don'；如果我关闭应用程序，则无法再次下载视频，如果它'；在iOS之前已经下载了

我正在开发一个应用程序，需要下载一段视频，然后对其进行编辑。因此，我正在使用NSURLSession下载一段视频，下载后，我关闭应用程序，然后再次打开。现在，我不再使用下载的视频，我需要检查它是否已经下载，然后获取URL。以下是我一直使用的代码：

    -(void)startDownloadingWithURLString:(NSString*)urlString{

    NSURL *url = [NSURL URLWithString:urlString];

  configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
  self.urlSession = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:[NSOperationQueue mainQueue]];

  task = [self.urlSession downloadTaskWithURL:url];
  [task resume];
}

#pragma mark - NSURLSession delagate methods

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

NSString *path = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];

NSFileManager *fileManager_ = [NSFileManager defaultManager];

NSURL *url = [NSURL URLWithString:[path stringByAppendingPathComponent:@"video.mp4"]];

if([fileManager_ fileExistsAtPath:[location path]]){

    [fileManager_ replaceItemAtURL:url withItemAtURL:location backupItemName:nil options:NSFileManagerItemReplacementUsingNewMetadataOnly resultingItemURL:nil error:nil];
    _videoURLPath = url;
}
UISaveVideoAtPathToSavedPhotosAlbum([url path
                                     ], nil, nil, nil);
dispatch_async(dispatch_get_main_queue(), ^{
self.progressView_.progress = 0.0;

});
}


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

    NSLog(@"%f",(double)totalBytesWritten/(double)totalBytesExpectedToWrite);
dispatch_async(dispatch_get_main_queue(), ^{
    self.progressView_.progress = (double)totalBytesWritten/(double)totalBytesExpectedToWrite;

});
}


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

[KSToastView ks_showToast:@"Download complete"];
NSData *data = [NSData dataWithContentsOfURL:[NSURL fileURLWithPath:_videoURLPath.path]];

[AppHelper saveToUserDefaults:_videoURLPath.path withKey:@"videoURL"];

dispatch_async(dispatch_get_main_queue(), ^{

    SAEditUploadViewController *editVC = [self.storyboard instantiateViewControllerWithIdentifier:@"SAEditUploadViewController"];

    editVC.videoPathData = data;

    [self.navigationController pushViewController:editVC animated:YES];

});
}