix goes out to wqq
I have looked into this quite a bit so I would love to be wrong here if someone knows something I don't, but technically, there is no documented way to tell the difference between locking the device, and sending to background.
One thing you can check however, is the UIApplicationState during the transition from foreground to background. Locking a device will give UIApplicationStateInactive and moving the App to the background will give UIApplicationStateBackground. But, since this behaviour is not officially documented it may change in the future.
A basic example:
- (void)applicationDidEnterBackground:(UIApplication *)application {
UIApplicationState state = [[UIApplication sharedApplication] applicationState];
NSLog(#"Device state: %#", state);
switch (state) {
case UIApplicationStateActive:
/* ... */
break;
case UIApplicationStateInactive:
/* Device was/is locked */
break;
case UIApplicationStateBackground:
/* User pressed home button or opened another App (from an alert/email/etc) */
break;
}
}
UIApplicationState - The running states of an application
typedef enum {
UIApplicationStateActive,
UIApplicationStateInactive,
UIApplicationStateBackground
}
UIApplicationState
Constants
UIApplicationStateActive - The application
is running in the foreground and currently receiving events. Available
in iOS 4.0 and later.
UIApplicationStateInactive - The application is running in the
foreground but is not receiving events. This might happen as a result
of an interruption or because the application is transitioning to or
from the background.
UIApplicationStateBackground - The application is
running in the background.
According to the UIApplicationDelegate Protocol Reference:
applicationWillResignActive:
didEnterBackground:
// ...
willEnterForeground:
applicationDidBecomeActive:
are the only methods that ever get called in both situations.
According to the iOS 4.3 to iOS 5.0 API Diff, these are the ONLY changes regarding UIApplication or UIApplicationDelegate, so I couldn't find where they documented any of these notification changes:
UIApplication.h
Added -[UIApplication setNewsstandIconImage:]
Added UIApplication.userInterfaceLayoutDirection
Added UIApplicationDelegate.window
Added UIApplication(UINewsstand)
Added UIApplicationLaunchOptionsNewsstandDownloadsKey
Added UIRemoteNotificationTypeNewsstandContentAvailability
Added UIUserInterfaceLayoutDirection
Added UIUserInterfaceLayoutDirectionLeftToRight
Added UIUserInterfaceLayoutDirectionRightToLeft
This is more of a workaround/hack, but according to my experience it's very reliable.
When the device is screen-locked (not just home button-ed, if that's a word :)) - bound network (UDP) sockets are broken.
I was using GCDAsyncUDPSocket (also AsyncUDPSocket before) and they both fire a network/broken pipe error reliably when the device is turned off.
In my case I need the UDP socket anyway, for other apps it might be a bit smelly, however, just binding/listening on a UDP socket without any action is not too terrible if you really need to differentiate here.
This note will [self destruct]; is 5 minutes (so Apple won't find out).
There’s a thread about this issue on Apple Developer Forums (registered developers only, sorry). The gist is that the new behaviour is by design. There are requests for a new API feature to distinguish between the two use cases, but nothing working yet.
Here is what Apple's iOS Programming Guide says:
Pressing the Sleep/Wake button is another type of interruption that
causes your app to be deactivated temporarily. When the user presses
this button, the system disables touch events, moves the app to the
background but sets the value of the app’s applicationState property
to UIApplicationStateInactive (as opposed to
UIApplicationStateBackground), and finally locks the screen.
http://developer.apple.com/library/ios/#DOCUMENTATION/iPhone/conceptual/iPhoneOSProgrammingGuide/ManagingYourApplicationsFlow/ManagingYourApplicationsFlow.html
So, you should check the UIApplication's applicationState property in applicationDidEnterBackground:. If it is UIApplicationStateBackground the user pressed the home button. But if it is UIApplicationStateInactive the user locked the device.
Which is the proper delegate to implement when an application is waking up from being in the background and you want it to prep it to be active?
applicationWillEnterForeground vs applicationDidBecomeActive -- What's the difference?
Which is the proper delegate to implement for when an application is going to sleep and you want to prep it to cleanup and save data?
applicationWillResignActive vs. applicationDidEnterBackground -- What's the difference?
Also, I've noticed that applicationWillResignActive gets called when an incoming SMS or call comes in but the user chooses to click Ok and continue. I don't want my app to take any action in these cases. I just want it to keep running without any intermediate cleanup since the user didn't exit the app. So, I would think it makes more sense to do cleanup work just in applicationDidEnterBackground.
I would appreciate your input on best practices to follow on choosing which delegates to implement for waking up and going to sleep as well as considering events like being interrupted by SMS/calls.
Thanks
When waking up i.e. relaunching an app (either through springboard, app switching or URL) applicationWillEnterForeground: is called. It is only executed once when the app becomes ready for use, after being put into the background, while applicationDidBecomeActive: may be called multiple times after launch. This makes applicationWillEnterForeground: ideal for setup that needs to occur just once after relaunch.
applicationWillEnterForeground: is called:
when app is relaunched
before applicationDidBecomeActive:
applicationDidBecomeActive: is called:
when app is first launched after application:didFinishLaunchingWithOptions:
after applicationWillEnterForeground: if there's no URL to handle.
after application:handleOpenURL: is called.
after applicationWillResignActive: if user ignores interruption like a phone call or SMS.
applicationWillResignActive: is called:
when there is an interruption like a phone call.
if user takes call applicationDidEnterBackground: is called.
if user ignores call applicationDidBecomeActive: is called.
when the home button is pressed or user switches apps.
docs say you should
pause ongoing tasks
disable timers
pause a game
reduce OpenGL frame rates
applicationDidEnterBackground: is called:
after applicationWillResignActive:
docs say you should:
release shared resources
save user data
invalidate timers
save app state so you can restore it if app is terminated.
disable UI updates
you have 5 seconds to do what you need to and return the method
if you don't return within ~5 seconds the app is terminated.
you can ask for more time with beginBackgroundTaskWithExpirationHandler:
The official documentation.
Managing Your App's Life Cycle is helpful to your questions. For quick concept, you can see Figures in that document.
You can also read the comment from the code generated by the XCode Wizard. Listed as follows:
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
// Override point for customization after application launch.
return YES;
}
- (void)applicationWillResignActive:(UIApplication *)application
{
/*
Sent when the application is about to move from active to inactive state.
This can occur for certain types of temporary interruptions (such as an
incoming phone call or SMS message) or when the user quits the application
and it begins the transition to the background state.
Use this method to pause ongoing tasks, disable timers, and throttle down
OpenGL ES frame rates. Games should use this method to pause the game.
*/
}
- (void)applicationDidEnterBackground:(UIApplication *)application
{
/*
Use this method to release shared resources, save user data, invalidate
timers, and store enough application state information to restore your
application to its current state in case it is terminated later.
If your application supports background execution, this method is called
instead of applicationWillTerminate: when the user quits.
*/
}
- (void)applicationWillEnterForeground:(UIApplication *)application
{
/*
Called as part of the transition from the background to the active state;
here you can undo many of the changes made on entering the background.
*/
}
- (void)applicationDidBecomeActive:(UIApplication *)application
{
/*
Restart any tasks that were paused (or not yet started) while the
application was inactive. If the application was previously in the
background, optionally refresh the user interface.
*/
}
- (void)applicationWillTerminate:(UIApplication *)application
{
/*
Called when the application is about to terminate.
Save data if appropriate.
See also applicationDidEnterBackground:.
*/
}
For more detailed explanations, please refer to official document for UIApplicationDelegate
I was still a bit confused with Dano's answer so I did a little test to get the flow of events in certain scenarios for my reference, but it might be useful to you too. This is for apps that DO NOT use UIApplicationExitsOnSuspend in their info.plist. This was conducted on an iOS 8 simulator + confirmed with iOS 7 device. Please excuse Xamarin's event handler names. They are very similar.
Initial and all subsequent launches from a not-running state:
FinishedLaunching
OnActivated
Interruption (phone call, top slide-down, bottom slide-up):
Home button double-press listing inactive apps, then reselecting our app:
OnResignActivation
OnActivated
Home button double-press listing inactive apps, selecting another app, then relaunching our app:
Home button single press, then relaunch:
Lock (on/off button), then unlock:
OnResignActivation
DidEnterBackground
WillEnterForeground
OnActivated
Home button double-press, and terminate our app: (subsequent relaunch is first case)
OnResignActivation
DidEnterBackground
DidEnterBackground (iOS 7 only?)
Yes, DidEnterBackground is called twice on iOS7 device. Both times UIApplication state is Background. However, iOS 8 simulator does not. This needs testing on iOS 8 device. I will update my answer when I get my hand on it, or someone else could confirm.
applicationWillEnterForeground is called:
when app is relaunched(comes from background to foreground)
This method is not invoked when app starts for the first time i.e when applicationDidFinishLaunch is called but only when comes from background
applicationDidBecomeActive
applicationDidBecomeActive is called
when app is first launched after didFinishLaunching
after applicationWillEnterForeground if there’s no URL to handle.
after application:handleOpenURL: is called.
after applicationWillResignActive if user ignores interruption like a phone call or SMS.
after disappearing of alertView anywhere from the application
applicationWillResignActive is called when system is asking for permissions. (in iOS 10). Just in case someone hit into the same trouble as me...
In iOS 8+ there is a subtle but important difference for taking phone call.
In iOS 7 if user takes phone call both applicationWillResignActive: and applicationDidEnterBackground: are called. But in iOS 8+ only applicationWillResignActive: is called.
For iOS 13+ the following methods will be executed:
- (void)sceneWillEnterForeground:(UIScene *)scene
- (void)sceneDidBecomeActive:(UIScene *)scene