Ruby/Rails类变量何时初始化?

此外,您不需要类变量。类实例变量足够好:

class Foo
   @available_ids = read_only_value
   def initialize
      @available_ids = self.class.instance_variable_get :@available_ids
   end
end

Rails具有内置的 cache ,因此沿着这些思路应该可以做到:(基于文档中的示例)

class Foo
  def available_ids
    Rails.cache.fetch('available_ids', expires_in: 24.hours) do
      API.fetch_ids
    end
  end
end

在你的代码中,API将在类被定义时被访问,这意味着当你启动应用程序时,它只能被访问一次。通过这个例子,您可以清楚地看到这一点:

puts "starting app"

def read_only_value
  puts "API accessed!"
  "my value"
end


class Foo
  @@available_ids = read_only_value
  def initialize
    @available_ids = @@available_ids
    puts "initializing with value: #{@available_ids}"
  end
end

puts "initializing two objects: "
Foo.new
Foo.new

#=> starting app
#=> API accessed!
#=> initializing two objects: 
#=> initializing with value: my value
#=> initializing with value: my value

当试图管理API访问时,最佳做法是将API封装在自己的类中。这样做可以独立于使用API的对象跟踪API调用和缓存值:

class API

  def self.instance
    @@instance ||= API.new
  end

  def read_only_value
    if @read_only_value.nil? || (Time.now - @cached_time) > 86400 # one day, in seconds
      puts "accessing api"
      @read_only_value = "access api here"
      @cached_time = Time.now
    end
    @read_only_value
  end

end


class Foo

  def initialize
    @available_ids = API.instance.read_only_value
    puts "initializing with value: #{@available_ids}"
  end

end

puts "initializing two objects: "

Foo.new
Foo.new

#=> initializing two objects: 
#=> accessing api
#=> initializing with value: access api here
#=> initializing with value: access api here