iOS 7 NSURLSession在后台下载多个文件

Question

我想使用NSUrlSession下载文件列表.

我有一个用于计算成功下载量的变量@property (nonatomic) int downloadsSuccessfulCounter;.在下载文件时我禁用了Download Button.当计数器等于下载列表大小时,我再次启用该按钮并将计数器设置为0.我在方法中执行此操作:

-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {

...

    [[NSOperationQueue mainQueue] addOperationWithBlock:^ {

        downloadsSuccessfulCounter++;

        if(downloadsSuccessfulCounter == self.downloadList.count) {
            NSLog(@"All downloads finished");

            [self.syncButton setEnabled:YES];

             downloadsSuccessfulCounter = 0;
        }
    }];

}

一切都运行正常,但当我再次打开ViewController时,我得到了消息A background URLSession with identifier com.myApp already exists!.计数器未设置为0,UI元素(UIButtons,UILabels)没有响应.

我想问题是因为NSURLSession仍然是开放的,但我不确定它是如何工作的.

我已经尝试了所有的教程,但其中99%仅用于下载1个文件,不超过1个...任何想法？

这是我的代码:

...    
@property (nonatomic, strong) NSURLSession *session;
...

    - (void)viewDidLoad {
        [super viewDidLoad];

        appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];

        self.downloadList = [[NSMutableArray alloc] init];

        NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration backgroundSessionConfiguration:@"com.myApp"];
        sessionConfiguration.HTTPMaximumConnectionsPerHost = 5;
        self.session = [NSURLSession sessionWithConfiguration:sessionConfiguration delegate:self delegateQueue:nil];
}

当我按下Download Button我调用此方法时(我有一个Downloadable包含a 的对象NSURLSessionDownloadTask):

-(void)startDownload {

    for (int i=0; i<[self.downloadList count]; i++) {
        Downloadable *d = [self.downloadList objectAtIndex:i];

        if (!d.isDownloading) {
            if (d.taskIdentifier == -1) {
                d.downloadTask = [self.session downloadTaskWithURL:[NSURL URLWithString:d.downloadSource]];

            }else {
                d.downloadTask = [self.session downloadTaskWithResumeData:fdi.taskResumeData];
            }

            d.taskIdentifier = d.downloadTask.taskIdentifier;
            [d.downloadTask resume];
            d.isDownloading = YES;
        }
    }
}

当应用程序处于后台时:

-(void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session{
    AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;

    [self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {

        if ([downloadTasks count] == 0) {
            if (appDelegate.backgroundTransferCompletionHandler != nil) {

                void(^completionHandler)() = appDelegate.backgroundTransferCompletionHandler;

                appDelegate.backgroundTransferCompletionHandler = nil;

                [[NSOperationQueue mainQueue] addOperationWithBlock:^{               
                    completionHandler();

                    UILocalNotification *localNotification = [[UILocalNotification alloc] init];
                    localNotification.alertBody = @"All files downloaded";
                    [[UIApplication sharedApplication] presentLocalNotificationNow:localNotification];

                }];
            }
        }
    }];
}

Answer 1

因此，正如我在评论中提到的，问题是每个文件都需要一个唯一的 NSURLSession，并且每个 NSURLSession 需要一个具有唯一标识符的 NSURLSessionConfiguration。

我认为你很接近 - 并且在某些方面可能比我更合适......你只需要创建一个结构来将唯一的 ID 传递到唯一的配置中，以填充唯一的会话（比如说快 10 倍）。

这就是我所做的：

/* * 检索要下载的文件列表 * 还使用该列表的大小来实例化项目 * 在我的例子中，我加载一个字符返回的文本文件，其中包含我要下载的文件的名称 */

- (void) getMediaList {

    NSString *list = @"http://myserver/media_list.txt";
    NSURLSession *session = [NSURLSession sharedSession]; // <-- BASIC session
    [[session dataTaskWithURL:[NSURL URLWithString:list]
            completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {

                NSString *stringFromData = [[NSString alloc] initWithData: data encoding: NSUTF8StringEncoding];

                // Populate Arrays
                REMOTE_MEDIA_FILE_PATHS = [stringFromData componentsSeparatedByString:@"\n"];
                [self instantiateURLSessions:[REMOTE_MEDIA_FILE_PATHS count]]; 


                // Start First File
                [self getFile:[REMOTE_MEDIA_FILE_PATHS objectAtIndex:downloadCounter]:downloadCounter]; // this variable is 0 at the start
            }]
     resume];
}

/* * 这会将配置和会话数组设置为适当的大小 * 它还为每个数组提供唯一的 ID */

- (void) instantiateURLSessions : (int) size {

    NSMutableArray *configurations = [NSMutableArray array];
    NSMutableArray *sessions = [NSMutableArray array];

    for (int i = 0; i < size; i++) {
        NSString *index = [NSString stringWithFormat:@"%i", i];
        NSString *UniqueIdentifier = @"MyAppBackgroundSessionIdentifier_";
        UniqueIdentifier = [UniqueIdentifier stringByAppendingString:index];

        [configurations addObject: [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:UniqueIdentifier]];
        [sessions addObject:[NSURLSession sessionWithConfiguration: [configurations objectAtIndex:i]  delegate: self delegateQueue: [NSOperationQueue mainQueue]]];
    }

    NSURL_BACKGROUND_CONFIGURATIONS = [NSArray arrayWithArray:configurations];
    NSURL_BACKGROUND_SESSIONS = [NSArray arrayWithArray:sessions];
}

/* * 这根据数组的索引为每个文件设置下载任务 * 它还连接到实际文件的路径 */

- (void) getFile : (NSString*) file :(int) index {
    NSString *fullPathToFile = REMOTE_MEDIA_PATH; // Path To Server With Files
    fullPathToFile = [fullPathToFile stringByAppendingString:file];

    NSURL *url = [NSURL URLWithString:fullPathToFile];
    NSURLSessionDownloadTask *downloadTask = [[NSURL_BACKGROUND_SESSIONS objectAtIndex:index ] downloadTaskWithURL: url];
    [downloadTask resume];

}

/* * 最后，在我的委托方法中，下载完成后（将文件从临时数据中移出后），我检查是否已完成，如果没有，则使用更新后的索引计数器再次调用 getFiles 方法 * /

-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location
{

    // Get the documents directory URL
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *dataPath = [documentsDirectory stringByAppendingPathComponent:LOCAL_MEDIA_PATH];
    NSURL *customDirectory = [NSURL fileURLWithPath:dataPath];

    // Get the file name and create a destination URL
    NSString *sendingFileName = [downloadTask.originalRequest.URL lastPathComponent];
    NSURL *destinationUrl = [customDirectory URLByAppendingPathComponent:sendingFileName];

    // Move the file
    NSError *error = nil;
    NSFileManager *fileManager = [NSFileManager defaultManager];
    if ([fileManager moveItemAtURL:location toURL:destinationUrl error: &error]) {

        // List
        [self listCustomDirectory];

        if(downloadCounter < [REMOTE_MEDIA_FILE_PATHS count] -1) {
            // Increment Counter
            downloadCounter++;

            // Start Next File
            [self getFile:[REMOTE_MEDIA_FILE_PATHS objectAtIndex:downloadCounter]:downloadCounter];
        }
        else {
            // FINISH YOUR OPERATION / NOTIFY USER / ETC
        }
    }
    else {
        NSLog(@"Damn. Error %@", error);
        // Do Something Intelligent Here
    }  
}