Multitasking and Background Execution

Transcription

1 Multitasking and Background Execution Fall, 2012 Prof. Massimiliano "Max" Pala

2 Introduction Spawning Threads and Tasks Background Execution User hits 'home' button the app is sent in the background Can continue to execute tasks, but no interaction with the user When the application is sent to the background and then to the foreground again, several delegate (UIApplicationDelegate) messages applicationdidenterbackground: applicationwillenterforeground: Several Messages are sent to the application as well UIApplicationDidEnterBackgroundNotification UIApplicationWillEnterForegroundNotification You can use the default notification center to register for these notifications

3 Multitasking Availability Not all the devices are capable of multitasking ios not updated, hardware not powerful enough To correctly handle the lack of multitasking capabilities you need to check for it Send the ismultitaskingsupported message to the UIDevice - (BOOL) ismultitaskingsupported { BOOL ret = NO; if ([[ UIDevice currentdevice] respondstoselector:@selector(ismultitaskingsupported:)]) { ret = [[UIDevice currentdevice] ismultitaskingsupported]; return ret;

4 Multitasking Availability (cont.) - (BOOL)applicationDidFinishLaunchingWithOptions: (NSDictionary *) launchoptions { self.mutitaskingok = [self ismultitaskingsupported]; self.window = [[UIWindow alloc] initwithframe:. if (self.multitaskingok == YES) { // Do something with it..

5 Blocks dispatch_queue_t queue = dispatch_get_global_queue(dispatch_queue_priority_default,0); dispatch_async(queue, ^(void) { ) Blocks are first class objects retain the value of the variables in the scope copy Do not need to setup NSInteger (^subtract)(nsinteger val, NSInteger from) = ^(NSInteger a, NSInteger b) { return a - b; ; subtract(5,2);

6 Shortcut for threads creation Problem: you want to create threads easily Solution: use the performselectorinbackgroundwithobjects instance method of NSObject [self ) withobject:nil]; -(void) firstcounter { NSUInteger counter = 0; for ( counter = 0; counter < 1000; counter++) { NSLog(@"Counter = %lu", (unsigned long)counter);

7 Executing tasks with GCD Grand Central Dispatch (CGD) is a low-level C API that works with block objects At the heart of CGD are dispatch queues run tasks sync, async, after a certain delay, etc.. All methods available in GCD start with dispatch_ keyword dispatch_async, dispatch_async_f, dispatch_after, etc. Different Queues: Main queue - main thread for UI operations Concurrent queues - use this instead of threads Serial queues - operate in FIFO mode (not on the main thread, though)

8 Long-running tasks in Background Application is sent to the background Still needs to finish a task Solution: borrow some time from ios to complete the task UIApplication has two useful instance method beginbackgroundtaskwithexpirationhandler: endbackgroundtask: The backgroundtimeremaining property of UIApplication contains the number of seconds the app has to finish the job DBL_MAX - when the app is in the foreground or fully suspended You can call the method as many times as you need each call must be followed by endbackgroundtask: failure to do so, may cause your app to be terminated! Background tasks should be short / small

9 Long-running tasks in Background Cleaning up tasks 1. End any threads or timers 2. End the background task by calling endbackgroundtask: method of UIApplication 3. Mark our task as ended Assign UIBackgroundTaskInvalid to the task identifiers One more thing to do When the app is put in the foreground again, we need to ensure that we get rid of the background task

10 Example #import mybackgroundappdelegate : UIResponder (nonatomic, strong) UIWindow (nonatomic, unsafe_unretained) UIBackgroundTaskIdentifier (nonatomic, strong) NSTimer #import mytimer; -(void) applicationdidenterbackground:(uiapplication *)application { if ([self ismultitaskingsupported] == NO) return; self.mytimer = [NSTimer scheduledtimerwithtimeinterval:1.0f target:self selector:@selector(timermethod:) userinfo:nil repeats:yes]; self.backtaskid = [application beginbackgroundtaskwithexpirationhandler:^(void) { [self endbackgroundtask]; ];

11 Example (cont.) -(void) timermethod:(nstimer *)sender { NSTimeInterval timeremaining = [[UIApplication sharedapplication] backgroundtimeremaining]; if (timeremaining == DBL_MAX) { NSLog(@"Background time.. undefined!"); else { NSLog(@"Remaining time = %.02f Seconds", timeremaining); -(void) endbackgroundtask { dispatch_queue_t mainqueue = dispatch_get_main_queue(); _weak mybackgroundappdelegate *weakself = self; dispatch_async(mainqueue, ^(void) { mybackgroundappdelegate *strongself = weakself; if (strongself!= nil) { [strongself.mytimer invalidate]; [[UIApplication sharedapplication] endbackgroundtask:self.backtaskid]; strongself.backtaskid = UIBackgroundTaskInvalid; );

12 Notifications in Background Problem: presenting an alert even when the application is not running Solution Instantiate a UILocalNotification object Schedule it to be delivered by using the schedulelocalnotification: instance method of UIApplication Use the cancelalllocalnotifications instance method of UIApplication to cancel the delivery of all pending local notifications If the application is running, then no notification is give to the user Pay attention to the locale - changes in the locale could affect the time, and, therefore, the delivery of your local notifications

13 Example #import myappdelegate : UIResponder (nonatomic, strong) UIWindow #import -(BOOL) applicationdidfinishlaunchingwithoptions:(nsdictionary *)opts { id schedulelocalnotification = [opts valueforkey:uiapplicationlaunchoptionslocalnotificationkey]; if (scheduledlocalnotification!= nil) { // Here our app is not running, but we received the local notification NSString *message Notification Woke Me Up!"; [[UIAlertView alloc] initwithtitle:@"notification" message:message delegate:nil cancelbuttontitle:@"ok" otherbuttontitles:nil] show]; else { (continue.)

14 Example else { // Here, our app was not awaken by a local notification // Let's schedule one! NSString *message new instant message is available. Would you like to read it?"; [self localnotificationwithmessage:message actionbuttontitle:@"yes" launchimage:nil applicationbadge:1 secondsfromnow:10.0f userinfo:nil]; message new local notification in 10 seconds!"; [[[UIAlertView alloc] initwithtitle:@"set Up" message:message delegate:nil cancelbuttontitle:@"ok" otherbuttontitles:nil] show]; return YES; -(void) didreceivelocalnotification:(uilocalnotification *)notification { NSString *message notification: delivered!"; [[[UIAlertView alloc] initwithtitle:@"local Notification" message:message delegate:nil cancelbuttontitle:@"ok" ] show];

15 Background Tasks Problem: you want the application to be able to continue to play audio files even when it's in the background Solution: edit the.plist file create a new key named UIBackgroundModes add the value 'audio' to it Now you can use the AV Foundation to play audio files also when the app is in the background <dict> <key> UIBackgroundModes</key> <array> <string> audio</string> </array> </dict>

16 Example #import <UIKit/UIKit.h> #import myappdelegate : UIResponder <UIApplicationDelegate, (nonatomic, strong) UIWindow (nonatomic, strong) AVAudioPlayer #import player; -(void) didfinishlaunchingwithoptions:(nsdictionary *)ops { dispatch_queue_t dispatchqueue = dispatch_get_global_queue(dispatch_queue_priority_default,0); dispatch_async(dispatchqueue, ^(void) { NSError *audiosessionerr = nil; AVAudioSession avsession = [AVAudioSession sharedinstance]; if ([audiosession setcategory:avaudiosessioncategoryplayback error:&audiosessionerr]) NSLog(@"Session is available"); (continue)

17 Example (cont.) NSBundle *mainbundle = [NSBundle mainbundle]; NSString *filepath = [mainbundle pathforresource:@"mysong" oftype:@"mp3"]; NSData *filedata = [NSData datawithcontentsoffile:filepath]; NSError *err = nil; ); self.audioplayer = [[AVAudioPlayer alloc] initwithdata:filedata error:&err]; if (self.audioplayer!= nil) { // Set the delegate self.audioplayer.delegate = self; // Start Playing if ([self.audioplayer preparetoplay] && [self.audioplayer play]) NSLog(@"Playing"); self.window = [[UIWindow ] ]; return YES;

18 Background Tasks 2 Problem: you want the application to be able to continue to use the location Solution: edit the.plist file create a new key named UIBackgroundModes add the value 'location' to it Now you can use the CLLocationManager to receive updates for the location even when the application is in background <dict> <key> UIBackgroundModes</key> <array> <string> location</string> </array> </dict>

19 Example #import <UIKit/UIKit.h> #import myappdelegate : UIResponder <UIApplicationDelegate, (nonatomic, strong) UIWindow (nonatomic, strong) CLLocationManager (nonatomic, unsafe_unretained, getter=isexecutinginbackground) BOOL #import executinginbackground; -(BOOL) isexecutinginbackground { return executinginbackground;

20 Example (cont.) -(void) didfinishlaunchingwithoptions:(nsdictionary *) opts { self.locmanager = [[CLLocationManager alloc] init]; self.locmanager.desiredaccuracy = kclllocationaccuracybest; self.locmanager.delegate = self; [self.locmanager startupdatinglocation]; self.window = [[UIWindow ] ]; return YES; -(void)applicationdidenterbackground:(uiapplication *)application{ self.executinginbackground = YES; // Let's reduce the accuracy while in the background self.locmanager.desiredaccuracy = kcllocationaccuracyhundredmeters; -(void)applicationwillenterforeground(uiapplication *)application { self.executinginbackground = NO; self.locmanager.desiredaccuracy = kcllocationaccuracybest;

21 Threads ios introduced access to multi threading for normal Apps in ios 4.0+ Operations needed for multi threading: A method to execute (eg., dosomething) An helper method where you create an NSOperationQueue Create a new NSInvocationOperation Add the operation to the queue Create a new autorelease pool in the dosomething: method (each thread needs its own) Use locks to access data shared among threads by to guarantee access to a block of code by a single thread

22 Threads myclass : NSObject { NSInvocationOperation *op; NSOperationQueue *queue; -(void) newthread: (id)data; myclass -(void) newthread:(id)data { queue = [[NSOperationQueue alloc] init]; op = [[[NSInvocationOperation alloc] initwithtarget:self selector:@selector(compute:) object:data] autorelease]; [queue addoperation: op];

23 Threads (cont.) -(void) compute: (id) data { NSAutoreleasePool *threadpool = [[NSAutoreleasePool alloc] init]; // Your { // Store the result in myresultobject.. [threadpool release]; -(void) submitbuttonclicked { [myobject newthread:data]; // Continue to be responsive to the UI..