public class BillingServiceProxy {
    public class billProjectRequest_element {
        public BillingServiceProxy.project project;
        private String[] project_type_info = new String[]{'project','http://salesforce.com/th/invoice-web-service',null,'1','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://salesforce.com/th/invoice-web-service','true','false'};
        private String[] field_order_type_info = new String[]{'project'};
    }
    public class project {
        public String username;
        public String password;
        public String projectid;
        public String projectRef;
        public Double billAmount;
        private String[] username_type_info = new String[]{'username','http://salesforce.com/th/invoice-web-service',null,'1','1','false'};
        private String[] password_type_info = new String[]{'password','http://salesforce.com/th/invoice-web-service',null,'1','1','false'};
        private String[] projectRef_type_info = new String[]{'projectRef','http://salesforce.com/th/invoice-web-service',null,'1','1','false'};
        private String[] projectid_type_info = new String[]{'projectid','http://salesforce.com/th/invoice-web-service',null,'1','1','false'};
        private String[] billAmount_type_info = new String[]{'billAmount','http://salesforce.com/th/invoice-web-service',null,'1','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://salesforce.com/th/invoice-web-service','true','false'};
        private String[] field_order_type_info = new String[]{'username','password','projectid','projectRef','billAmount'};
    }
    public class billProjectResponse_element {
        public String status;
        private String[] status_type_info = new String[]{'status','http://salesforce.com/th/invoice-web-service',null,'1','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://salesforce.com/th/invoice-web-service','true','false'};
        private String[] field_order_type_info = new String[]{'status'};
    }
    public class InvoicesPortSoap11 {
        public String endpoint_x = 'http://sb-integration-bs.herokuapp.com:80/ws';
        public Map<String,String> inputHttpHeaders_x;
        public Map<String,String> outputHttpHeaders_x;
        public String clientCertName_x;
        public String clientCert_x;
        public String clientCertPasswd_x;
        public Integer timeout_x;
        private String[] ns_map_type_info = new String[]{'http://salesforce.com/th/invoice-web-service', 'BillingServiceProxy'};
        public String billProject(BillingServiceProxy.project project) {
            BillingServiceProxy.billProjectRequest_element request_x = new BillingServiceProxy.billProjectRequest_element();
            request_x.project = project;
            BillingServiceProxy.billProjectResponse_element response_x;
            Map<String, BillingServiceProxy.billProjectResponse_element> response_map_x = new Map<String, BillingServiceProxy.billProjectResponse_element>();
            response_map_x.put('response_x', response_x);
            WebServiceCallout.invoke(
              this,
              request_x,
              response_map_x,
              new String[]{endpoint_x,
              '',
              'http://salesforce.com/th/invoice-web-service',
              'billProjectRequest',
              'http://salesforce.com/th/invoice-web-service',
              'billProjectResponse',
              'BillingServiceProxy.billProjectResponse_element'}
            );
            response_x = response_map_x.get('response_x');
            return response_x.status;
        }
    }
}



APex CLASS:

public class BillingCalloutService {

    //method to run asynchronously or in future, invoked from trigger.
    @future(callout=true)
    public static void callBillingService(String projectRef, Decimal billingAmount){
        //gets the custom settings values.
        ServiceCredentials__c servCred = ServiceCredentials__c.getValues('BillingServiceCredential');
        //creating the project object to be passed in argument below.
        BillingServiceProxy.project project = new BillingServiceProxy.project();
        project.username = servCred.Username__c;
        project.password = servCred.Password__c;
        project.projectid = projectRef;
        project.projectRef = projectRef;
        project.billAmount = billingAmount;
                                //synchronous call to the Billing Invoice system.
        BillingServiceProxy.InvoicesPortSoap11 invoiceCall = new BillingServiceProxy.InvoicesPortSoap11();
        //getting the response back from billing system.
        String response = invoiceCall.billProject(project);
                                //if the response is ok, need to change the billing status to 'Billed'
        List<Project__c> proj;
        system.debug('response: '+response);
        if (response != null && response.equalsIgnoreCase('OK')){
                                                proj = [SELECT Status__c FROM Project__c WHERE ProjectRef__c =: projectRef];
            if(proj.size() > 0){
                proj[0].Status__c = 'Billed';
            }
            update proj;
        }
    }
}


APEX CLASS:

@isTest
global class BillingCalloutServiceMock implements WebServiceMock {
   global void doInvoke(
           Object stub,
           Object request,
           Map<String, Object> response,
           String endpoint,
           String soapAction,
           String requestName,
           String responseNS,
           String responseName,
           String responseType) {
       BillingServiceProxy.billProjectResponse_element respElement =
           new BillingServiceProxy.billProjectResponse_element();
       respElement.status = 'OK';
       response.put('response_x', respElement);
   }
}

APEX CLASS:

@isTest
global class BillingCalloutServiceMockFailure implements WebServiceMock {
   global void doInvoke(
           Object stub,
           Object request,
           Map<String, Object> response,
           String endpoint,
           String soapAction,
           String requestName,
           String responseNS,
           String responseName,
           String responseType) {
       BillingServiceProxy.billProjectResponse_element respElement =
           new BillingServiceProxy.billProjectResponse_element();
       respElement.status = 'UNAUTHORIZED';
       response.put('response_x', respElement);
   }
}

APEX CLASS:

@isTest
private class BillingCalloutServiceTest {

    @testSetup static void setupProject(){

        Opportunity oppo = new Opportunity();
        oppo.Name = 'TestOpp';
        oppo.CloseDate = Date.today();
        oppo.StageName = 'Prospecting';
        insert oppo;

        Project__c proj = new Project__c();
        proj.Name = 'TestProj';
        proj.Billable_Amount__c = 1000;
        proj.ProjectRef__c = 'TestRef';
        proj.Status__c = 'Running';
        proj.Opportunity__c = oppo.Id;
        insert proj;

        ServiceCredentials__c servCred = new ServiceCredentials__c();
        servCred.Name = 'BillingServiceCredential';
        servCred.Username__c = 'user1';
        servCred.Password__c = 'pass1';
        insert servCred;

    }

    @isTest static void testCalloutSuccess(){
                                Test.setMock(WebServiceMock.class, new BillingCalloutServiceMock());
        List<Project__c> prof = [SELECT Status__C FROM Project__c WHERE ProjectRef__c = 'TestRef'];
        System.assertEquals(1, prof.size());
        Test.startTest();
        prof[0].Status__c = 'Billable';
        update prof;
        Test.stopTest();
        prof = [SELECT Status__C FROM Project__c WHERE ProjectRef__c = 'TestRef'];
        System.assertEquals(1, prof.size());
        System.assertEquals('Billed', prof[0].Status__C);
    }

    @isTest static void testCalloutFailure(){
                                Test.setMock(WebServiceMock.class, new BillingCalloutServiceMockFailure());
        List<Project__c> prof = [SELECT Status__C FROM Project__c WHERE ProjectRef__c = 'TestRef'];
        System.assertEquals(1, prof.size());
        Test.startTest();
        prof[0].Status__c = 'Running';
        update prof;
        Test.stopTest();
        prof = [SELECT Status__C FROM Project__c WHERE ProjectRef__c = 'TestRef'];
        System.assertEquals(1, prof.size());
        System.assertEquals('Running', prof[0].Status__C);
    }

}


APEX TRIGGER:

trigger ProjectTrigger on Project__c (after update) {
    //after trigger to fire if the Status is set to Billable.
    If (Trigger.isAfter && Trigger.isUpdate){
        for(Project__c proj : Trigger.new){
            if(proj.Status__c.equals('Billable')){
                BillingCalloutService.callBillingService(proj.ProjectRef__c, proj.Billable_Amount__c);
            }
        }
    }
}