How can download audio? objective-c
<p>I have audio player. I download audio in this way</p>

<pre><code>- (void) song{
if (_index == 0) {

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

NSString *filePath = [documentsDirectory stringByAppendingPathComponent:@"1.mp3"];
BOOL fileExists = [[NSFileManager defaultManager] fileExistsAtPath:filePath isDirectory:NO];

if (!fileExists) {
    NSString *stringURL = @"https://drive.google.com/uc?export=download&amp;id=0B6zMam2kAK39VHZ1cUZsM3BhQXM";
    NSURL  *url = [NSURL URLWithString:stringURL];
    NSData *urlData = [NSData dataWithContentsOfURL:url];
    [urlData writeToFile:filePath atomically:YES];
}

self.audioPlayer = [[AVAudioPlayer alloc] initWithContentsOfURL:[NSURL URLWithString:filePath] error:nil];
}
}
</code></pre>

<p>But when audio downloading I can’t do anything. User interface stop.
How can download audio and do anything in user interface simultaneously?</p>