Test Class in apex - triggers

Question

So i am new to salesforce and i finished my training and now im working on a project. But on my project i have stumbled on a test class that i am not finding a way to write it, so i would appreciate if anyone can help me figure out a way to write it. Here is the code:
public class AP01_Opportunity
{
//Method to create a new service contract when opportunity = Gagné
public static void CreateContract(List<Opportunity> listOpp, Map<Id, Opportunity> oldMap)
{
//Variable Declaration
ServiceContract sc;
List<ServiceContract> listSCToAdd = new List<ServiceContract>();
List<ContractLineItem> listContractItems = new List<ContractLineItem>();
List<Opportunity> listOppGagne = new list<Opportunity>();
//Loop in list of opportunities
for(Opportunity opp : listOpp)
{
if(opp.StageName == Label.ClotureGagne && !oldMap.get(opp.Id).isWon)
{
listOppGagne.add(opp);
}
}
//check if list has opportunity becoming won
if(listOppGagne.size() > 0){
Map<Id, Opportunity> mapOppGagne = new Map<Id, Opportunity> ([SELECT Id,
Name,
StageName,
Pricebook2Id,
Account.Name,
(SELECT Id,
PricebookEntryId,
PricebookEntry.Name,
Quantity,
UnitPrice
FROM OpportunityLineItems)
FROM Opportunity
WHERE Id in :listOppGagne]);
for( Opportunity opp : listOppGagne )
{
//Create new service contract
sc = new ServiceContract();
sc.Name = opp.Name;
sc.ApprovalStatus = Label.Activated;
sc.OpportunityId__c = Id.valueOf(opp.Id);
sc.Pricebook2Id = opp.Pricebook2Id;
sc.StartDate = Date.today();
listSCToAdd.add(sc);
}
if(listSCToAdd.size() > 0){
insert listSCToAdd;
Opportunity currentOpp;
ContractLineItem cli;
Id oppId;
for(ServiceContract servcont : listSCToAdd)
{
oppId = servcont.OpportunityId__c;
if(mapOppGagne.containsKey(oppId))
{
currentOpp = mapOppGagne.get(oppId);
//copy the oppLineItems per opportunity to the respective Service Contract
for(OpportunityLineItem items : currentOpp.OpportunityLineItems)
{
cli = new ContractLineItem();
cli.PricebookEntryId = items.PricebookEntryId;
cli.Quantity = items.Quantity;
cli.UnitPrice = items.UnitPrice;
cli.ServiceContractId = servcont.Id;
listContractItems.add(cli);
}
}
}
if(listContractItems.size() > 0)
{
insert listContractItems;
}
}
}
}
}
this code is a trigger that creates a new service contract record with contract line items copied from the opportunity line items, when the opportunity stage changes to "Cloturé Gagné" which means closed won in french.
Thank you in advance.

Answer 1

In order to write a simple test class I would recommend you to use the following guide: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_qs_test.htm
The idea is simple: Lets say you create an Opportunity in your Test class and make an insert or update in your case - your trigger class will automatically fire and run the code from your AP01_Opportunity class. You can put some
System.debug('some message');
to check if your logic works as expected and also which code blocks are executed