controller===>
public class actionFunctionController{
    public Position__c poc{get;set;}
    public Boolean show{get;set;}
    public Boolean showmessage{get;set;}
    
    public actionFunctionController(ApexPages.StandardController stdController){
        String pid = ((Position__c)stdcontroller.getRecord()).Id;
        poc=[select id,name,Hire_By__c,Hiring_Manager__c,Job_Level__c,Functional_Area__c,Job_Description__c,Location__c,Max_Pay__c,
             Min_Pay__c,Open_Date__c,Responsibilities__c,Travel_Required__c,Type__c,Status__c,Cancellation_Reason__c from Position__c where id=: pid];
             show = false;
    }
    public PageReference Savefun(){
        try{
            if(poc.Status__c =='Cancelled' && poc.Cancellation_Reason__c == null){
                showmessage = true;    
                return null;
                }  
            else{
                update poc;
                showmessage=false;
                PageReference newPage = new PageReference('https://ap2.salesforce.com/'+ApexPages.currentPage().getParameters().get('id'));
                return newPage;
                }
            }
        catch(Exception e){
            ApexPages.addMessages(e);
            return null;
        }
        return null;        
    }
    public PageReference priorityChanged(){
        if(poc.Status__c == 'Cancelled'){
            show = true;
        }
        else{
            show = false;
        }
        return null;
    }
}


test class-->
@istest
public class Test_Action_contrller {
    Static TestMethod void test_Method(){
        Position__c poc = new Position__c();
        poc.name = 'Hari';
        poc.Hire_By__c = Date.newInstance(2017, 12,30);
        poc.Status__c='Cancelled';
        poc.Cancellation_Reason__c='Cancelled';
        insert poc;
        Position__c poc1 = new Position__c();
        poc1.name = 'Hari';
        poc1.Hire_By__c = Date.newInstance(2017, 12,30);
        poc1.Status__c='Cancelled';
        poc1.Cancellation_Reason__c='';
        insert poc1;
        Position__c poc2 = new Position__c(name = 'Hari',Hire_By__c = Date.newInstance(2017, 11,30),Status__c='Canceld',Cancellation_Reason__c='not need');
        insert poc2;
        ApexPages.StandardController sc= new  ApexPages.StandardController(poc);
        ApexPages.StandardController sc1= new  ApexPages.StandardController(poc1);
         ApexPages.StandardController sc2= new  ApexPages.StandardController(poc2);
        actionFunctionController afc=new actionFunctionController(sc);
        actionFunctionController afc1=new actionFunctionController(sc1);
        actionFunctionController afc2=new actionFunctionController(sc2);
           Test.startTest();
        afc.priorityChanged();
        afc.Savefun();
        afc1.Savefun();
        afc2.priorityChanged();
        afc2.Savefun();
        Test.stopTest();
           }    
}