dependency_inversion.rb

# The Dependency Inversion Principle has to do with high-level (think business
# logic) objects not depending on low-level (think database querying and IO)
# implementation details. This can be achieved with duck typing and the
# Dependency Inversion Principle. Often this pattern is used to achieve the
# Open/Closed Principle that we discussed above. In fact, we can even reuse
# that same example as a demonstration of this principle.

# Now there is a formatter class, but I've hardcoded it on the Report class,
# thus creating a dependency from the Report to the JSONFormatter. Since the
# Report is a more abstract (high-level) concept than the JSONFormatter, we're
# effectively breaking the DIP.

class Report
  def body
    # Implementation
  end

  def print
    JSONFormatter.new.format(body)
  end
end

class JSONFormatter
  def format(body)
    # Implementation
  end
end

# Solution

# This way the Report does not depend on the JSONFormatter and can use any type
# of formatter that has a method called format (this is known as duck typing).
# Another thing of note is that we've used, once again, dependency injection to
# solve a problem. This technique is a very powerful one when our goal is
# decoupling objects, and even though it has the same initials as the dependency
# inversion principle (vs dependency injection pattern), they are completely
# different concepts.

class Report
  def body
    # Implementation
  end

  def print(formatter: JSONFormatter.new)
    formatter.format body
  end
end