how to show uialertview in another thread in objective c - objective-c

Question

I created another thread and its functionality is working well. But only case is in new thread we cannot use UIControllers. As an example I couldn't use UIAlerview in new thread. How can I slove it?
My tried code is bellow.
- (IBAction)btnCopyImage:(id)sender
{
[NSThread detachNewThreadSelector:#selector(DownloadCheck) toTarget:self withObject:nil];
// [self performSelectorOnMainThread:#selector(DownloadCheck) withObject:nil waitUntilDone:NO];
NSLog(#"My name is ");
int sum =0;
for (int i=1; i<=1000; i++)
{
sum =sum+i;
}
NSLog(#"sum %d",sum);
}
-(void)DownloadCheck
{
[UIApplication sharedApplication].networkActivityIndicatorVisible = YES;
NSString *documentsDirectory = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];
NSString *path = [documentsDirectory stringByAppendingPathComponent:#"Download"];
NSString* saveDialogURL = #"/Volumes/Inbox/7. Debugging and Troubleshooting.zip";
NSString* fileNameWithExtension = saveDialogURL.lastPathComponent;
NSLog(#"path %# ",fileNameWithExtension);
NSString *pathWithExtention=[NSString stringWithFormat:#"%#/%#", path,fileNameWithExtension];
NSLog(#"path %#",pathWithExtention);
//Remove existing file
NSFileManager *filemgr;
filemgr = [NSFileManager defaultManager];
NSError *error;
[filemgr removeItemAtPath:pathWithExtention error:&error];
//Copy file to i phone created directory
if ([filemgr copyItemAtPath: saveDialogURL toPath: pathWithExtention error: NULL] == YES)
{
UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:#"Download"
message:#"Downloaded Sucessfully"
delegate:self
cancelButtonTitle:nil
otherButtonTitles:#"OK", nil];
[alertView show];
NSLog (#"Copy successful");
}
else
{
UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:#"Error"
message:#"Download Faild"
delegate:self
cancelButtonTitle:nil
otherButtonTitles:#"OK", nil];
[alertView show];
NSLog (#"Copy Faild");
}
[UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
}
[self performSelectorOnMainThread:#selector(DownloadCheck) withObject:nil waitUntilDone:NO]; this cannot be used because it's working on same thread.
So what should I do by using same code?