在iPhone上执行后台任务

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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: ^
            {
                [selfrequestServerHowManyUnreadMessages];
            }
            ]
           )
        {
            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 = [NSDatedateWithTimeIntervalSinceNow:15];
            alarm.timeZone = [NSTimeZonedefaultTimeZone];
            alarm.repeatInterval = 0;
            alarm.soundName = UILocalNotificationDefaultSoundName;
            alarm.alertBody = @"Time to request MOA2 Server!";
            [app scheduleLocalNotification:alarm];
        }
    }
    elseif([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。

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