The document provides an overview of iOS multitasking support from a developer's perspective, noting that quitting an app moves it to the background instead of terminating it. When interrupted, an app resigns active state and moves to background. Developers must handle transitions between foreground and background and save state before the app is suspended. Not all devices support multitasking and earlier SDKs do not provide multitasking capabilities.

1. iPHONE iOS4 / SDK4.0
multitasking support
from a developer’s perspective
(a dump of my own development notes while writing PlaceTrack)
Nico Tranquilli
nico@tranquilli.org
www.tranquilli.org

2. iPHONE MULTITASKING
quitting (pressing Home) a running application on iOS 4.0 causes it to
become inactive and be moved to the background. Quitting from the
task bar is equivalent to a force quit/termination (kill -9)
an application interrupted by a system event (incoming call, SMS,
calendar notification) resigns the active state (ie. looses focus). Once
the user accepts the interruption, it moves to the background and
suspends
app.delegate’s methods get called on transitions between foreground
and background states (and should be handled appropriately by the
developer); other objects can observe appropriate notifications
older applications, linked against SDK earlier than 4.0 or running on
hw with no multitasking support, always SUSPEND, are KEPT IN
RAM (that’s for Fast App Switching) but are NOT going to be USING
CPU CYCLES
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

3. MULTITASKING SUPPORT
iOS4 on 3G and earlier iPhone models doesn’t support multitasking
iOS earlier than 4.0 doesn’t support multitasking
any app built against a pre-4.0 SDK still behaves as it did on earlier OS
(terminates when you leave and gets the same notifications)
when you leave an app built against a 4.0 SDK, it receives
applicationDidEnterBackground. On (older) devices with no
multitasking support, applicationWillTerminate is called
afterwards.
developers can explicitly opt out by setting
UIApplicationExitsOnSuspend to YES in Info.plist
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

4. APPLICATION’S STATES
‣ executing code in the foreground
ACTIVE ‣ receiving events
‣ application has focus
still executing code in the foreground but not receiving events
applications stay here:
INACTIVE ‣ on their way to the background state
‣ when the system is waiting for the user to respond to events
‣ when the user presses the Sleep/Wake button
executing code in the background
applications stay here:
BACKGROUND
‣ on their way to the suspended state (briefly)
‣ if they asked for a (limited) extra processing time
‣ frozen in the background: not executing code but still in memory
‣ push notifications still delivered when the user taps alert’s btn
SUSPENDED ‣ events coalesced and delivered later when it resumes
‣ wakes up and moves to the background on registered
background events (eg. Significant Location Change)
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

5. BACKGROUND EXECUTION
once in the background, an application can...
suspend, shortly after entering this state and be woken up later by
events delivered by system background services (Significant Location
Change, VoIP) or when user taps a notification’s view button
execute your code for a short amount of time, continuing what it was
doing before becoming INACTIVE and suspending once the task is
complete (or the 600s time limit is reached)
keep running in the background for unlimited time, if it was
registered for one of the permitted background task (eg. continuos
location or background audio)
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

6. ENTERING BACKGROUND...
applicationWillResignActive is called on app.delegate (and
UIApplicationWillResignActiveNotification is posted to
registered objects)
➡ delivery of touch events is suspended by the system (inactive
applications run but do not dispatch incoming events)
➡ you should pause ongoing tasks and wait to transition to either
the active or background state
➡ an active application resigns active state...
‣ on incoming phone call/sms or calendar event
‣ when user presses the Sleep/Wake button while running
‣ when user presses the Home button (if background is supported)
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

7. ENTERING BACKGROUND...
applicationDidEnterBackground is called on app.delegate (and
UIApplicationDidEnterBackgroundNotification is posted to
registered objects)
➡ application is now in the background
➡ system data objects (cache, etc) are automatically released
➡ you should always save user data and state information here (so
you can restore it later, just in case it gets terminated)
➡ you should release as much memory as possible to prevent
termination
➡ just five seconds to perform any tasks and return (additional time
can be requested) before being suspended
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

8. ACTIVE
USER INTERRUPT
PRESSES (incoming call,
HOME BUTTON sms, event)
 save user data
 save application state
… USER GETS
NOTIFIED
applicationWillResignActive:
INACTIVE
 register for bg tasks YES NO
 ask for exec. time
 schedule local notif.
ACCEPT ?
 suspend
applicationDidEnterBackground:
BACKGROUND
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

9. SAVE DATA BEFORE ENTERING
BACKGROUND!
apps running in the background still get incoming messages delivered:
observe low-memory warnings as usual
applicationDidReceiveMemoryWarning: (in your app.delegate’s)
didReceiveMemoryWarning (in UIViewController)
UIApplicationDidReceiveMemoryWarningNotification
when suspended, it won’t get any notice in case of termination: save data
and context beforehand!
may be purged from memory and terminated during low-memory
conditions: always save context and release as much memory as possibile
to prevent termination before moving to the background
applicationWillTerminate gets called as usual if the application is
currently running
no termination notice is delivered if the application is suspended
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

10. BACKGROUND APPLICATIONS
SHOULD…
postpone any UI updates
NEVER make OpenGL ES calls
not use system shared resources (contacs db, etc)
did I tell you to save state and user data when
entering background ?
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

11. BACKGROUND APPLICATIONS
CAN…
continue to run, ie. request permission to run in the
background and provide an expiration handler (for final
cleanup, etc) to be called when your time limit is reached.
If your job requires the thread for a long time it makes
sense to schedule the work on a dispatch queue so that
the run loop is not blocked.
respond to registered background events
schedule local notifications (sound, alerts, badges)
SUSPEND ;-)
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

12. RUNNING IN THE BACKGROUND
background applications have a limited amount of execution time, except
for certain (permitted) background tasks
you should wrap any long-running (critical) tasks with
beginBackgroundTaskWithExpirationHandler: and endBackgroundTask:
calls
‣ you can do this while the app is still in foreground
‣ you can start any bg task at quit time (in your didEnterBackground delegate)
‣ you can start any task later when your app is woken up by a registered bg event
‣ limit for task completion is 600secs. Time left to run is in the
backgroundTimeRemaining property of UIApplication
always provide an expiration handler for each task and call the
endBackgroundTask: from there (application gets terminated rather then
suspended if you leave outstanding background tasks!)
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

13. SOME BACKGROUND TYPES ARE
DECLARED
Support for some types of background execution must be
declared in advance including the UIBackgroundModes key
in your Info.plist. Possible strings in its array value:
audio
location – for continuos, high-accuracy, location updates
(drains batteries!). Do not use this if all you need are
Significant Location Change notifications!
voip – VoIp applications are automatically started in the
background upon system boot, suspended, woken up by the
system on incoming connection (socket handed over)
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

14. BACKGROUND LOCATION
significant location change: suspended application is woken up
(or relaunched if not running) on significant location changes.
Does NOT need to be declared as a background application.
Low-power, recommended
standard location updates: location updates are delivered (as
usual) to a running fg/bg application. The application gets
suspended once background time expires and nothing is
delivered when application is suspended or terminated
continuous background location: an application registered as a
continuos background location app is never suspended by the
system and gets continuous location updates in the backgound
(turn-by-turn apps and the like). Power-intensive!
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

15. EARLIER iOS SUPPORT
set the deployment target to the lowest you want to
support
set the base and active SDK to the latest stable available
weakly link frameworks that may not exist on the
deployment platform
at runtime, make sure code that needs OS 4.0 features
(including C blocks) doesn't get executed when running
on earlier OS versions: NSClassFromString, [UIDevice
currentDevice].multitaskingSupported, respondsToSelector ,
instancesRespondesToSelector are your friends
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

16. SOME CODE SNIPPETS
checking multitasking availability
-(BOOL)isMultitaskingOS
{
BOOL bgSupport=NO;
if ([[UIDevice currentDevice] respondsToSelector:@selector(isMultitaskingSupported)])
! ! bgSupport=[UIDevice currentDevice].multitaskingSupported;
return bgSupport;
}
checking the runtime state of the app
-(BOOL)isForeground
{
! if (![self isMultitaskingOS])
! ! return YES;
! UIApplicationState state = [UIApplication sharedApplication].applicationState;
! //return (state==UIApplicationStateActive || state==UIApplicationStateInactive );
! return (state==UIApplicationStateActive);
}
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

17. SOME CODE SNIPPETS
making sure an app is not suspended while completing a task in
the background (uses C blocks and the Grand Central Dispatch)
UIApplication *app=[UIApplication sharedApplication];
NSAssert(self->bgTask == UIInvalidBackgroundTask, nil);
// beginBackgroundTaskWithExpirationHandler: marks the beginning of
// a new long-running background task.
bgTask = [app beginBackgroundTaskWithExpirationHandler: ^{
! dispatch_async(dispatch_get_main_queue(), ^{
! ! [app endBackgroundTask:self->bgTask];
! ! self->bgTask = UIInvalidBackgroundTask;
! });
}];
!
dispatch_async(dispatch_get_main_queue(), ^{
! // do something!
! // eg:while ([app backgroundTimeRemaining] > 1.0) { ; }
! !
! [app endBackgroundTask:self->bgTask];
! self->bgTask = UIInvalidBackgroundTask;
});
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

18. SOME CODE SNIPPETS
background app relaunch by os due to a location event
- (BOOL)application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
! // ....
if ([launchOptions objectForKey:@"UIApplicationLaunchOptionsLocationKey"]]) {
// application was (re)launched in response to an incoming location event
! }
! // ....
!
}
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org

19. iPHONE iOS4 / SDK4.0
multitasking support
from a developer’s perspective
PlaceTrack has been available
on the App Store since 09/2010
Nico Tranquilli
nico@tranquilli.org
www.tranquilli.org
Nico Tranquilli • July 2010 “iOS4 multitasking development notes” www.tranquilli.org