How handle movenext and moveprevious for Business Process Flow in 2015.

Hi All,

https://msdn.microsoft.com/en-in/library/dn817878.aspx#BKMK_moveNext

I am unable to handle movenext and moveprevious button.

these methods calling on both button (move next and back)

Please let me know. I want to call some javascript after coming to next stage.

Below is the sample i am using on onload event.

calling this function on onload;;

function CallOnload()
{
Xrm.Page.data.process.addOnStageChange(handler);

}

function handler()
{
Xrm.Page.data.process.moveNext(function(){alert('callbackfunction');});
}

the method call when i click on nextstage button, and also call if i click on previous button. while it should not call on previous button. 

Kindly let me know how to handle them.