iOS7中的多任务II

1 aps {  
2      content-available: 1
3      alert: {...}
4    }

 1 - (NSURLSession *)backgroundSession
 2 {
 3     //Use dispatch_once_t to create only one background session. If you want more than one session, do with different identifier
 4     static NSURLSession *session = nil;
 5     static dispatch_once_t onceToken;
 6     dispatch_once(&onceToken, ^{
 7         NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfiguration:@"com.yourcompany.appId.BackgroundSession"];
 8         session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
 9     });
10     return session;
11 }

 1 //@property (nonatomic) NSURLSession *session;
 2 //@property (nonatomic) NSURLSessionDownloadTask *downloadTask;
 3 
 4 - (NSURLSession *)backgroundSession
 5 {
 6     //...
 7 }
 8 
 9 - (void) beginDownload
10 {
11     NSURL *downloadURL = [NSURL URLWithString:DownloadURLString];
12     NSURLRequest *request = [NSURLRequest requestWithURL:downloadURL];
13     self.session = [self backgroundSession];
14     self.downloadTask = [self.session downloadTaskWithRequest:request];
15     [self.downloadTask resume];
16 }

 1 //AppDelegate.m
 2 - (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier
 3   completionHandler:(void (^)())completionHandler
 4 {
 5     //Check if all transfers are done, and update UI
 6     //Then tell system background transfer over, so it can take new snapshot to show in App Switcher
 7     completionHandler();
 8 
 9     //You can also pop up a local notification to remind the user
10     //...
11 }