Expose out and err for commands

lib/dry/cli.rb

@@ -97,6 +97,10 @@ def call(arguments: ARGV, out: $stdout, err: $stderr)
     # @api private
     def perform_command(arguments)
       command, args = parse(kommand, arguments, [])
+
+      command.instance_variable_set(:@err, err) unless command.instance_variable_defined?(:@err)
+      command.instance_variable_set(:@out, out) unless command.instance_variable_defined?(:@out)
+
       command.call(**args)
     end
 
@@ -113,6 +117,9 @@ def perform_registry(arguments)
 
       command, args = parse(result.command, result.arguments, result.names)
 
+      command.instance_variable_set(:@err, err) unless command.instance_variable_defined?(:@err)
+      command.instance_variable_set(:@out, out) unless command.instance_variable_defined?(:@out)
+
       result.before_callbacks.run(command, args)
       command.call(**args)
       result.after_callbacks.run(command, args)

lib/dry/cli/command.rb

@@ -379,6 +379,39 @@ def self.superclass_options
         optional_arguments
         subcommands
       ] => "self.class"
+
+      protected
+
+      # The error output used to print error messaging
+      #
+      # @example
+      #   class MyCommand
+      #     def call
+      #       out.puts "Hello World!"
+      #       exit(0)
+      #     rescue StandardError => e
+      #       err.puts "Uh oh: #{e.message}"
+      #       exit(1)
+      #     end
+      #   end
+      #
+      # @since unreleased
+      # @return [IO]
+      attr_reader :err
+
+      # The standard output object used to print messaging
+      #
+      # @example
+      #   class MyCommand
+      #     def call
+      #       out.puts "Hello World!"
+      #       exit(0)
+      #     end
+      #   end
+      #
+      # @since unreleased
+      # @return [IO]
+      attr_reader :out
     end
   end
 end

spec/unit/dry/cli/cli_spec.rb

@@ -183,5 +183,45 @@
         )
       end
     end
+
+    it "exposes @out and @err to command without overriding pre-existing ivars" do
+      # @out and @err are exposed by default
+      command_class = Class.new(Dry::CLI::Command) do
+        def call(**)
+          @out.puts "out"
+          @err.puts "err"
+        end
+      end
+      cli = Dry.CLI(command_class.new)
+
+      default_out = StringIO.new
+      default_err = StringIO.new
+      cli.call(arguments: [], out: default_out, err: default_err)
+
+      expect(default_out.string).to eq("out\n")
+      expect(default_err.string).to eq("err\n")
+
+      # @out and @err do not override pre-existing ivars
+      custom_command_class = Class.new(command_class) do
+        define_method(:initialize) do |out, err|
+          super()
+          @out = out
+          @err = err
+        end
+      end
+
+      custom_out = StringIO.new
+      custom_err = StringIO.new
+      cli = Dry.CLI(custom_command_class.new(custom_out, custom_err))
+
+      default_out = StringIO.new
+      default_err = StringIO.new
+      cli.call(arguments: [], out: default_out, err: default_err)
+
+      expect(custom_out.string).to eq("out\n")
+      expect(custom_err.string).to eq("err\n")
+      expect(default_out.string).to eq("")
+      expect(default_err.string).to eq("")
+    end
   end
 end