Ruby 的延遲計算大家已經(jīng)用得很多了, 下面就是一個范例

class ApplicationControllerdef current_user@current_user ||= User.where(:id => session[:user_id]).firstend endclass FooController < ApplicationControllerdef foocurrent_usercurrent_userend end

但這個范例有一個問題, 如果 current_user 不存在, 那么每次調(diào)用 current_user 時都會額外查詢一次數(shù)據(jù)庫, 優(yōu)化的方式就是用 instance_variable_defined? 先查詢一下變量是否已經(jīng)被定義, 測試范例如下

class Adef fooputs " expensive computing"nilenddef failed_lazy@foo ||= fooenddef successful_lazyreturn @bar if instance_variable_defined? "@bar"@bar = fooenddef testputs "use failed lazy computing"(1..10).each {failed_lazy}puts "use successful lazy computing"(1..10).each {successful_lazy}end endA.new.test

測試結(jié)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: 第一段的代碼范例有錯誤: "||" 應(yīng)該改為 "||="?

轉(zhuǎn)載于:https://www.cnblogs.com/lidaobing/archive/2011/02/26/optimize-lazy-evaluation-in-ruby.html

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