Salesforce - Override Standard Approval Process Page

Sometimes requirement comes to make Rejection Reasons mandatory when Reject button is clicked. Follow the below approach to over-ride standard approval process page -
1. Create a custom visualforce page as a replica of standard approval/rejection page.
2. Create a controller to have methods for approve/reject/cancel. Approval Process api is available in apex reference book.
3. Create a home page component in which a java-script will be written to over-ride "Approve/Reject" button on Lead.
4. Add this home page component in home page layout.

Done..!!

Visualforce Page



<apex:page controller="ProcessInstanceController" tabStyle="Lead">
   <apex:form >
   <apex:sectionHeader title="Lead" subtitle="{!objLead.Name}"/>
   <apex:pageBlock title="Approve/Reject Approval Request">
       <apex:pageBlockButtons location="bottom"> 
           <apex:commandButton value="Approve" action="{!approve}"/>
           <apex:commandButton value="Reject" action="{!reject}"/>
           <apex:commandButton value="Cancel" action="{!cancel}"/>
       </apex:pageBlockButtons>   
       <apex:pageBlockSection columns="1">
           <apex:pageBlockSectionItem >
               Name <apex:outputField value="{!objLead.Name}"/>
           </apex:pageBlockSectionItem>
           <apex:pageBlockSectionItem >
               Lead Owner <apex:outputField value="{!objLead.Owner.Name}"/>
           </apex:pageBlockSectionItem>
           <apex:pageBlockSectionItem >
               Rejection Reason <font color="red">(Mandatory while Rejection)</font><apex:inputField value="{!objLead.Rejection_Reason__c}"/>
           </apex:pageBlockSectionItem>
           <apex:pageBlockSectionItem >
              Comments <font color="red">(Mandatory while Rejection)</font> <apex:inputTextArea value="{!objLead.Comments__c}" rows="5" cols="100"/>
           </apex:pageBlockSectionItem>
       </apex:pageBlockSection>
   </apex:pageBlock>
   </apex:form>
</apex:page>


Controller



public class ProcessInstanceController {
    public String processId;
    public String leadId;
    public ProcessInstance objProcessInstance;
    public Lead objLead {get; set;}
    public PageReference redirectPage;
    public ProcessInstanceController(){
        processId = ApexPages.currentPage().getParameters().get('id');
        leadId = ApexPages.currentPage().getParameters().get('leadId');
        objLead = [select Name,Owner.Name,Rejection_Reason__c,Comments__c from Lead where id =:leadId];
        redirectPage = new PageReference('/'+leadId);
    }
    
    public PageReference Approve(){
        Approval.ProcessWorkitemRequest req = new Approval.ProcessWorkitemRequest();
        req.setComments(objLead.Comments__c);
        req.setAction('Approve');
        req.setWorkitemId(processId);
        Approval.ProcessResult result =  Approval.process(req);
        update objLead;
        return redirectPage ;
    }
    
    public PageReference Reject(){
        Approval.ProcessWorkitemRequest req = new Approval.ProcessWorkitemRequest();
        req.setComments(objLead.Comments__c);
        req.setAction('Reject');
        req.setWorkitemId(processId);
        Approval.ProcessResult result =  Approval.process(req);
        update objLead;
        return redirectPage ;       
    }
    
     public PageReference Cancel(){
     return redirectPage ;
    }
}


Home Page Component



<div id="demoContainer"><script language="JavaScript">try{var pa=document.getElementById("demoContainer").parentNode.parentNode; pa.style.display = "none";}catch(err){alert('There was an error on this webpage='+err.description);}</script><script language="JavaScript">function CustomApproval(){try{var str=location.href;var leadId=str.substr(str.indexOf(".com/")-0 + 5,15);var divid=leadId+'_RelatedProcessHistoryList_body';var Approvalprocessdiv=document.getElementById(divid);if(Approvalprocessdiv!=null){var originalHTML=Approvalprocessdiv.innerHTML;var newHtml=originalHTML.replace('/p/process/ProcessInstanceWorkitemWizardStageManager?','/apex/ProcessInstanceWorkitemWizard?LeadId='+leadId+'&');Approvalprocessdiv.innerHTML=newHtml;}}catch(err){alert(err);}}var oldonload = window.onload; if (typeof window.onload != 'function'){window.onload = oldonload;} else { window.onload = function() {if (oldonload) {oldonload();}CustomApproval();}} </script></div>