Allow adding a subclass of an already added formatter

[argos83]

duplicate_formatter_exists? in formatters.rb prevents the user from adding the same formatter for the same output twice by checking the formatter instance's class. However the check is done in a way that it doesn't only avoid the same class to be added but also subclasses.

If a user implements a more specific formatter. E.g. a SpecializedJsonFormatter which extends from RSpec::Core::Formatters::JsonFormatter and adds some extra functionality, and he/she wants to register both then, depending on the registration order, the SpecializedJsonFormatter may not be added.

For instance:

RSpec.configure do |config|
  ....
  formatters.each { |formatter| config.add_formatter(formatter) }
  ....

Will work for:

formatters = [SpecializedJsonFormatter, RSpec::Core::Formatters::JsonFormatter]

But not for:

formatters = [RSpec::Core::Formatters::JsonFormatter, SpecializedJsonFormatter]

I believe both cases used to work with RSpec 3.2.0

[JonRowe]

I think I'd prefer this if the classes were declared as local variables / lets inline rather than using ones prepared in another file, especially as for this spec it's completely unnecessary to have an implementation.

e.g.

a_general_formatter = Struct.new(:output)
a_more_specific_formatter = Class.new(a_general_formatter)

before { loader.add a_general_formatter, output }

it "adds a subclass of that formatter for the same output target" do
  expect {
    loader.add a_more_specific_formatter, output
  }.to change { loader.formatters.length }
end

[argos83]

I've tried that, but it doesn't work. None of the formatters are added.

and if you do instead

generic_formatter = BaseFormatter
specific_formatter = Class.new(generic_formatter)

Only the generic formatter is added (looks like classes don't get added unless they've registered).

The following works though (and doesn't require the custom formatter classes)

specific_formatter = RSpec::Core::Formatters::JsonFormatter
generic_formatter = specific_formatter.superclass

I'll add those changes.

[JonRowe]

Ah yep, you do need to register otherwise you're considered a legacy formatter, still the new way is better, thanks!

[JonRowe]

Good catch @argos83, I'd like to refactor the specs a little before this is merged but it's looking good otherwise, thanks.

[JonRowe]

Picked to 3-3-maintenance