Ruby面試精選30題 - Day10 Ruby invoke method調用方法?
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?
Explain how Ruby looks up a method to invoke?
每當不知從何下筆時，我的起手式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 | ||= method |
(or-equals)條件判斷 a||=b是 a || a = b縮寫，意思為條件運算式 a ? a : a = b |
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。
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裡的方法.protected及private
day6.protected #=> undefined method `protected' (NoMethodError)
day6.private #=> undefined method `private' (NoMethodError)
在[Day 7]Symbol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，我決定用英文整理出這10天的學習紀錄：
| 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 | If object is nil, true, or false, it returns NilClass, TrueClass, or FalseClass. (可以直接存取，而不需要實體化此類別的物件。) |
| 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| (TrueClass method) |
Or—Returns true. As obj is an argument to a method call, it is always evaluated |
感想：
為了做Ruby如何invoke method的表格，我竟然把Ruby API的Object ，Kernal ，BasicObject 頁面翻閱了一遍，了解輸入的參數怎麼用、已經輸出的物件會是什麼形式。這是我過去從來沒想過自己可以做得到的事（感覺翻手冊是高手才看得懂的境界啊！）經過這10天成長收穫巨大！
明天來繼續研究更多method!!!! :)
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