如果任何后台任务运行时间超过 10 分钟,则该任务将被挂起,并调用 beginBackgroundTaskWithExpirationHandler 指定的代码块来清理该任务。后台剩余时间可以通过 [[UIApplication sharedApplication] backgroundTimeRemaining] 检查。最初,当应用程序处于前台时,backgroundTimeRemaining 设置为更大的值。当应用程序进入后台时,您可以看到 backgroundTimeRemaining 值从 599.XXX(1o 分钟)减少。一旦 backgroundTimeRemaining 变为零,后台任务将被挂起。
//1)Creating iOS Background Task
__block UIBackgroundTaskIdentifier background_task;
background_task = [application beginBackgroundTaskWithExpirationHandler:^ {
//This code block is execute when the application’s
//remaining background time reaches ZERO.
}];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//### background task starts
//#### background task ends
});
//2)Making background task Asynchronous
if([[UIDevice currentDevice] respondsToSelector:@selector(isMultitaskingSupported)])
{
NSLog(@"Multitasking Supported");
__block UIBackgroundTaskIdentifier background_task;
background_task = [application beginBackgroundTaskWithExpirationHandler:^ {
//Clean up code. Tell the system that we are done.
[application endBackgroundTask: background_task];
background_task = UIBackgroundTaskInvalid;
}];
**//Putting All together**
//To make the code block asynchronous
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//### background task starts
NSLog(@"Running in the background\n");
while(TRUE)
{
NSLog(@"Background time Remaining: %f",[[UIApplication sharedApplication] backgroundTimeRemaining]);
[NSThread sleepForTimeInterval:1]; //wait for 1 sec
}
//#### background task ends
//Clean up code. Tell the system that we are done.
[application endBackgroundTask: background_task];
background_task = UIBackgroundTaskInvalid;
});
}
else
{
NSLog(@"Multitasking Not Supported");
}