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中关村村草 发表于 2011-02-28 10:38

Ruby: 延迟计算与优化

转：LI Daobing


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

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