From 5c58e9f7116fcb8d0c826b453e093b00a6d76ba1 Mon Sep 17 00:00:00 2001
From: kwatch <kwatch@gmail.com>
Date: Sat, 19 Nov 2016 09:01:16 +0900
Subject: [PATCH 3/9] feat(psych): add hook point for mapping key and value

This is necessary to generate custom object as mapping instead of Hash
according to context. For example:

    TableObj  = Struct.new('TableObj', 'name', 'columns')
    ColumnObj = Struct.new('ColumnObj', 'name', 'type', 'pkey')

    class CustomVisitor < Psych::Visitors::ToRuby
      def initialize(*args)
        super
        @key_path = []   # ex: [] -> ['tables'] -> ['tables', 'columns']
      end
      def accept_key(k)    # push keys
        key = super k
        @key_path << key
        return key
      end
      def accept_value(v)  # pop keys
        value = super v
        @key_path.pop()
        return value
      end
      def empty_mapping(o)  # generate custom object instead of Hash
        case @key_path.last
        when 'tables'   ; return TableObj.new
        when 'columns'  ; return ColumnObj.new
        else            ; return super o
        end
      end
    end

    ## example to generate custom object according to context
    input = <<'END'
    tables:
      - name: admin_users
        columns:
          - name:  id
            type:  int
            pkey:  true
    END
    tree = Psych.parse(input)
    visitor = CustomVisitor.create
    ydoc = visitor.accept(tree)
    p ydoc['tables'][0].class                #=> Struct::TableObj
    p ydoc['tables'][0]['columns'][0].class  #=> Struct::ColumnObj
---
 ext/psych/lib/psych/visitors/to_ruby.rb | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/ext/psych/lib/psych/visitors/to_ruby.rb b/ext/psych/lib/psych/visitors/to_ruby.rb
index 5ecb7e7..9eb11bf 100644
--- a/ext/psych/lib/psych/visitors/to_ruby.rb
+++ b/ext/psych/lib/psych/visitors/to_ruby.rb
@@ -336,11 +336,19 @@ def register_empty object
         list
       end
 
+      def accept_key k
+        accept(k)
+      end
+
+      def accept_value v
+        accept(v)
+      end
+
       SHOVEL = '<<'
       def revive_hash hash, o
         o.children.each_slice(2) { |k,v|
-          key = accept(k)
-          val = accept(v)
+          key = accept_key(k)
+          val = accept_value(v)
 
           if key == SHOVEL && k.tag != "tag:yaml.org,2002:str"
             case v
-- 
2.9.3 (Apple Git-75)