How to mock App Exchange package's API response?

Question

I am using an app exchange package in my code, they have exposed a few global methods that I can use to get my job done. It all works beautifully. The issue comes with testing.

I want to test positive, negative test scenarios to see my code handle's App Exchange packages' response elegantly.

That app exchange package does callout and parses, I don't know the actual HTTP Response JSON structure or mock so I cannot use Test.setMock.

The next thing I investigated is to use Stub API. Which sounded perfect but when I tried this mock I got an exception.

System.TypeException: Test.createStub() can only be called with classes in the current namespace

AppExchange Package Code:

AppExchangePackage.Api api = new AppExchangePackage.Api();
AppExchangePackage.ApiData.ScreenResponse response = api.screen(entityId); //Can be PersonAccount , Lead, Contact Id

Is there an elegant way to mock api.screen method so that I get my desired ScreenResponse in test context?

Answer 1

I cannot mock app-exchange package method, but I can mock my method, which calls the package. This will stop the code entering package and get me to do what I want.

public class APIProcessingService
{

    public ppExchangePackage.ApiData.ScreenResponse submit(Id entityId){
        AppExchangePackage.Api api = new AppExchangePackage.Api();
        AppExchangePackage.ApiData.ScreenResponse response = api.screen(entityId); //Can be PersonAccount , Lead, Contact Id
        return response;
    }

} 

Test:

@isTest
public class APIProcessingServiceTest{


    public class APIProcessingMockProvider implements System.StubProvider {

        public Object handleMethodCall(Object stubbedObject, String stubbedMethodName,
                Type returnType, List<Type> listOfParamTypes, List<String> listOfParamNames,
                List<Object> listOfArgs) {


            if(stubbedMethodName =='submit'){
                return new AppExchangePackage.ApiData.ScreenResponse(); //My Mock for success or failure response

            }

            return null;
        }
    }

    @isTest
    public static void testMethodPositive(){

        Account acc =  [SELECT  Id FROM  Account LIMIT  1];

        APIProcessingService rs = (APIProcessingService) Test.createStub(APIProcessingService.class , new APIProcessingMockProvider());
        Object resp =rs.submit(acc.Id);
        System.debug(resp);



    }
}

The only downside is, as the code is not entering the package, the package level validations won't work.