{"id":4878,"date":"2010-11-10T12:41:53","date_gmt":"2010-11-10T04:41:53","guid":{"rendered":"http:\/\/ihower.tw\/blog\/?p=4878"},"modified":"2011-04-09T01:46:14","modified_gmt":"2011-04-08T17:46:14","slug":"rails3-activesupport-class_attribute","status":"publish","type":"post","link":"https:\/\/ihower.tw\/blog\/4878-rails3-activesupport-class_attribute","title":{"rendered":"\u6df1\u5165Rails3: ActiveSupport \u7684 class_attribute"},"content":{"rendered":"<p>\u5982\u679c\u4f60\u5c0d Ruby Object Model \u7a0d\u52a0\u8a8d\u8b58\uff0c\u5c31\u6703\u77e5\u9053\u9664\u4e86 class variable \u548c instance variable \u4e4b\u5916\uff0c\u9084\u6709\u4e00\u7a2e\u8b8a\u6578\u53eb\u505a class instance variable\uff0c\u4e4b\u524d\u6211\u5728\u7814\u7a76\u6642\u6709<a href=\"\/blog\/archives\/1696\">\u64b0\u6587\u89e3\u91cb<\/a>\u904e\uff0c\u8b80\u8005\u53ef\u4ee5\u8907\u7fd2\u4e00\u4e0b\u3002<\/p>\n<p>\u5728 Rails3 ActiveSupport Core Extension \u4e2d\uff0c\u5c31\u6709\u5e7e\u500b\u65b9\u6cd5\u662f\u5728\u8655\u7406\u9019\u4ef6\u4e8b\u60c5\uff0c\u8b93\u6211\u5011\u53ef\u4ee5\u5f88\u65b9\u4fbf\u5730\u5b9a\u7fa9\u5b58\u53d6\u65b9\u6cd5\uff0c\u8b93\u6211\u5011\u4f86\u770b\u770b\u3002<\/p>\n<h2>cattr_* \u7cfb\u5217<\/h2>\n<p>Ruby \u8a9e\u8a00\u672c\u8eab\u5c31\u6709\u91dd\u5c0d instance variable \u63d0\u4f9b attr_accessor, attr_reader, attr_writer \u7b49\u65b9\u6cd5\uff0c\u9019\u4e9b\u6703\u5efa\u7acb @ \u958b\u982d\u7684\u5be6\u4f8b\u8b8a\u6578\u4e26\u63d0\u4f9b\u5b58\u53d6\u65b9\u6cd5\u3002\u800c ActiveSupport \u7684\u9019\u500b\u64f4\u5145\u5247\u662f\u91dd\u5c0d class variable \u4e5f\u63d0\u4f9b\u985e\u4f3c\u7684\u529f\u80fd\uff0c\u5b83\u6703\u5efa\u7acb @@ \u958b\u982d\u7684\u985e\u5225\u8b8a\u6578\u53ca\u63d0\u4f9b\u5b58\u53d6\u65b9\u6cd5\u3002<\/p>\n<pre>\r\n<code>\r\nclass A\r\n  cattr_accessor :x\r\nend\r\n  \r\nclass B < A\r\nend\r\n  \r\nA.x = 1\r\nA.x # => 1\r\nB.x = 2\r\nB.x # => 2\r\nA.x # => 2 \u8ddf\u8457\u6539\u4e86\r\n<\/code>\r\n<\/pre>\n<p>\u6ce8\u610f\u5230\u6574\u500b\u7e7c\u627f\u9ad4\u7cfb A, B \u90fd\u5171\u7528\u4e86 @@x\uff0c\u6240\u4ee5\u5982\u679c\u6539\u4e86 B.x\uff0c\u90a3\u4e5f\u6703\u9023\u52d5 A.x\u3002\u5f88\u591a\u6642\u5019\uff0c\u9019\u4e0d\u662f\u6211\u5011\u8981\u7684\uff0c\u4f8b\u5982\u5728 Rails \u4e2d\u6240\u6709 Model \u90fd\u7e7c\u627f\u81ea ActiveRecord\uff0c\u65bc\u662f\u6703\u5171\u7528 class variable\uff0c\u5982\u679c\u8981\u5404\u81ea Model \u9700\u8981\u6709\u81ea\u5df1\u7684 class \u5c6c\u6027\u5c31\u4e0d\u5408\u7528\u4e86\u3002\u6240\u4ee5\u8aaa\u8a8d\u8b58 class instance variable \u53ef\u4ee5\u8aaa\u662f\u5beb ActiveRecord Plugin \u7684\u5fc5\u5099\u77e5\u8b58 (\u751a\u81f3\u4e5f\u6709\u4eba\u8aaa Ruby \u7684 class variable \u8a2d\u8a08\u932f\u8aa4\uff0c\u7576\u521d\u5c31\u61c9\u8a72\u628a\u884c\u70ba\u8a2d\u8a08\u6210 class instance variable \u6bd4\u8f03\u5be6\u7528)<\/p>\n<h2>class_attribute<\/h2>\n<p>\u4e0d\u50cf class variable \u6574\u500b\u7e7c\u627f\u9ad4\u7cfb\u5171\u7528\u985e\u5225\u8b8a\u6578\uff0cclass instance variable \u662f\u4e0d\u540c class \u5206\u5225\u7368\u7acb\u7684\uff0c\u4e5f\u5c31\u662f\u985e\u5225 A \u7684 class instance variable \u548c B < A \u7684 class instance variable \u662f\u7368\u7acb\u7684\u3002\n\n\n\n<pre>\r\n<code>\r\nclass A\r\n  @x = 1\r\nend\r\n\r\nclass B < A\r\nend\r\n\r\nA.instance_eval { @x } # => 1\r\nB.instance_eval { @x } # => nil \u56e0\u70ba\u8ddf A \u7684 @x \u662f\u7368\u7acb\u7684\uff0c\u4e0d\u6703\u7e7c\u627f\u4e0b\u4f86\r\n<\/code>\r\n<\/pre>\n<p>\u9019\u500b\u7279\u6027\u8b93\u6211\u5011\u53ef\u4ee5\u5be6\u4f5c\u51fa\u771f\u6b63\u5be6\u7528\u7684\u884c\u70ba\uff0c\u4e5f\u5c31\u662f &#8220;\u5c6c\u6027\u53ef\u4ee5\u7e7c\u627f\uff0c\u4f46\u662f\u5982\u679c\u6709\u4fee\u6539\uff0c\u4e0d\u6703\u5f71\u97ff\u5230 parent class \u7684\u503c&#8221;\u3002\u9019\u5c31\u662f ActiveSupport \u7684 class_attribute \u63d0\u4f9b\u7684\u529f\u80fd\uff1a<\/p>\n<pre>\r\n<code>\r\nclass A\r\n  class_attribute :x\r\nend\r\n  \r\nclass B < A\r\nend\r\n  \r\nA.x = 1\r\nB.x # => 1 \u7e7c\u627f\u81ea A\r\nB.x = 2\r\nB.x # => 2\r\nA.x # => 1 \u4e0d\u8b8a\r\n<\/code>\r\n<\/pre>\n<p>\u6700\u5f8c\u7684 A.x \u9084\u662f\u4fdd\u6301\u672c\u4f86\u7684\u503c\u4e0d\u53d7\u5f71\u97ff\u3002<\/p>\n<p>\u4e0d\u904e\u4f7f\u7528\u4e0a\u6709\u500b\u7d30\u7bc0\u8981\u6ce8\u610f\uff1a\u5982\u679c\u9019\u500b\u503c\u662f\u6703\u8b8a\u52d5\u7684\u7d50\u69cb(\u7269\u4ef6)\uff0c\u4f8b\u5982 Array \u6216 Hash\uff0c\u90a3\u9ebc child class \u7b2c\u4e00\u6b21\u4f7f\u7528\u6642\u5c31\u4e0d\u9069\u5408\u7528 in-place \u985e\u578b\u7684\u65b9\u6cd5\uff0c\u4f8b\u5982\uff1a<\/p>\n<pre>\r\n<code>\r\nA.x = []\r\nB.x << :foo\r\nA.x # => [:foo] \u4e5f\u8ddf\u8457\u6539\u4e86\uff0c\u4e0d\u5c0d\u554a\u554a\u554a!!\r\nB.x # => [:foo]\r\n<\/code>\r\n<\/pre>\n<p>\u8981\u6539\u6210\u7528 setter \u985e\u578b\u7684\u65b9\u6cd5:<\/p>\n<pre>\r\n<code>\r\nA.x = []\r\nB.x += :foo # \u7b2c\u4e00\u6b21\u8a2d\u5b9a\u5fc5\u9808\u4f7f\u7528 setter \u985e\u578b\u7684\u65b9\u6cd5\r\nA.x # => [] \u4e0d\u8b8a\r\nB.x # => [:foo]\r\n\r\nB.x << :bar\r\nA.x # => []\r\nB.x # => [:foo,:bar]\r\n<\/code>\r\n<\/pre>\n<p>\u6703\u9020\u6210\u9019\u7a2e\u884c\u70ba\u7684\u539f\u56e0\u662f\uff0cActiveSupport \u4e26\u4e0d\u662f\u8907\u88fd A.x \u7d66 B.x\uff0c\u800c\u662f\u5982\u679c B.x \u6c92\u8a2d\u5b9a\uff0c\u5c31\u53bb\u8b80 A.x (\u9019\u9ede\u8ddf\u4e0b\u8ff0\u7684 class_inheritable_* \u7528\u8907\u88fd\u7684\u4f5c\u6cd5\u5c31\u4e0d\u540c) \u3002<\/p>\n<p>ActiveSupport cattr_* \u9084\u63d0\u4f9b\u4e86 query \u662f\u5426\u70ba nil \u7684\u65b9\u6cd5\uff0c\u4e5f\u5c31\u662f A.x? \u548c B.x?<\/p>\n<p>\u6700\u5f8c\uff0cActiveSupport cattr_* \u7684\u884c\u70ba\u4e5f\u9069\u7528\u65bc\u5be6\u4f8b\u5316\u6642\uff0c\u4e0d\u6703\u5f71\u97ff\u5230 parent class\uff1a<\/p>\n<pre>\r\n<code>\r\nA.x = 1\r\nobject = A.new\r\nobject.x = 2 \r\nobject.x # => 2\r\nA.x # => 1 \u4fdd\u6301\u4e0d\u8b8a\r\n<\/code>\r\n<\/pre>\n<h2>class_inheritable_* \u7cfb\u5217<\/h2>\n<p>ActiveSupport \u9084\u6709\u4e00\u5957\u53e4\u65e9\u7684 class_inheritable_* \u65b9\u6cd5\uff0c\u5b83\u7684\u4f5c\u7528\u8ddf\u4e0a\u8ff0\u7684 class_attribute \u662f\u5dee\u4e0d\u591a\u7684\uff0c\u53ea\u662f\u5167\u90e8\u7684\u5be6\u4f5c\u4e0d\u540c\u3002\u662f\u8aaa class_attribute \u662f Rails3 \u624d\u65b0\u5beb\u7684\uff0c\u6548\u80fd\u8f03\u4f73\uff0c\u6703\u7559\u8457 class_inheritable_* \u4e3b\u8981\u662f\u56e0\u70ba\u5411\u4e0b\u76f8\u5bb9\u6027(\u6709\u5f88\u591a\u7684 Plugins \u4f7f\u7528\u4e86\u9019\u500b\u65b9\u6cd5)\u3002<\/p>\n<h2>\u53c3\u8003\u8cc7\u6599<\/h2>\n<ul>\n<li><a href=\"https:\/\/github.com\/rails\/rails\/tree\/master\/activesupport\/lib\/active_support\/core_ext\/class\/\">active_support\/core_ext\/class \u539f\u59cb\u78bc<\/a><\/li>\n<li><a href=\"http:\/\/blog.obiefernandez.com\/content\/2010\/04\/tr3w-highlights-activesupport-class-class-attribute.html\">The Rails 3 Way Highlights: ActiveSupport&#8217;s Class.class_attribute<\/a><\/li>\n<li><a href=\"http:\/\/tr3w.com\/\">The Rails3 Way: ActiveSupport API Reference<\/a><\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>\u5982\u679c\u4f60\u5c0d Ruby Object Model \u7a0d\u52a0\u8a8d\u8b58\uff0c\u5c31\u6703\u77e5\u9053\u9664\u4e86 class variable \u548c ins &hellip; <\/p>\n<p class=\"link-more\"><a href=\"https:\/\/ihower.tw\/blog\/4878-rails3-activesupport-class_attribute\" class=\"more-link\">\u95b1\u8b80\u5168\u6587<span class=\"screen-reader-text\">\u3008\u6df1\u5165Rails3: ActiveSupport \u7684 class_attribute\u3009<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[53,31],"tags":[],"class_list":["post-4878","post","type-post","status-publish","format-standard","hentry","category-rails","category-ruby","entry"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_shortlink":"https:\/\/wp.me\/p1q6tG-1gG","jetpack_sharing_enabled":true,"jetpack_likes_enabled":true,"_links":{"self":[{"href":"https:\/\/ihower.tw\/blog\/wp-json\/wp\/v2\/posts\/4878","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ihower.tw\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ihower.tw\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ihower.tw\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/ihower.tw\/blog\/wp-json\/wp\/v2\/comments?post=4878"}],"version-history":[{"count":25,"href":"https:\/\/ihower.tw\/blog\/wp-json\/wp\/v2\/posts\/4878\/revisions"}],"predecessor-version":[{"id":5508,"href":"https:\/\/ihower.tw\/blog\/wp-json\/wp\/v2\/posts\/4878\/revisions\/5508"}],"wp:attachment":[{"href":"https:\/\/ihower.tw\/blog\/wp-json\/wp\/v2\/media?parent=4878"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ihower.tw\/blog\/wp-json\/wp\/v2\/categories?post=4878"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ihower.tw\/blog\/wp-json\/wp\/v2\/tags?post=4878"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}