For complex use cases we may want to pass data to views in order to present it to our users. Hanami puts emphasis on explicitness: data isn’t shared between the controller action and the view unless we tell it to do so.

We use a simple and powerful mechanism to achieve our goal: exposures. Each exposure first looks for a method matching the given name. If no method is found, a getter is created with #attr_reader. Only the whitelisted exposures are made available to the corresponding view.

# apps/web/controllers/dashboard/index.rb
module Web
  module Controllers
    module Dashboard
      class Index
        include Web::Action
        expose :greeting
        def call(params)
          @greeting = "Hello"
          @foo      = 23
        end
      end
    end
  end
end

In the example above we have exposed :greeting, but not :foo. Only greeting can be used from the view and template.

# apps/web/views/dashboard/index.rb
module Web
  module View
    module Dashboard
      class Index
        include Web::View
        def welcome_message
          greeting + " and welcome"
        end
      end
    end
  end
end

If we try to use foo, Ruby will raise a NoMethodError.