```
// Preserve first element for "oncontactschange" way event listener.
//
// Compared with addEventListener, the main difference is there is only one
// oncontactschange callback will be invoked. While addEventListener can 
// add multiple callbacks when event "contactschanged"arrives.
//
// This listener can be set as "oncontactschange = function(){...};"
// and can be unset as "oncontactschange = null;"
_listeners[0] = null;
var _next_listener_id = 1;
```