Day10 - Ruby 如何調用方法(invoke method)?

2019鐵人賽 ruby method invoke

貓貓工程師 / PM Ting

2018-10-13 05:23:43

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前情提要:

不知不覺到了第10天囉！。鐵人賽進度1/3（挺胸！）接下來應該會進入學習的深水區，但是我會越戰越勇。路遙知馬力，日久見人心！?‍♀️

Ruby經典面試題目 #10

Ruby如何引入方法? Please explain how Ruby looks up a method to invoke?

每當不知從何下筆時，起手式就是開始回顧之前的文章，盤點我已經走了多遠、以及基礎觀念到底掌握了多少：

鐵人賽足跡	面試問題	我的總結
Day 1	class	class建立物件實體，以method和資料互動
Day 2	class與 module	class可繼承，module不可繼承
Day 3	module	module裡的method可被include和extend
Day 4	instance method與class method	include用於instance method, extend用於class method
Day 5	self物件與singleton method	singleton method是singleton class的instence method
Day 6	public, protected, private method	在classs外無法取得protected或private method
Day 7	symbol與string	`:符號` symbol class的物件實體，object_id相同 / `字串:` string變數指向字串物件，object_id不同
Day 8	`concat` 與 `+=` method	以`concat`串接， object_id相同 / `+=` object_id不同
Day 9	`

洋洋灑灑地條列出這麼多方法之後，我們好奇的問，

Ruby到底是怎麼尋找這些方法的呢？

Ruby最先尋找的地方是物件的eigenclass（特徵類別，物件上層的隱藏類別)method會直接定義在裡面，如同Day 5提到的singleton method(類別方法)。

如果Ruby沒有辦法在物件的eigenclass找到，它會尋找此物件class所屬的上一層(ancestor)class、層層往上搜尋，深入到Object、到Kernal、最後去BasicObject搜尋method是否在裡面。

圖片來源

如果都找不到Method的話呢？

不用擔心，Ruby就像Google Map一樣給予提示，它會在內部搜尋另一個:method_missing method給這個物件的class，提供Ruby工程師解bug的線索：

undefined method `某方法名稱' (NoMethodError)

這個線索我們並不陌生，因為我們已經有多次經驗了：

在Day 6，無法使用class裡的.protected及.private方法

day6.protected #=> undefined method `protected' (NoMethodError)
day6.private #=> undefined method `private' (NoMethodError)

在Day 7Symbol找不到[]=方法

:tingsmessage[1]= "Z"
#undefined method `[]=' for :tingsmessage:Symbol (NoMethodError)

我從https://ruby-doc.org/core-2.5.1/列出我在前十篇文章所用到的實體方法(Public Instance Methods)，整理表格如下：

Object	Kernal	BasicObject
`class` →class	`puts`(obj, ...) → nil	`object_id` →integer
`extend`(module) →obj	`String(arg)` → string	`send`(symbol [, args...]) → obj
`singleton_method`(sym) → method	`Hash(arg)` → hash	`new()`這個是Public Class Methods！

為了更清楚釐清自己的觀念，以及未來尋找海外工作、面試語言的需要，我決定用英文整理出這10天的學習紀錄，並附上手冊連結：

觀念	解釋
class	Classes in Ruby are first-class objects.
module	Modules serve two purposes in Ruby, namespacing and mix-in functionality.
class method	Class methods (methods on a module) may be called directly.
instance method	Instance methods defined in a module are only callable when included.
include	when the module is included, istance methods appear as methods in a class. (module methods do not.)
extend	Adds to obj the instance methods from each module given as a parameter.
self	Self refers to the object that defines the current scope. (it will change when it a different method or a new module) .
singleton class	Returns an array of the names of singleton methods for obj. (If object is nil, true, or false, it returns NilClass, TrueClass, or FalseClass.)
singleton method	The behavior of an instance is determined by its class.
public method	With no arguments, sets the default visibility for subsequently defined methods to public. With arguments, sets the named methods to have public visibility.
projected method	If a method has protected visibility, it is callable only where self of the context is the same as the method.
private method	With no arguments, sets the default visibility for subsequently defined methods to private. With arguments, sets the named methods to have private visibility.
symbol	Symbol objects represent names and some strings inside the Ruby interpreter.
string	A String object holds and manipulates an arbitrary sequence of bytes
concat (string method)	Concatenates the given object(s) to str. If an object is an Integer, it is converted to a character before concatenation.
+ (string method)	Concatenation — Returns a new String containing
[true`	`](https://ruby-doc.com/core/String.html#method-i-2B) (TrueClass method)