The Doctrine of Useful Objects - Get rid of mocking

# The Doctrine of<br />Useful Objects

## Get rid of mocking

---

# Who am I?

.pull-left.avatar[![Picture of me](https://avatars.githubusercontent.com/u/945115?v=4)]

- Chris Wawer

- Ruby on Rails Developer

- Improving legacy applications

- https://wawer.it

---

# Service Object

```ruby
class FindMovie
  def initialize(title)
    @title = title
  end

def call
    Tmdb::Movie.find(@title)
  end
end
```

---

# Unit Testing

---

# External dependencies

Dealing with external dependencies:

```ruby
# stubbing
allow(Tmdb::Movie).to receive(:find).with(...)

# mocking
movie_service = double(:movie_service)
allow(movie_service).to receive(:find).with(...)
```

---

---

# Usefule Objects

- dependency injection
- single responsibility
- ...

---

# Service Object

```ruby
class FindMovie
  def initialize(title, movie_service)
    @title = title
    @movie_service = movie_service
  end

def call
    @movie_service.call(@title)
  end
end
```

Everything OK?

---

# Incorrect DI

```ruby
class FindMovie
  def initialize(title, movie_service)
    @title = title
    @movie_service = movie_service
  end

def call
    @movie_service.call(@title)
  end
end

FindMovie.new("Ruby on Rails: The Documentary", nil)
```

---

# Usefule Objects

- dependency injection
- single responsibility
- don't allow to intialize object with empty dependency (1st law)
- ...

---

# Service Object

```ruby
class FindMovie
  attr_reader :title

dependency :movie_service, MovieService

def self.build(title)
    new(title).tap do |instance|
      MovieService.configure(instance)
    end
  end

def initialize(title)
    @title = title
  end

def call
    movie_service.call(title)
  end
end
```

---

# "Empty instance"

```ruby
find_movie = FindMovie.new
find_movie.movie_service # => #<Mimic::Class::MovieService_42:0x...>

find_movie.movie_service.call(title) # => #<Mimic::Void:0x...>

find_movie.movie_service.invoked?(:call)
```

---

# Substitutes

---

# Substitutes

```ruby
class FindMovie
  dependency :movie_service, MovieService
end

class MovieService
  # ...

module Substitute
    def self.build
      MovieService.new
    end

class MovieService < ::MovieService
      def call
        # ...
      end

def set(result)
        # ...
      end
    end
  end
end
```

---

# Substitutes

```ruby
class FindMovie
  dependency :movie_service, MovieService
end

class MovieService
  module Substitute
    def self.build
      MovieService.new
    end

class MovieService < ::MovieService
      include Mimic::Recorder::Predicate

record def call
        # ...
      end

def set(result)
        # ...
      end
    end
  end
end
```

---

# Questions ???