Allow to pass initializer arguments to the encoder of a flow

lib/rackstash/flow.rb

@@ -178,17 +178,17 @@ module Rackstash
     # Get or set the encoder for the log {#adapter}. If this value is not
     # explicitly defined, it defaults to the #{adapter}'s default encoder.
     #
-    # @param encoder [#encode, nil] if given, set the flow's encoder to this
-    #   object
+    # @param encoder_spec (see Rackstash::Encoder.build)
+    # @param encoder_args (see Rackstash::Encoder.build)
+    # @param block (see Rackstash::Encoder.build)
     # @raise [TypeError] if the given `encoder` does not respond to the `encode`
     #   method
     # @return [#encode] the newly set encoder (if given) or the currently
     #   defined one
     # @see #encoder=
-    def encoder(encoder = nil)
-      return @encoder if encoder.nil?
-      self.encoder = encoder
-      @encoder
+    def encoder(encoder_spec = nil, *encoder_args, &block)
+      return @encoder if encoder_spec.nil?
+      @encoder = Rackstash::Encoder.build(encoder_spec, *encoder_args, &block)
     end
 
     # Set the encoder for the log {#adapter}. You can use any object which

spec/rackstash/flow_spec.rb

@@ -211,6 +211,18 @@ RSpec.describe Rackstash::Flow do
       # The encoder is persisted and is returned afterwards
       expect(flow.encoder).to equal encoder
     end
+
+    it 'allows to set an encoder spec' do
+      expect { flow.encoder(:json) }.to change { flow.encoder }
+        .from(instance_of(Rackstash::Encoder::Raw))
+        .to(instance_of(Rackstash::Encoder::JSON))
+    end
+
+    it 'allows to set arguments to an encoder' do
+      flow.encoder(:message, tagged: ['tags'])
+      expect(flow.encoder).to be_instance_of(Rackstash::Encoder::Message)
+      expect(flow.encoder.tagged).to eql ['tags']
+    end
   end
 
   describe '#encoder=' do