Peer terminate call event by WebRTC on iOS

I have run ios/NativeDemo project on my iPhone, and it worked perfectly.
However, I find that when my WebRTC peer taps 'Hangup' button to terminate the call, I will be noticed this thing by EventSource, not WebRTC itself.
In Javascript, we have WebRTC disconnect event to know when the peer disconnects like below.

```
pc.oniceconnectionstatechange = function() {
if(pc.iceConnectionState == 'disconnected') {
console.log('Disconnected');
}
}
```

So, I think there should be a way to implement it like Javascript.