@yongsheng is right.

In presentation/device capabilities api implementation, we need to notify the display availability change event from native side to all extension instances on JavaScript side.

The system event (e.g. display arrival/removal event) is random and should be shared by the whole app. For example, we add a message listener for such an event in JavaScript:
```
etension.setMessageListener(function(message) {
  if (message == "system_event") {
    // invoke the callbacks
  }
});
```
Each JavaScript context (aka. extension instance) will be injected the above code. When a system event is fired, we should broadcast such an event to all JavaScript contexts, not a specific context, that is why broadcastMessage is needed.

Does it answer your concern? @cmarcelo