[ruby/prism] Dynamically register events to dispatch

Instead of requiring the consumer to provide a list of all events which
they wish to handle, we can give them to option of dynamically detecting
them, by scanning the listener's public methods.

This approach is similar to that used by Minitest (scanning for `test_`
methods) and Rails generators (running all public methods in the order
they are defined).

While this is slower than specifying a hard coded list, the penalty is
only during registration. There is no change the the behaviour of
dispatching the events.

ruby/prism@781ebed743

Author: sambostock

prism/templates/lib/prism/dispatcher.rb.erb

@@ -44,6 +44,19 @@ module Prism
     #
     # def register: (Listener, *Symbol) -> void
     def register(listener, *events)
+      register_events(listener, events)
+    end
+
+    # Register all public methods of a listener that match the pattern
+    # `on_<node_name>_(enter|leave)`.
+    #
+    # def register_public_methods: (Listener) -> void
+    def register_public_methods(listener)
+      register_events(listener, listener.public_methods(false).grep(/\Aon_.+_(?:enter|leave)\z/))
+    end
+
+    # Register a listener for the given events.
+    private def register_events(listener, events)
       events.each { |event| (listeners[event] ||= []) << listener }
     end
 

test/prism/ruby/dispatcher_test.rb

@@ -25,9 +25,12 @@ def on_integer_node_enter(node)
     end
 
     def test_dispatching_events
-      listener = TestListener.new
+      listener_manual = TestListener.new
+      listener_public = TestListener.new
+
       dispatcher = Dispatcher.new
-      dispatcher.register(listener, :on_call_node_enter, :on_call_node_leave, :on_integer_node_enter)
+      dispatcher.register(listener_manual, :on_call_node_enter, :on_call_node_leave, :on_integer_node_enter)
+      dispatcher.register_public_methods(listener_public)
 
       root = Prism.parse(<<~RUBY).value
         def foo
@@ -36,11 +39,17 @@ def foo
       RUBY
 
       dispatcher.dispatch(root)
-      assert_equal([:on_call_node_enter, :on_integer_node_enter, :on_integer_node_enter, :on_integer_node_enter, :on_call_node_leave], listener.events_received)
 
-      listener.events_received.clear
+      [listener_manual, listener_public].each do |listener|
+        assert_equal([:on_call_node_enter, :on_integer_node_enter, :on_integer_node_enter, :on_integer_node_enter, :on_call_node_leave], listener.events_received)
+        listener.events_received.clear
+      end
+
       dispatcher.dispatch_once(root.statements.body.first.body.body.first)
-      assert_equal([:on_call_node_enter, :on_call_node_leave], listener.events_received)
+
+      [listener_manual, listener_public].each do |listener|
+        assert_equal([:on_call_node_enter, :on_call_node_leave], listener.events_received)
+      end
     end
   end
 end