嘗試瞭解雙派遣模式

Question

我一直在努力訓練double-dispatch pattern並且很難。我終於嘗試了一個示例程序來幫助我理解。 Here's的要點。但後來我決定嘗試一下without Double dispatch，解決方案看起來不像平常那​​麼糟糕。我究竟做錯了什麼？

編輯：根據建議，我已發佈此問題here。保持此鏈接重定向。

Answer 1

在單個調度---你在大多數現代OO語言中看到---一種方法是根據單個對象的運行時類型調度。這顯示爲點運算符（在ruby，java，javascript等）或箭頭運算符（perl，C++）。

# look, ma single dispatch! 
# method on obj's run-time type that is called 
dentist.work_on(patient) 

雙調度，然後，將基於對象的運行時類型。這看起來有幾種方式;該方法應該活在哪個對象上？

# Hmm, this looks weird. 
# Is the method in to dentist.class or patient.class? 
(dentist, patient).do_dentistry() 


# okay, this looks more familiar; the method lives on obj1.class 
# This only works in static-typed languages which support double dispatch 
# in which you declare the type of the method parameters. 
dentist.work_on(patient) 

class Dentist 
    def work_on(Adult patient); ...; end 
    def work_on(Child patient); ...; end 
end 

像groovy這樣的語言有多個調度概括上面的第二個例子;他們在選擇運行哪種方法時考慮所有參數的運行時類型。有關groovy和多次調度，請參閱this blog post。

大多數現代OO語言只有單個派遣和多派遣派遣模式是試圖獲得多種派遣語言的好處。它甚至適用於像Ruby這樣的動態語言。它通過連續執行單個調度兩次而工作。第一個方法調用將調用第二個對象的方法。

class Dentist 
    def work_on(patient) 
     patient.dispatch_work(self) 
    end 
    def work_on_adult(patient) 
     drill_as_hard_as_you_can(patient) 
    end 
    def work_on_child(patient) 
     use_bubble_gum_toothpaste(patient) 
     give_toothbrush_to(patient) 
    end 
end 

class Doctor 
    def work_on(patient) 
     patient.dispatch_work(self) 
    end 
    def work_on_adult(patient) 
     do_checkup(patient) 
    end 
    def work_on_child(patient) 
     assure_presence_of(patient.guardian) 
     ask_questions_to(patient.guardian) 
     do_checkup(patient) 
     give_cheap_toy_to(patient) 
    end 
end 



class Adult 
    def dispatch_work(dentist) 
     dentist.work_on_adult(self) 
    end 
end 

class Child 
    def dispatch_work(dentist) 
     dentist.work_on_child(self) 
    end 
end 

雙重派遣模式就是我所說的一個低級別的模式，因爲其他的模式是建立在它。例如，訪問者模式嚴重依賴於雙重調度模式。

---

更新只看見自己的要旨。你的第一要義並不是真的在做雙重調度。當然，你要派遣兩次，但你並沒有改變第二次派遣的行爲。把它改成雙派遣我會做這樣的事情。

class Chicken 
    def make_dispatch dish 
    dish.make_with_chicken self 
    end 
end 

class Beef 
    def make_dispatch dish 
    dish.make_with_beef self 
    end 
end 


module Dish 
    def make meat 
    meat.make_dispatch self 
    end 
end 

class Sandwich 
    include Dish 

    def make_with_chicken chicken 
    puts "Grilled Chicken Sandwich" 
    end 

    def make_with_beef beef 
    puts "Roast Beef Sandwich" 
    end 
end 

class Stew 
    include Dish 

    def make_with_chicken chicken 
    puts "Thai curry" 
    end 

    def make_with_beef beef 
    puts "Beef stew" 
    end 
end 

class Casserole 
    include Dish 

    def make_with_chicken chicken 
    puts "Chicken Pot Pie--or something" 
    end 

    def make_with_beef beef 
    puts "Shepard's Pie" 
    end 
end 

Sandwich.new.make(Chicken.new) 
Stew.new.make(Chicken.new) 
Casserole.new.make(Beef.new)