Synchronize Salesforce project data with Square Peg's external billing system|| challenge 8

[@Salesforcebuddy1711]

BillingCalloutService :-
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public class BillingCalloutService {
@future(callout = true)
public static void callBillingService(String projectRef, Decimal billingAmount){
ServiceCredentials__c srvcCrd = ServiceCredentials__c.getValues('BillingServiceCredential');

BillingServiceProxy.project projectInst = new BillingServiceProxy.project();
projectInst.username = srvcCrd.Username__c;
projectInst.password = srvcCrd.Password__c;
projectInst.billAmount = billingAmount;

BillingServiceProxy.InvoicesPortSoap11 invPortSoapInst = new BillingServiceProxy.InvoicesPortSoap11();
String response = invPortSoapInst.billProject(projectInst);

List lstOfProjects = new List();
if(response != null && response.equalsIgnoreCase('OK')){
List lstOfPrjts = [SELECT Status__c FROM Project__c WHERE ProjectRef__c = :projectRef];
for(Project__c prjt : lstOfPrjts){
prjt.Status__c = 'Billed';

lstOfProjects.add(prjt);
}

UPDATE lstOfProjects;
}
}
}
ProjectTrigger :-
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trigger ProjectTrigger on Project__c (after update) {
if(Trigger.isAfter && Trigger.isUpdate){
for(Project__c prjt : Trigger.new){
if(prjt.Status__c != null && prjt.Status__c.equals('Billable')){
BillingCalloutService.callBillingService(prjt.ProjectRef__c, prjt.Billable_Amount__c);
}
}
}
}