I'm having trouble cacheing the path of an existing file

Here is my code, I was using a QLPreviewController for display a detail view with a pdf file which I download and stock at the beginning of the program.

It throw a exception with telling the path was null.

 -(id<QLPreviewItem>)previewController:(QLPreviewController *)previewController previewItemAtIndex:(NSInteger)index{
        NSString *name=[selectedItem.source lastPathComponent];
        NSFileManager *fileManager=[NSFileManager defaultManager];
        if([fileManager fileExistsAtPath:name] ){
            NSLog(@"%@ exsit! ",name);  //In!!! 
        }


        name = [name stringByDeletingPathExtension];
           NSLog(@"name for QL: %@",name);  //Name ok!!!
        NSString *path = [[NSBundle mainBundle]pathForResource:name ofType:@"pdf"];
         NSLog(@"path: %@",path);   //Path: null !!!




        return [NSURL fileURLWithPath:path]; //error!!
    }
开发者_Python百科

Files don't download to [NSBundle mainBundle]. Try changing that line to

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *filePathString = [documentsDirectory stringByAppendingPathComponent:fileName];` 

rather than reading from [NSBundle mainBundle]. You can't download files to mainBundle. It's read only.