Nsarray count +1 from current item & loop - cocoa

Question

I am writing a audio player App using AVAudio Player to play mp3's from the NSDocs Dir loading them with ALAssetsLibrary subclass & AVURLAsset When I select a mp3 from the tableview it will push to a player & play but Im struggling with the skip & previous buttons
I can skip to the last track with the below code, although I'm unsure if this is correct approach sending one AVURLAsset to play rather than a queue array.
Im using apple sample project AVPlayerDemo as the foundation of the ALAssetsLibrary subclass
- (IBAction)nextButtonTapped:(id)sender {
NSArray *sourceitems = [[finalSources objectAtIndex:_selectedPath.section] items];
[sourceitems enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
id next = nil;
next = [next objectAtIndex:_selectedPath.row];
if (idx + 1 < sourceitems.count) {
next = [sourceitems objectAtIndex:idx + 1];
AssetBrowserItem *item = next;
AVURLAsset *asset = (AVURLAsset*)item.asset;
[self setURL:asset.URL];
NSLog(#"next %#", next);
//here it continues to the last object and plays,
//but i just want to skip one track and play, then click again and move
//to the next one
}
}];
}
- (void)setURL:(NSURL*)URL
{
if (mURL != URL)
{
mURL = [URL copy];
/*
Create an asset for inspection of a resource referenced by a given URL.
Load the values for the asset keys "tracks", "playable".
*/
AVURLAsset *asset = [AVURLAsset URLAssetWithURL:mURL options:nil];
NSArray *requestedKeys = [NSArray arrayWithObjects:kTracksKey, kPlayableKey, nil];
/* Tells the asset to load the values of any of the specified keys that are not already loaded. */
[asset loadValuesAsynchronouslyForKeys:requestedKeys completionHandler:
^{
dispatch_async( dispatch_get_main_queue(),
^{
/* IMPORTANT: Must dispatch to main queue in order to operate on the AVPlayer and AVPlayerItem. */
[self prepareToPlayAsset:asset withKeys:requestedKeys];
[self syncLabels:mURL];
});
}];
}
}
Any pointers would be greatly appreciated.
Thanks in advance Gav.