Ruby: 延迟计算与优化

Ruby 的延迟计算大家已经用得很多了, 下面就是一个范例

class ApplicationController
  def current_user
    @current_user ||= User.where(:id => session[:user_id]).first
  end
end

class FooController < ApplicationController
  def foo
    current_user
    current_user
  end
end

但这个范例有一个问题, 如果 current_user 不存在, 那么每次调用 current_user 时都会额外查询一次数据库, 优化的方式就是用 instance_variable_defined? 先查询一下变量是否已经被定义, 测试范例如下

class A
  def foo
    puts "  expensive computing"
    nil
  end

  def failed_lazy
    @foo ||= foo
  end

  def successful_lazy
    return @bar if instance_variable_defined? "@bar"
    @bar = foo
  end

  def test
    puts "use failed lazy computing"
    (1..10).each {failed_lazy}
    puts "use successful lazy computing"
    (1..10).each {successful_lazy}
  end
end

A.new.test

测试结果如下

use failed lazy computing
  expensive computing
  expensive computing
  expensive computing
  expensive computing
  expensive computing
  expensive computing
  expensive computing
  expensive computing
  expensive computing
  expensive computing
use successful lazy computing
  expensive computing

• update1: 第一段的代码范例有错误: "||" 应该改为 "||="