iPhone的后台执行任务

iPhone的后台执行任务

当App进入到后台时，可以有一段时间做处理工作。

或者，对于某些服务，可以长时间运行，比如播放音乐。

对于长时间运行的任务，需要在Info.plist添加一行，键为UIBackgroundModes，值为一个数组，可以包含如下几个字符串：

audio
location
voip
newsstand-content
external-accessory
bluetooth-central



然后，在后台就可以做一些事情了：

[cpp] view
plaincopy

- (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.

UIDevice *device = [UIDevicecurrentDevice];

BOOL backgroundSupported = NO;

if ([device respondsToSelector:@selector(isMultitaskingSupported)]) {

backgroundSupported = YES;

}

__blockUIBackgroundTaskIdentifier bgTaskId = [application beginBackgroundTaskWithExpirationHandler:^{

[application endBackgroundTask:bgTaskId];

bgTaskId = UIBackgroundTaskInvalid;

}];

if (backgroundSupported) {

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0),^{

//

});

}

}

步骤：

1.在info.plist里加入UIBackgroundModes键，其值为数组，数组之一为voip字符串：

<key>UIBackgroundModes</key><array><string>voip</string></array>

2.在程序启动的时候调用- (void)setupBackgroundHandler函数，函数体如下：

#pragma mark - VoIP - ( void )setupBackgroundHandler { if ( UIUDeviceIsBackgroundSupported() ) { if ( [[UIApplication sharedApplication] setKeepAliveTimeout:600 handler: ^ { [ self requestServerHowManyUnreadMessages]; } ] ) { UDLog(@ "Set Background handler successed!" ); } else { //failed UDLog(@ "Set Background handler failed!" ); } } else { UDLog(@ "This Deviece is not Background supported." ); } } - ( void )requestServerHowManyUnreadMessages { UIApplication* app = [UIApplication sharedApplication]; if ([app applicationState] == UIApplicationStateBackground) { NSArray * oldNotifications = [app scheduledLocalNotifications]; if ([oldNotifications count] > 0) [app cancelAllLocalNotifications]; UILocalNotification* alarm = [[[UILocalNotification alloc] init] autorelease]; if (alarm) { alarm.fireDate = [ NSDate dateWithTimeIntervalSinceNow:15]; alarm.timeZone = [ NSTimeZone defaultTimeZone]; alarm.repeatInterval = 0; alarm.soundName = UILocalNotificationDefaultSoundName; alarm.alertBody = @ "Time to request MOA2 Server!" ; [app scheduleLocalNotification:alarm]; } } else if ([app applicationState] == UIApplicationStateActive) { UIAlertView *alertView = [[[UIAlertView alloc] init] autorelease]; [alertView setTitle:@ "alert" ]; [alertView setMessage:@ "Time to request MOA2 Server!" ]; [alertView addButtonWithTitle: NSLocalizedString (@ "cancel" , nil )]; [alertView setDelegate: nil ]; [alertView show]; } }

解说：

- (BOOL)setKeepAliveTimeout:(NSTimeInterval)timeout handler:(void (^)(void))keepAliveHandler

函数功能：app每隔timeout唤醒一次。

0.要成功调用该函数，就必须在Info.plist里设UIBackgroundModes键的array值之一voip字符串.

1.timeout必须>=600

2.唤醒app的时间间隔是不精准的。

3.唤醒后只有10秒执行时间。即handler里的代码要在10秒类执行完。10秒后app再次被阻塞。

（可以用-backgroundTimeRemaining属性来返回剩余时间）

4.该函数成功调用后，在程序生命周期内有效。

该函数的效果在回到前台的状况下,依然有效。（因此可以把它当timer使.）

5.clearKeepAliveTimeout函数用来清除handler。