save the audio from url locally for play later - iphone

Question

How can i save the audio from url locally for play later. I can stream the audio using avplayer but can't save it locally and play later .I tried with avurlasset. Can anyone help me?
This is my code by which i tried to save locally:
here i streamed the audio using avplayer
//code for saving the locally
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
if ([keyPath isEqualToString:#"status"])
{
AVPlayerItem * item = (AVPlayerItem *)object;
if (item.status == AVPlayerItemStatusReadyToPlay)
{
NSArray *paths = NSSearchPathForDirectoriesInDomains (NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *myPathDocs = [documentsDirectory stringByAppendingPathComponent: #"audio.m4a"];
asset=player.currentItem.asset;
NSURL *url1 = [NSURL fileURLWithPath:myPathDocs];
// 5 - Create exporter
AVAssetExportSession *assetEx = [[AVAssetExportSession alloc] initWithAsset:player.currentItem.asset presetName:AVAssetExportPresetAppleM4A];//tried with asset2 also
assetEx.outputURL=[NSURL fileURLWithPath:myPathDocs];
assetEx.outputFileType = AVFileTypeAppleM4A;
assetEx.shouldOptimizeForNetworkUse = YES;
[assetEx exportAsynchronouslyWithCompletionHandler:^{
int status = assetEx.status;
dispatch_async(dispatch_get_main_queue(), ^{
[self exportDidFinish:assetEx];
});
}];
}
}
}
but there is no file created in the documents directory.