quicklook QLPreviewController不适用于pdfs

Question

我有一个应用程序，用户可以浏览他的保管箱帐户的文件。如果他选择一个文件，应用程序首先将其本地下载到文档目录中，然后使用QLPreviewController显示它的预览。除pdf文档外，它工作正常。它不是显示pdf，而只是一个白色的屏幕。这是代码：

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

    UINavigationController *controller = self.navigationController;
    DBMetadata *file = [self.currentData.contents objectAtIndex: [indexPath row]];

    if( file.isDirectory ) {
        //get name of new path
        NSString *path = [NSString stringWithFormat: @"%@%@/", self.currentData.path, file.filename];

        SubLevelViewController *slController = [[SubLevelViewController alloc] initWithStyle:UITableViewStylePlain];
        slController.dirPath = path;
        [controller pushViewController: slController animated: YES];
    }
    else {
        //save file into documents directory
        NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
        NSString *documentsDirectory = [paths objectAtIndex:0];
        currentFilePath = [documentsDirectory stringByAppendingPathComponent: [NSString stringWithString: file.filename]];
        NSString *dbPath = [NSString stringWithFormat: @"%@%@/", self.currentData.path, file.filename];
        [self.restClient loadFile:dbPath intoPath:currentFilePath];
    }
}

//rest client delegate methods
- (void)restClient:(DBRestClient *)client loadedFile:(NSString *)localPath
       contentType:(NSString *)contentType metadata:(DBMetadata *)metadata {
    NSLog(@"File loaded into path: %@", localPath);

    //load the file preview and push onto the nav controller
    QLPreviewController *previewController = [[QLPreviewController alloc] init];
    previewController.dataSource = self;
    previewController.currentPreviewItemIndex = 0;
    [[self navigationController] pushViewController:previewController animated:YES];
}

- (id)previewController:(QLPreviewController *)controller previewItemAtIndex:(NSInteger)index
{
    NSURL *fileURL = nil;
    fileURL = [NSURL fileURLWithPath:currentFilePath];
    return fileURL;
}