NSURLSessionDownloadTask - downloads but ends with

问题:

I am trying to download a pdf file. earlier when i used the completion handler block i was able to see the file in the tmp location. I then wanted to show download progress so i implemented the delegate methods. But i can now see the progress bar working and the file being downloaded. but once the download is complete (bytes written/total bytes = 1) the error delegate is called and there is no file in the tmp location. what am i missing ? below is my code. I have uploaded the project at https://www.dropbox.com/s/vn5zwfwx9izq60a/trydownload.zip

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
    NSURLSession *session = [NSURLSession sessionWithConfiguration:sessionConfig delegate:self delegateQueue:nil];
    NSURLSessionDownloadTask *downloadTask = [session downloadTaskWithURL:[NSURL URLWithString:@"http://aayudham.com/URLLoadingSystem.pdf"]];
    [downloadTask resume];

}

-(void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
{
    NSLog(@"%@",[error localizedDescription]);
}
-(void) URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
    dispatch_async(dispatch_get_main_queue(), ^{
        _progressBar.progress = (double)totalBytesWritten/(double)totalBytesExpectedToWrite;
        double value =(double)totalBytesWritten/(double)totalBytesExpectedToWrite;
        NSLog(@"%f",value);
    });
}

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

}

-(void) URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location
{
       NSError *error;
//getting docs dir path
NSArray * tempArray = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *docsDir = [tempArray objectAtIndex:0];

//adding folder path
NSString *appDir = [docsDir stringByAppendingPathComponent:@"/Reader/"];

NSFileManager *fileManager = [NSFileManager defaultManager];

if(![fileManager fileExistsAtPath:appDir])
{
    [fileManager createDirectoryAtPath:appDir withIntermediateDirectories:NO attributes:nil error:&error];
}


BOOL fileCopied = [fileManager moveItemAtPath:[location path] toPath:[appDir stringByAppendingString:@"/demo.pdf"] error:&error];

NSLog(fileCopied ? @"Yes" : @"No");
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

回答1:

@Rob thank you for your prompt replies and that helped me a lot. Here is my code that worked. Hope it helps someone. I am able to get the actual file name and move the file to my documents directory using the original name.

-(void) URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location
{
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSError *error;

    //getting application's document directory path
    NSArray * tempArray = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *docsDir = [tempArray objectAtIndex:0];

    //adding a new folder to the documents directory path
    NSString *appDir = [docsDir stringByAppendingPathComponent:@"/Reader/"];

    //Checking for directory existence and creating if not already exists
    if(![fileManager fileExistsAtPath:appDir])
    {
        [fileManager createDirectoryAtPath:appDir withIntermediateDirectories:NO attributes:nil error:&error];
    }

    //retrieving the filename from the response and appending it again to the path
    //this path "appDir" will be used as the target path 
    appDir =  [appDir stringByAppendingFormat:@"/%@",[[downloadTask response] suggestedFilename]];

    //checking for file existence and deleting if already present.
    if([fileManager fileExistsAtPath:appDir])
    {
        NSLog([fileManager removeItemAtPath:appDir error:&error]?@"deleted":@"not deleted");
    }

    //moving the file from temp location to app's own directory
    BOOL fileCopied = [fileManager moveItemAtPath:[location path] toPath:appDir error:&error];
    NSLog(fileCopied ? @"Yes" : @"No");

}

回答2:

In didFinishDownloadingToURL you should move the file from location to some place more permanent (e.g. your Documents folder). If you're looking for that file in the temporary location later, I'm not surprised it's no longer there.

As the documentation says, the location is defined as such:

A file URL for the temporary file. Because the file is temporary, you must either open the file for reading or move it to a permanent location in your app’s sandbox container directory before returning from this delegate method.

You must move the file to its new location before returning from didFinishDownloadingToURL.

回答3:

Just in case someone had the same issue that I did I thought I would post my solution here.

My problem was that the predicate methods are firing on a background thread so I was dispatching to my "file io" thread which handles any Writing to files, deleting etc within the app.

The problem with this is that the temporary file gets deleted as soon as the delegate method ends which was occurring at the exact moment I switched threads. So when I tried to access the file in my file io thread it had already been deleted.

My solution was to parse the file into NSData in the delegate method, then use the NSData to write to the filesystem in my file io thread.