- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    [application setMinimumBackgroundFetchInterval:UIApplicationBackgroundFetchIntervalMinimum];
    NSLog(@"Launched in background %d",UIApplicationStateBackground == application.applicationState);
    return YES;
}
// 1.利用Background Fetch
- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler {
    NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration defaultSessionConfiguration];
    NSURLSession *session = [NSURLSession sessionWithConfiguration:sessionConfiguration];
    NSURL *url = [[NSURL alloc] initWithString:@"http://127.0.0.1/data.json"];
    NSURLSessionDataTask *task = [session  dataTaskWithURL:url
                                         completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
                                             if (error) {
                                                 completionHandler(UIBackgroundFetchResultFailed);
                                                 return;
                                             }
                                             completionHandler(UIBackgroundFetchResultNewData);
                                         }];
    [task resume];
}
// 2.利用Remote Notification
/*
 推送的内容
 {
 "aps" : {
 "content-available" : 1
 },
 "content-id" : 42
 }
 */
- (void)application:(UIApplication *)application
didReceiveRemoteNotification:(NSDictionary *)userInfo
fetchCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
    NSLog(@"Received remote notification with userInfo %@", userInfo);
    
    NSNumber *contentID = userInfo[@"content-id"];
    NSString *downloadURLString = [NSString stringWithFormat:@"http://yourserver.com/downloads/%d.mp3", [contentID intValue]];
    NSURL* downloadURL = [NSURL URLWithString:downloadURLString];
    
    NSURLRequest *request = [NSURLRequest requestWithURL:downloadURL];
    NSURLSessionDownloadTask *task = [[self backgroundURLSession] downloadTaskWithRequest:request];
    task.taskDescription = [NSString stringWithFormat:@"Podcast Episode %d", [contentID intValue]];
    [task resume];
    completionHandler(UIBackgroundFetchResultNewData);
}

- (NSURLSession *)backgroundURLSession
{
    static NSURLSession *session = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSString *identifier = @"io.objc.backgroundTransferExample";
        NSURLSessionConfiguration* sessionConfig = [NSURLSessionConfiguration backgroundSessionConfiguration:identifier];
        session = [NSURLSession sessionWithConfiguration:sessionConfig
                                                delegate:self
                                           delegateQueue:[NSOperationQueue mainQueue]];
    });
    return session;
}