TRigger
---------------------------------
trigger OpportunityTrigger on Opportunity (After insert,After update)
{
    if(trigger.isAfter )
   
    {
        if(trigger.isInsert){
            List<OpportunityContactRole> lstContactRoletoUpdate = new List<OpportunityContactRole>();
            Set<String> setOppId = new Set<String>();
                    system.debug('!!!!After insert');
                    List<OpportunityContactRole> lstOppContactRolePrimary = new List<opportunityContactRole>();
                    for(Opportunity o:trigger.new)
                    {
                        setOppId.add(String.valueOf(o.id));
                        system.debug('@@@@@setOppId '+(String.valueOf(o.id)) );
                    }
           
                    lstOppContactRolePrimary = [select id,contactId,opportunityId from OpportunityContactRole where opportunityId in :setOppId];
                    //no primaryContact role found at all have make from scratch
                    system.debug('lstOppContactRolePrimary '+lstOppContactRolePrimary );
                    if(lstOppContactRolePrimary.size() > 0)
                    {
                        Map<String,OpportunityContactRole> mapOppIdConrole = new Map<String,OpportunityContactRole>();
                        for(OpportunityContactRole conRole:lstOppContactRolePrimary)
                        {
                            mapOppIdConrole.put(String.valueOf(conRole.opportunityId),conRole);
                        }
                        for(String id:setOppId)
                        {
                            if(mapOppIdConrole.containsKey(id))
                            {
                                Opportunity oppTeamp = trigger.newMap.get(id);
                                if(oppTeamp.buyer__c == mapOppIdConrole.get(id).contactId)
                                {
                                    //both are equal , we will just have to update the role
                                    OpportunityContactRole tempRole = mapOppIdConrole.get(id);
                                    tempRole.Role = 'Buyer';
                                    lstContactRoletoUpdate.add(tempRole);
                                }
                                else
                                {
                                    //we have the contact role but the contacts are different
                                    OpportunityContactRole tempRole = mapOppIdConrole.get(id);
                                    tempRole.Role = 'Buyer';
                                    tempRole.contactId = oppTeamp.buyer__c;
                                    lstContactRoletoUpdate.add(tempRole);
                                }
                            }else
                            {
                                // we have to make contact role
                                System.debug('####Creating absolutely new one 1');
                                OpportunityContactRole tempRole = new OpportunityContactRole();
                                tempRole.Role = 'Buyer';
                                tempRole.contactId = trigger.newMap.get(id).buyer__c;
                                tempRole.isPrimary = true;
                                lstContactRoletoUpdate.add(tempRole);
                            }
                        }
                    }else
                    {
                        System.debug('####Creating absolutely new one 2');
                        opportunityTriggerHandler objopportunityTriggerHandler = new opportunityTriggerHandler();
                        objopportunityTriggerHandler.createNewContact(trigger.new);
                    }
                if(lstContactRoletoUpdate.size() > 0)
                {
                    upsert lstContactRoletoUpdate;
                }  
           
        }
    }
    /*
    if(trigger.isBefore)
    {
        if(trigger.isInsert)
        {
                    system.debug('!!!!Before Insert');
                    for(Opportunity o:trigger.new)
                {
                    List<OpportunityContactRole> lstOppContactRolePrimary = [select id,contactId from OpportunityContactRole where opportunityId =: o.id];
                    System.debug('!!!!Before insert contact id'+o);
                    System.debug('!!!!Before insert buyer id'+o.buyer__c);
                    System.debug('!!!!Before lstOppContactRolePrimary = >'+lstOppContactRolePrimary);
                }
           
        }
    }
    */ 
        //opportunityTriggerHandler objopportunityTriggerHandler = new opportunityTriggerHandler();
        //objopportunityTriggerHandler.createNewContact(trigger.new);
   
   
    //code added by deepak dhingra on 13/6/2014 to update the line items
     if(trigger.isAfter && trigger.isUpdate)
     {
        Set<string> qualifiedOppId = new Set<string>();
        for(Opportunity o:Trigger.new)
        {
            if(o.Agency_Discount_Percentage__c != Trigger.Oldmap.get(o.id).Agency_Discount_Percentage__c)
            {
                qualifiedOppId.add(String.valueOf(o.id).left(15));
            }
           
        }
       
        if(qualifiedOppId.size() > 0)
        {
            List<OpportunityLineItem> lstLineItems = new List<OpportunityLineItem>();
            lstLineItems  = [select id from OpportunityLineItem where opportunityid in :qualifiedOppId];
            if(lstLineItems.size() > 0)
            {
                update lstLineItems;
            }
        }


_______________________________

TRACKER CLASSSSSSS



@isTest
public class OpportunityTriggerHandlerTest
{
static testMethod void insertOpportunity()
{
Account objAccount = new Account();
objAccount.Name = 'Vivek';
objAccount.Type = 'Supplier';
objAccount.Phone = '1234';
objAccount.CurrencyIsoCode = 'AUD';
insert objAccount;

Contact objcontact = new contact();
objcontact.LastName = 'Singh';
objcontact.CurrencyIsoCode = 'AUD';
objcontact.Accountid = objAccount.id ;
insert objContact;

opportunity objopportunity= new opportunity ();
objopportunity.Name = 'Hindi2014';
objopportunity.CloseDate = System.Today();
objopportunity.Accountid = objAccount.id;
objopportunity.Buyer__c = objcontact.id;
objopportunity.Vendor__c = objaccount.id;
objopportunity.StageName = 'Prposal Sent';
objopportunity.Billing_Terms__c = 'On Completion';              
objopportunity.Type = 'New Business';
insert Objopportunity;

OpportunityContactRole objOpportunityContactRole=new OpportunityContactRole();
   objOpportunityContactRole.ContactId = Objopportunity.Buyer__c;
   objOpportunityContactRole.OpportunityId = Objopportunity.Id;
   objOpportunityContactRole.Role = 'Buyer';
  insert objOpportunityContactRole;

  List<OpportunityContactRole> lstOpportunityContactRole = new List<OpportunityContactRole>();
           //OpportunityContactRole = [select id from OpportunityContactRole where OpportunityContactRole ='Buyer'];

}
static testMethod void updateOpp()
{
  opportunity objopportunity2= new opportunity ();
  objopportunity2.Name = 'Hindi2015';
  //upadte Objopportunity2;
}
}