QLPreviewController无法打开下载的文件

Question

我正在尝试将下载的文件预览到系统，并且在执行名为previewController：previewItemAtIndex：的方法时程序崩溃。从我假设是在显示之前释放预览控制器，但是当文件已经在文档文件夹中时不会发生此错误。只有在下载后立即尝试打开文件时才会发生这种情况。

以下是代码：

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{

NSUInteger row = [indexPath row];
NSUInteger count = [listSessionsST count];

[tableView deselectRowAtIndexPath:indexPath animated:YES];


    Signature *object = [listSessionsST objectAtIndex:row];


        UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
        UILabel *lblInComplete = (UILabel*)[cell viewWithTag:4];
        UIImageView *imgCheck = (UIImageView*)[cell viewWithTag:3];

        //Sets the File Name
        self.File = [[[NSString stringWithFormat:@"%@_%@.%@",object.FileName,object.ModDate,object.Extension] 
                         componentsSeparatedByCharactersInSet: [NSCharacterSet whitespaceCharacterSet]] componentsJoinedByString: @""];

        //Shows the loading screen while the file is being downloaded
        [DejalBezelActivityView activityViewForView:self.view withLabel:@"Downloading" width:0 LoadingType:@"Full"];
        [DejalActivityView currentActivityView].showNetworkActivityIndicator = YES;

        //Calls the method that will begin downloading the file 
        [self performSelector:@selector(startDownload:) withObject:object.Location afterDelay:0.2];

}


- (void)startDownload:(NSString *)strLocation{

NSString *filePath = [NSString stringWithFormat:@"%@/tmp/%@", NSHomeDirectory(),self.File];
BOOL fileExists = [[NSFileManager defaultManager] fileExistsAtPath:filePath];

if (fileExists)
{
    [DejalBezelActivityView removeViewAnimated:YES];
    //QuickLook APIs directly to preview the document 
    QLPreviewController *previewController = [[QLPreviewController alloc] init];
    previewController.dataSource = self;
    previewController.delegate = self;
    [[self navigationController] pushViewController:previewController animated:YES];
    [previewController release];
}
else
{
    NSDictionary *dictionary = [Signature downloadFile:self.File Path:strLocation];

    if ([dictionary valueForKey:@"Error"] == nil)
    {
        //If no error has occurred while downloading then preview the file.

        //QuickLook APIs directly to preview the document 
        QLPreviewController *previewController = [[QLPreviewController alloc] init];
        previewController.dataSource = self;
        previewController.delegate = self;
        [[self navigationController] pushViewController:previewController animated:YES];
        [previewController release];
        [DejalBezelActivityView removeViewAnimated:YES];
    }
    else 
    {
        //If an error occurred while downloading then display message to user.
        [DejalBezelActivityView removeViewAnimated:YES];
        UIAlertView *alert = [[UIAlertView alloc]                        
                              initWithTitle:@"" 
                              message:[dictionary valueForKey:@"Response"]
                              delegate:self 
                              cancelButtonTitle:@"Ok" 
                              otherButtonTitles:nil];
        [alert show];
        [alert release];
    }
  }
}

// Returns the number of items that the preview controller should preview
- (NSInteger)numberOfPreviewItemsInPreviewController:(QLPreviewController *)previewController{
    return 1;
}

// returns the item that the preview controller should preview
- (id)previewController:(QLPreviewController *)controller previewItemAtIndex:(NSInteger)index{

     NSString *path = [NSString stringWithFormat:@"%@/tmp/%@", NSHomeDirectory(),self.File];
    return [NSURL fileURLWithPath:path];
}


The method that downloads the file and saves it to the tmp folder 
---------------------------------------------------------------
+ (NSDictionary *)downloadFile:(NSString *)strFile Path:(NSString *)strPath{

float freeSpace = 0.0f;  
NSError *error = nil;  
NSDictionary *dictionary = [[NSFileManager defaultManager] attributesOfFileSystemForPath:[NSString stringWithFormat:@"%@/tmp", NSHomeDirectory()]
                                                                                   error: &error];  
if (dictionary) 
{  
    NSNumber *fileSystemFreeSizeInBytes = [dictionary objectForKey: NSFileSystemFreeSize];  
    freeSpace = [fileSystemFreeSizeInBytes floatValue];  
}
else
{ 
    //Handle error
    NSLog(@"Error;%@",error);
    return [NSDictionary dictionaryWithObjectsAndKeys:@"An error has occurred while downloading the file.",@"Response",@"Y",@"Error", nil];
}  

if (freeSpace > 0.0f)
{
    NSDictionary *dict = nil;
    BOOL blnHasErroroccurred = NO;

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

    NSString *encoded = (NSString *)CFURLCreateStringByAddingPercentEscapes(NULL,
                                                                            (CFStringRef)strPath,
                                                                            NULL,
                                                                            (CFStringRef)@"!*'\"();:@&=+$,/?%#[]%",
                                                                            kCFStringEncodingUTF8 );

    NSString *strURL = [NSString stringWithFormat:@"%@/DataTransfer/DownloadFile?EMP_ID=%@&FilePath=%@", 
                        appDelegate.ServerAddress,
                        appDelegate.UserId,
                        encoded];

    [encoded release];

    NSURL *url = [NSURL URLWithString:strURL];

    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url 
                                                           cachePolicy: NSURLRequestUseProtocolCachePolicy 
                                                       timeoutInterval:20.0];

    [request setHTTPMethod:@"POST"];
    [request setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"Content-Type"];

    NSURLResponse* response = nil;
    NSError* resultError = nil;

    NSData* dataResult = [NSURLConnection sendSynchronousRequest:request 
                                         returningResponse:&response 
                                                     error:&resultError];

    NSHTTPURLResponse* httpResponse = (NSHTTPURLResponse*)response;

    if ([httpResponse statusCode] == 200) 
    {
        @try 
        {
            if (dataResult.length < freeSpace) 
            {
                //Get the tmp directory
                NSFileManager *fileManager =[NSFileManager defaultManager];
                NSString *fileName = [NSString stringWithFormat:@"%@/tmp/%@", NSHomeDirectory(),strFile];

                //Write the data to using the tmp directory
                BOOL filecreationSuccess = [fileManager createFileAtPath:fileName contents:dataResult attributes:nil];       

                if(filecreationSuccess == YES)
                {
                    dict = [NSDictionary dictionaryWithObjectsAndKeys:@"",@"Response",nil,@"Error", nil];
                }   
                else
                {
                    blnHasErroroccurred = YES;
                }
            }
        }
        @catch (NSException *exception)
        {
            blnHasErroroccurred = YES;
        }
    }
    else 
    {
        blnHasErroroccurred = YES;
    }

    if(blnHasErroroccurred == YES)
    {
        dict  = [NSDictionary dictionaryWithObjectsAndKeys:@"An error has occurred while downloading the file.", @"Response", @"Yes",@"Error",nil];
    }

    [dataResult release];
    return dict;
}
else
{
    return [NSDictionary dictionaryWithObjectsAndKeys:@"Not enough disk space to download file.",@"Response",@"Y",@"Error", nil];
}
}