mock object in unit testing

Question

This may be overasked, but I just couldn't grasp what is it for, and when/why I use it. Since I'm learning dart, I'm gonna give my example in dart and I hope I'll get answer in dart too. Also I'm new with unit testing.

Imagine I have 3 class, a Person, a Translator, a Db. Person can do query, Translator translates the query given by Person, so Db can process it.

class Person {
  query(String args, Translator translator) => 
      translator.query(args);
}

class Translator {
  Db db;

  Translator(this.db);

  query(args) => db.query(args); // assuming Translator knows what the db type is
}

class Db {
  String type;

  Db(this.type);

  query(args) => // query with args, return the result;
}

Now I want to test if the Translator is doing good:

void main() {
  var mongoDB = new Db('mongodb'),
      translator = new Translator(mongoDB),
      person = new Person(),
      result = person.query('give me 1 Person named Andy', translator); // get the result

  expect(result['name'], equals('Andy'));
}

From what I have read, mocking is used if I have a case like above, Translator's dependency with Db and a Db io.

Which class should I mock, Translator or Db ?

In what way/how do I mock the class?

What benefit I get from mocking?

TGH · Accepted Answer · 2013-11-19 05:45:06Z

1

The main purpose of a mocking framework is to simulate the integration to external systems in your code. If you don't mock the dependencies you are writing an integration test, and not a unit test. A big benefit of this is that you don't have dependencies on existing data in the external systems (data in the DB etc). There is also a performance benefit since you don't have to make potentially slow calls to external systems.

answered Nov 19, 2013 at 5:45

TGH

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