Rails源码研究之ActionView:三，form_helper

ActionView还带有许多helpers，让我们开发起来更快速更顺手，让我们来看看form中常用的一些helper方法

form_helper.rb:

module ActionView
  module Helpers
    module FormHelper

      def form_for(object_name, *args, &proc)
        raise ArgumentError, "Missing block" unless block_given?
        options = args.last.is_a?(Hash) ? args.pop : {}
        concat(form_tag(options.delete(:url) || {}, options.delete(:html) || {}), proc.binding)
        fields_for(object_name, *(args << options), &proc)
        concat('</form>', proc.binding)
      end

      def fields_for(object_name, *args, &block)
        raise ArgumentError, "Missing block" unless block_given?
        options = args.last.is_a?(Hash) ? args.pop : {}
        object  = args.first
        builder = options[:builder] || ActionView::Base.default_form_builder
        yield builder.new(object_name, object, self, options, block)
      end

      def text_field(object_name, method, options = {})
        InstanceTag.new(object_name, method, self, nil, options.delete(:object)).to_input_field_tag("text", options)
      end

      def password_field(object_name, method, options = {})
        InstanceTag.new(object_name, method, self, nil, options.delete(:object)).to_input_field_tag("password", options)
      end

      def hidden_field(object_name, method, options = {})
        InstanceTag.new(object_name, method, self, nil, options.delete(:object)).to_input_field_tag("hidden", options)
      end

      def file_field(object_name, method, options = {})
        InstanceTag.new(object_name, method, self, nil, options.delete(:object)).to_input_field_tag("file", options)
      end

      def text_area(object_name, method, options = {})
        InstanceTag.new(object_name, method, self, nil, options.delete(:object)).to_text_area_tag(options)
      end

      def check_box(object_name, method, options = {}, checked_value = "1", unchecked_value = "0")
        InstanceTag.new(object_name, method, self, nil, options.delete(:object)).to_check_box_tag(options, checked_value, unchecked_value)
      end

      def radio_button(object_name, method, tag_value, options = {})
        InstanceTag.new(object_name, method, self, nil, options.delete(:object)).to_radio_button_tag(tag_value, options)
      end

    end

    class InstanceTag

      def to_input_field_tag(field_type, options = {})
        options = options.stringify_keys
        options["size"] ||= options["maxlength"] || DEFAULT_FIELD_OPTIONS["size"]
        options = DEFAULT_FIELD_OPTIONS.merge(options)
        if field_type == "hidden"
          options.delete("size")
        end
        options["type"] = field_type
        options["value"] ||= value_before_type_cast(object) unless field_type == "file"
        add_default_name_and_id(options)
        tag("input", options)
      end

      def to_radio_button_tag(tag_value, options = {})
        options = DEFAULT_RADIO_OPTIONS.merge(options.stringify_keys)
        options["type"]     = "radio"
        options["value"]    = tag_value
        if options.has_key?("checked")
          cv = options.delete "checked"
          checked = cv == true || cv == "checked"
        else
          checked = self.class.radio_button_checked?(value(object), tag_value)
        end
        options["checked"]  = "checked" if checked
        pretty_tag_value    = tag_value.to_s.gsub(/\s/, "_").gsub(/\W/, "").downcase
        options["id"]     ||= defined?(@auto_index) ?             
          "#{@object_name}_#{@auto_index}_#{@method_name}_#{pretty_tag_value}" :
          "#{@object_name}_#{@method_name}_#{pretty_tag_value}"
        add_default_name_and_id(options)
        tag("input", options)
      end

      def to_text_area_tag(options = {})
        options = DEFAULT_TEXT_AREA_OPTIONS.merge(options.stringify_keys)
        add_default_name_and_id(options)
        if size = options.delete("size")
          options["cols"], options["rows"] = size.split("x")
        end
        content_tag("textarea", html_escape(options.delete('value') || value_before_type_cast(object)), options)
      end

      def to_check_box_tag(options = {}, checked_value = "1", unchecked_value = "0")
        options = options.stringify_keys
        options["type"]     = "checkbox"
        options["value"]    = checked_value
        if options.has_key?("checked")
          cv = options.delete "checked"
          checked = cv == true || cv == "checked"
        else
          checked = self.class.check_box_checked?(value(object), checked_value)
        end
        options["checked"] = "checked" if checked
        add_default_name_and_id(options)
        tag("input", options) << tag("input", "name" => options["name"], "type" => "hidden", "value" => unchecked_value)
      end

    end

    class FormBuilder

      class_inheritable_accessor :field_helpers
      self.field_helpers = (FormHelper.instance_methods - ['form_for'])

      attr_accessor :object_name, :object, :options

      def initialize(object_name, object, template, options, proc)
        @object_name, @object, @template, @options, @proc = object_name, object, template, options, proc        
      end
      
      (field_helpers - %w(check_box radio_button)).each do |selector|
        src = <<-end_src
          def #{selector}(method, options = {})
            @template.send(#{selector.inspect}, @object_name, method, options.merge(:object => @object))
          end
        end_src
        class_eval src, __FILE__, __LINE__
      end
      
      def check_box(method, options = {}, checked_value = "1", unchecked_value = "0")
        @template.check_box(@object_name, method, options.merge(:object => @object), checked_value, unchecked_value)
      end
      
      def radio_button(method, tag_value, options = {})
        @template.radio_button(@object_name, method, tag_value, options.merge(:object => @object))
      end

    end

  end

  class Base
    cattr_accessor :default_form_builder
    self.default_form_builder = ::ActionView::Helpers::FormBuilder
  end

end

form_for方式引入，form_tag可以丢弃了，因为form_for是对form_tag更简洁更高级的封装

<% form_for :person, @person, :url => { :action => "update" } do |f| %>
  First name: <%= f.text_field :first_name %>
  Last name : <%= f.text_field :last_name %>
  Biography : <%= f.text_area :biography %>
  Admin?    : <%= f.check_box :admin %>
<% end %>

而text_field/password_field/hidden_field/file_field/text_area/check_box/radio_button以及form_options_helpers里的select这八项
form里的基本元素都有方便的helper方法定义，实际上是通过InstanceTag的to_input_field_tag/to_text_area_tag等方法来生成html标签
而InstanceTag的to_input_field_tag/to_text_area_tag等方法又是通过tag_helper.rb里定义的content_tag方法来生成html标签，都很简单

我们可以定制自己的FormBuilder来满足form样式一样等要求，详见每天一剂Rails良药之Creating a Custom Form Builder