IOS 後臺傳輸

創建後臺會話

- (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 backgroundSessionConfigurationWithIdentifier:@"com.example.apple-samplecode.SimpleBackgroundTransfer.BackgroundSession"];
		session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
	});
	return session;
}

創建後臺下載任務並啓動

NSURL *downloadURL = [NSURL URLWithString:DownloadURLString];
	NSURLRequest *request = [NSURLRequest requestWithURL:downloadURL];
	self.downloadTask = [self.session downloadTaskWithRequest:request];
    [self.downloadTask resume];

處理下載進度

- (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;
        });
    }
}

處理下載任務完成回調

- (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]);
    }
}

處理下載任務失敗回調

- (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;
}

處理下載任務由於網絡原因失敗後被系統恢復的回調

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

處理後臺任務事件

當APP不在運行時，因爲後臺下載任務是重新開啓一個進程處理，所以當任務完成或者請求證書驗證，系統會重新啓動並進入後臺處理後臺會話事件。在這裏，你應該保存completionHandler以便在處理完事件時告訴系統重新進入暫停狀態。你還可以使用後臺會話標識來重新創建後臺會話，這個會話會關聯之前的會話進程。

- (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier
  completionHandler:(void (^)(void))completionHandler
{
    BLog();
    /*
     Store the completion handler. The completion handler is invoked by the view controller's checkForAllDownloadsHavingCompleted method (if all the download tasks have been completed).
     */
	self.backgroundSessionCompletionHandler = completionHandler;
}

注意：後臺會話設置（NSURLSessionConfiguration）不能夠重用，因爲共用一個標識的後臺會話可能會出現不確定的問題。

處理完成後臺任務事件的回調

在完成相關後臺任務事件，例如，下載完成或者請求證書驗證。需要執行之前保存的completionHandler來指示系統可以重新進入暫停狀態。

/*
 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
{
    dispatch_async(dispatch_get_main_queue(), ^{
        APLAppDelegate *appDelegate = (APLAppDelegate *)[[UIApplication sharedApplication] delegate];
        if (appDelegate.backgroundSessionCompletionHandler) {
            void (^completionHandler)() = appDelegate.backgroundSessionCompletionHandler;
            appDelegate.backgroundSessionCompletionHandler = nil;
            completionHandler();
        }
        
        NSLog(@"All tasks are finished");
    });
}

注意：completionHandler必須在主線程執行，如果你要更新UI界面，必須在它之前更新。