When writing tests in RSpec, it is often necessary to stub out certain methods in order to isolate the behavior of a specific class or object. This can be particularly useful when testing code that relies on external dependencies or complex logic. In this article, we will explore how to stub an instance method that is called by a constructor in RSpec.
First, let's start with a basic understanding of constructors in RSpec. Constructors are special methods that are automatically called when an object is created. They are responsible for initializing the object's state and setting up any necessary dependencies. In RSpec, constructors are typically defined using the `initialize` method.
Now, let's consider a scenario where we have a `User` class that has a method called `full_name`, which combines the user's first and last name. This method is called by the `initialize` method in order to set the `@name` instance variable. Our goal is to test the `full_name` method in isolation, without relying on the `initialize` method. This is where stubbing comes in handy.
To stub an instance method, we can use the `allow` method provided by RSpec. This method takes two arguments: the object we want to stub and the method we want to stub. In our case, we want to stub the `full_name` method of the `User` class. We can do this by writing the following code in our test:
```
allow(User).to receive(:new).and_return("John Smith")
```
This code tells RSpec to stub the `new` method of the `User` class and return the string "John Smith" instead of actually calling the `full_name` method. This allows us to test the behavior of the `full_name` method without worrying about the `initialize` method.
Next, we need to ensure that the `full_name` method is actually called when we create a new instance of the `User` class. This can be done by using the `expect` method, which allows us to set expectations for our tests. We can write the following code to ensure that the `full_name` method is called when we create a new `User` object:
```
expect(User).to receive(:new)
```
This code tells RSpec to expect the `new` method of the `User` class to be called. If the `full_name` method is not called, the test will fail.
Putting it all together, our test should look something like this:
```
RSpec.describe User do
describe "#full_name" do
it "combines the first and last name of the user" do
allow(User).to receive(:new).and_return("John Smith")
expect(User).to receive(:new)
user = User.new("John", "Smith")
expect(user.full_name).to eq("John Smith")
end
end
end
```
In this test, we are stubbing the `new` method of the `User` class to return the string "John Smith" instead of actually creating a new instance of the `User` class. We are also setting an expectation that the `new` method will be called when we create a new `User` object. Finally, we are calling the `full_name` method on the `user` object and expecting it to return the correct full name.
