Downloading images when the app is in idle state usingNSURLSession iOS

Question

I have a requirement to download some of the large images to the app while the app is in idle state.I am planing to make it done using NSURLSession.Tutorials and the sample code available worked and confirmed the background download is possible.Is this the best method for my requirement?Also what happens if the app is removed from the background when the download is not completed and only several bytes are got.Can i resume the download from where it stopped?Again can i use this in iOS6?These are the delegate methods i am using.

- (NSURLSession *)backgroundSession
{
/*
 Using disptach_once here ensures that multiple background sessions with the same identifier are not created in this instance of the application. If you want to support multiple background sessions within a single process, you should create each session with its own identifier.
 */
    static NSURLSession *session = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfiguration:@"com.example.apple-samplecode.SimpleBackgroundTransfer.BackgroundSession"];
        session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
    });
    return session;
}


- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
    BLog();

    /*
     Report progress on the task.
     If you created more than one task, you might keep references to them and report on them individually.
     */

    if (downloadTask == self.downloadTask)
    {
        double progress = (double)totalBytesWritten / (double)totalBytesExpectedToWrite;
        BLog(@"DownloadTask: %@ progress: %lf", downloadTask, progress);
        dispatch_async(dispatch_get_main_queue(), ^{
            self.progressView.progress = progress;
        });
    }
}


- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)downloadURL
{
    BLog();

    /*
     The download completed, you need to copy the file at targetPath before the end of this block.
     As an example, copy the file to the Documents directory of your app.
    */
    NSFileManager *fileManager = [NSFileManager defaultManager];

    NSArray *URLs = [fileManager URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask];
    NSURL *documentsDirectory = [URLs objectAtIndex:0];

    NSURL *originalURL = [[downloadTask originalRequest] URL];
    NSURL *destinationURL = [documentsDirectory URLByAppendingPathComponent:[originalURL lastPathComponent]];
    NSError *errorCopy;

    // For the purposes of testing, remove any esisting file at the destination.
    [fileManager removeItemAtURL:destinationURL error:NULL];
    BOOL success = [fileManager copyItemAtURL:downloadURL toURL:destinationURL error:&errorCopy];

    if (success)
    {
        dispatch_async(dispatch_get_main_queue(), ^{
            UIImage *image = [UIImage imageWithContentsOfFile:[destinationURL path]];
            self.imageView.image = image;
            self.imageView.hidden = NO;
            self.progressView.hidden = YES;
        });
    }
    else
    {
        /*
         In the general case, what you might do in the event of failure depends on the error and the specifics of your application.
         */
        BLog(@"Error during the copy: %@", [errorCopy localizedDescription]);
    }
}


- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
{
    BLog();

    if (error == nil)
    {
        NSLog(@"Task: %@ completed successfully", task);
    }
    else
    {
        NSLog(@"Task: %@ completed with error: %@", task, [error localizedDescription]);
    }

    double progress = (double)task.countOfBytesReceived / (double)task.countOfBytesExpectedToReceive;
    dispatch_async(dispatch_get_main_queue(), ^{
        self.progressView.progress = progress;
    });

    self.downloadTask = nil;
}


/*
 If an application has received an -application:handleEventsForBackgroundURLSession:completionHandler: message, the session delegate will receive this message to indicate that all messages previously enqueued for this session have been delivered. At this time it is safe to invoke the previously stored completion handler, or to begin any internal updates that will result in invoking the completion handler.
 */
- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session
{
    APLAppDelegate *appDelegate = (APLAppDelegate *)[[UIApplication sharedApplication] delegate];
    if (appDelegate.backgroundSessionCompletionHandler) {
        void (^completionHandler)() = appDelegate.backgroundSessionCompletionHandler;
        appDelegate.backgroundSessionCompletionHandler = nil;
        completionHandler();
    }

    NSLog(@"All tasks are finished");
}


-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didResumeAtOffset:(int64_t)fileOffset expectedTotalBytes:(int64_t)expectedTotalBytes
{
    BLog();
}

Jacob Davis Cherussery · Answer 1 · 2014-06-12T10:30:59.567

NSURLSession API is available from ios7.

Resuming Data in NSURLSession-n some cases, you can resume a download that was canceled or that failed while in progress. To do so, first make sure your original download doesn’t delete its data upon failure by passing NO to the download’s setDeletesFileUponFailure: method. If the original download fails, you can obtain its data with the resumeData method. You can then initialize a new download with the initWithResumeData:delegate:path: method. When the download resumes, the download’s delegate receives the download:willResumeWithResponse:fromByte: message.

You can resume a download only if both the protocol of the connection and the MIME type of the file being downloaded support resuming. You can determine whether your file’s MIME type is supported with the canResumeDownloadDecodedWithEncodingMIMEType: method

If you schedule the download in a background session, the download continues when your app is not running. If you schedule the download in a standard or ephemeral session, the download must begin anew when your app is relaunched.

During the transfer from the server, if the user tells your app to pause the download, your app can cancel the task by calling the cancelByProducingResumeData: method. Later, your app can pass the returned resume data to either the downloadTaskWithResumeData: or downloadTaskWithResumeData:completionHandler: method to create a new download task that continues the download.

If the transfer fails, your delegate’s URLSession:task:didCompleteWithError: method is called with an NSError object. If the task is resumable, that object’s userInfo dictionary contains a value for the NSURLSessionDownloadTaskResumeData key. Your app should use reachability APIs to determine when to retry, and should then call downloadTaskWithResumeData: or downloadTaskWithResumeData:completionHandler: to create a new download task to continue that download.

Downloading images when the app is in idle state usingNSURLSession iOS

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