如何在子類中的方法之前和之後運行代碼？

Question

我的第一個想法是有些事情是這樣的：

class AbstractBuilder 
    attr_reader :time_taken 

    def build_with_timer 
    started_at = Time.now 
    build 
    @time_taken = Time.now - started_at 
    end 

    def build 
    raise 'Implement this method in a subclass' 
    end 
end 

class MyBuilder < AbstractBuilder 
    def build 
    sleep(5) 
    end 
end 

builder = MyBuilder.new.build_with_timer 
puts builder.time_taken 

我懷疑有它提供了更好的靈活性，比如理想，我想稱之爲「打造」的MyBuilder的一個實例，而不是一個更好的辦法'build_with_timer'並始終記錄執行時間。

我確實考慮過使用alias_method從初始化或甚至使用模塊mixin代替類繼承，它會覆蓋在中間調用super的構建方法（不知道這是否可行）。在我放下兔子洞之前，我想我會看看是否有一個確定的練習。

Answer 1

我與alias_method玩：

module Timeable 
    def time_methods *meths 
    meths.each do |meth| 
     alias_method "old_#{meth}", meth 

     define_method meth do |*args| 
     started_at = Time.now 
     res = send "old_#{meth}", *args 
     puts "Execution took %f seconds" % (Time.now - started_at) 
     res 
     end 
    end 
    end 

end 

class Foo 
    def bar str 
    puts str 
    end 
end 

Foo.extend Timeable 
Foo.time_methods :bar 
Foo.new.bar('asd') 
#=>asd 
#=>Execution took 0.000050 seconds 

Answer 2

聽起來就像您正在尋找掛鉤到對象生命週期事件。您必須將其構建到您的基礎對象中，並提供一個小小的DSL - 我認爲您是在類似ActiveRecord Callbacks之後。下面是我們如何修改例如允許這樣的事情：

class AbstractBuilder 
    attr_reader :time_taken 

    def construct! # i.e., build, and also call your hooks 
    @@prebuild.each { |sym| self.send(sym) } 
    build 
    @@postbuild.each { |sym| self.send(sym) } 
    end 

    def construct_with_timer 
    started_at = Time.now 
    construct! 
    @time_taken = Time.now - started_at 

    puts "!!! Build time: #@time_taken" 
    end 

    class << self 
    def before_build(fn); @@prebuild ||= []; @@prebuild << fn; end 
    def after_build(fn); @@postbuild ||= []; @@postbuild << fn; end 
    end 
end 

class MyBuilder < AbstractBuilder 
    before_build :preprocess 
    after_build :postprocess 

    def build; puts "BUILDING"; sleep(3); end 
    def preprocess; puts "Preparing to build..."; end 
    def postprocess; puts "Done building. Thank you for waiting."; end 
end 

builder = MyBuilder.new 
builder.construct_with_timer 

# => Preparing to build... 
# => BUILDING 
# => Done building. Thank you for waiting. 
# => !!! Build time: 3.000119 

Answer 3

我有一個刺在一個版本，以達到你想要的東西。該版本不要求子類有任何額外的代碼。

class AbstractBuilder 

    @@disable_override = false 

    def before_method 
    puts "before" 
    end 

    def after_method 
    puts "after" 
    end 

    def self.method_added name 
    unless @@disable_override 
     if name == :build 
     @@disable_override = true # to stop the new build method 
     self.send :alias_method, :sub_build, :build 
     self.send :remove_method, :build 
     self.send :define_method, :build do 
      before_method 
      sub_build 
      after_method 
     end 
     @@disable_override = false 
     else 
     puts "defining other method #{name}" 
     end 
    end 
    end 

end 

class MyBuilder < AbstractBuilder 

    def build 
    puts "starting build" 
    sleep(5) 
    puts "built." 
    end 

    def unnaffected_method 
    # this method won't get redefined 
    end 

end 

b = MyBuilder.new 
b.build 

輸出

defining other method unnaffected_method 
before 
starting build 
built. 
after 

Answer 4

這是一本教科書定義使用情況Aspect-Oriented Programming。它通常提供更清晰的關注點分離。在這個舞臺上，Ruby提供了Aquarium和AspectR。但是，您可能不想向項目添加其他依賴項。因此，您可能仍然考慮使用其他方法之一。