ios - 从前台 NSURLSession 移动到后台 NSURLSession - 处理进程中的任务

Question

我正在尝试在应用程序进入后台(例如从主页按钮按下)的情况下正确处理进程内 NSURLSessionTasks。我目前正在采用将进程中的任务复制到后台队列的方法(请参见下面的代码)。然而，我发现后台任务表现不稳定，并不总是完成。任何人都可以发现我可能做错了什么/建议最好的方法吗？

- (void)appWillResignActive : (NSNotification *)notification {
    UIApplication *app = [UIApplication sharedApplication];

    // Register expiring background task
    __block UIBackgroundTaskIdentifier bgTaskId =
    [app beginBackgroundTaskWithExpirationHandler:^{
        bgTaskId = UIBackgroundTaskInvalid;
    }];

    [self switchToBackground];
    [app endBackgroundTask:bgTaskId];

}

- (void)appWillBecomeActive : (NSNotification *)notification {
    [self switchToForeground];
}

- (void)switchToBackground
{
    NSLog(@"Switch to background line 217 Network Manager");
    if ([state isEqualToString: kdownloadManagerStateForeground]) {
        [urlSession getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
            for (NSURLSessionDownloadTask *downloadTask in downloadTasks) {
                [downloadTask cancelByProducingResumeData:^(NSData *resumeData) {
                    NSURLSessionDownloadTask *downloadTask = [self.backgroundSession downloadTaskWithResumeData:resumeData];
                    [downloadTask resume];
                }];
            }
        }];

        state = kdownloadManagerStateBackground;
    }
}

- (void)switchToForeground
{
    if ([state isEqualToString: kdownloadManagerStateBackground]) {
        [backgroundSession getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
            for (NSURLSessionDownloadTask *downloadTask in downloadTasks) {
                [downloadTask cancelByProducingResumeData:^(NSData *resumeData) {
                    NSURLSessionDownloadTask *downloadTask = [self.urlSession downloadTaskWithResumeData:resumeData];
                    [downloadTask resume];
                }];
            }
        }];

        state = kdownloadManagerStateForeground;
    }
}

Answer 1

Background sessions let you perform uploads and downloads of content in the background while your app is not running. You can create a background session configuration by calling the backgroundSessionConfiguration: method on the NSURLSessionConfiguration class.

NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL];
NSURLSessionConfiguration *sessionConfig;
float timeout = 5 * 60.0f;

BOOL iOS8OrNew = [[[UIDevice currentDevice] systemVersion] floatValue] >= 8.0;
if (iOS8OrNew) {
   sessionConfig = [NSURLSessionConfiguration       backgroundSessionConfigurationWithIdentifier:identifier];
   request.timeoutInterval = timeout;
}
else {
   sessionConfig = [NSURLSessionConfiguration        backgroundSessionConfiguration:identifier];
  sessionConfig.timeoutIntervalForRequest = timeout;
}

sessionConfig.HTTPMaximumConnectionsPerHost = 10;
AFURLSessionManager *manager = [[AFURLSessionManager alloc] initWithSessionConfiguration:sessionConfig];

NSURLSessionDownloadTask *downloadTask = [manager downloadTaskWithRequest:request];


[manager setDidFinishEventsForBackgroundURLSessionBlock:^(NSURLSession * _Nonnull session) {
AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
if (appDelegate.backgroundSessionCompletionHandler) {
    void (^completionHandler)() = appDelegate.backgroundSessionCompletionHandler;
    appDelegate.backgroundSessionCompletionHandler = nil;
    completionHandler();
}
NSLog(@"All tasks are finished");
}];

有关更多信息，请参阅我的回答:How to keep downloading new images in background even if user force quits the app in iOS objective C?