##Online Demo http://jsfiddle.net/bzr5npdj/
####Why do you need this?
When you need to have multiple view models communicate with each other, you usually have to do something like this data-bind="click: $parent[2].action"
This method is messy and unreliable, because if the order of the view models change, then your view will break.
Instead, you can send a message from the HTML and have the ViewModel subscribe to it:
####HTML File
data-bind="message: {topic: 'topicName', event: 'click'}"
####JS File
postbox.subscribe(function() {
alert("hello");
}, self, "topicName");
Look at demo.html