【iOS开发】在程序被送入后台时，开启一个长期任务（voip）。

#pragma mark - 程序进入后台 << 开启socket后台长连接 voip(网络电话) >>
- (void)keepLongConnectionOfSocketWhenApplicationDidEnterBackground:(UIApplication *)application
{
// 允许后台开启一个长期任务，记得配置voip(网络电话)
__block UIBackgroundTaskIdentifier backgroundTaskIdentifier;
backgroundTaskIdentifier = [application beginBackgroundTaskWithExpirationHandler:^{
dispatch_async(dispatch_get_main_queue(), ^{

if (backgroundTaskIdentifier != UIBackgroundTaskInvalid)
{
backgroundTaskIdentifier = UIBackgroundTaskInvalid;
}
});
}];

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
dispatch_async(dispatch_get_main_queue(), ^{

if (backgroundTaskIdentifier != UIBackgroundTaskInvalid)
{
backgroundTaskIdentifier = UIBackgroundTaskInvalid;
}
});
});

NSLog(@"后台backgroundTaskIdentifier--------  %lu",(unsigned long)backgroundTaskIdentifier);

}