|
1 | | -# Learn-Rails-by-Reading-Source-Code |
| 1 | +# Learn-Rails-by-Reading-Source-Code |
| 2 | + |
| 3 | +### Before you research Rails 5 source code |
| 4 | +1) I suggest you learn Rack [http://rack.github.io/](http://rack.github.io/) first. You need to know that an object respond to `call` method is the most important convention. |
| 5 | + |
| 6 | +So which is the object with `call` method in Rails? |
| 7 | + |
| 8 | +I will answer this question later. |
| 9 | + |
| 10 | +2) You need a good IDE with debugging function. I use [RubyMine](https://www.jetbrains.com/). |
| 11 | + |
| 12 | +### Follow the process of Rails when starting |
| 13 | +As Rack described, `config.ru` is the entry file. |
| 14 | +```ruby |
| 15 | +# ./config.ru |
| 16 | +# This file is used by Rack-based servers to start the application. |
| 17 | + |
| 18 | +require_relative 'config/environment' |
| 19 | + |
| 20 | +run Rails.application # We can guess 'Rails.application' has a 'call' method. |
| 21 | + |
| 22 | +puts Rails.application.respond_to?(:call) # Returned 'true'. Bingo! |
| 23 | +``` |
| 24 | + |
| 25 | +Let's dig deeper for `Rails.application`. |
| 26 | +```ruby |
| 27 | +module Rails |
| 28 | + class << self |
| 29 | + @application = @app_class = nil |
| 30 | + |
| 31 | + attr_accessor :app_class |
| 32 | + def application # Oh, 'application' is a class method for module 'Rails'. It is not an object. |
| 33 | + # But it returns an object which is an instance of 'app_class'. |
| 34 | + # So it is important for us to know what class 'app_class' is. |
| 35 | + @application ||= (app_class.instance if app_class) |
| 36 | + end |
| 37 | + end |
| 38 | +end |
| 39 | +``` |
| 40 | + |
| 41 | +Because `Rails.application.respond_to?(:call) # Returned 'true'.`, `app_class.instance` has a `call` method. |
| 42 | + |
| 43 | +When was `app_class` set? |
| 44 | +```ruby |
| 45 | +module Rails |
| 46 | + class Application < Engine |
| 47 | + class << self |
| 48 | + def inherited(base) # This is a hooked method. |
| 49 | + Rails.app_class = base # This line set the 'app_class'. |
| 50 | + end |
| 51 | + end |
| 52 | + end |
| 53 | +end |
| 54 | +``` |
| 55 | +
|
| 56 | +When `Rails::Application` is inherited like below, |
| 57 | +```ruby |
| 58 | +# ./config/application.rb |
| 59 | +module YourProject |
| 60 | + class Application < Rails::Application # Here the hooked method `inherited` defined in eigenclass of 'Rails::Application' is invoked. |
| 61 | + end |
| 62 | +end |
| 63 | +``` |
| 64 | +`YourProject::Application` will become the `Rails.app_class`. Let's replace `app_class.instance` to `YourProject::Application.instance`. |
| 65 | +
|
| 66 | +But where is the `call` method? `call` method should be a method of `YourProject::Application.instance`. |
| 67 | +
|
| 68 | +Then Rack can `run YourProject::Application.new` (equal to `run Rails.application`). |
| 69 | +
|
| 70 | +The `call` method processes every request. Here it is. |
| 71 | +```ruby |
| 72 | +# ../gems/railties/lib/rails/engine.rb |
| 73 | +module Rails |
| 74 | + class Engine < Railtie |
| 75 | + def call(env) # This method will process every request. It is invoked by Rack. So it is very important. |
| 76 | + req = build_request env |
| 77 | + app.call req.env |
| 78 | + end |
| 79 | + end |
| 80 | +end |
| 81 | + |
| 82 | +# ../gems/railties/lib/rails/application.rb |
| 83 | +module Rails |
| 84 | + class Application < Engine |
| 85 | + end |
| 86 | +end |
| 87 | + |
| 88 | +# ./config/application.rb |
| 89 | +module YourProject |
| 90 | + class Application < Rails::Application |
| 91 | + end |
| 92 | +end |
| 93 | + |
| 94 | +``` |
| 95 | +
|
| 96 | +Ancestor's chain is `YourProject::Application < Rails::Application < Rails::Engine < Rails::Railtie`. |
| 97 | +
|
| 98 | +So `YourProject::Application.new.respond_to?(:call) # Will return 'true'`. |
| 99 | + |
| 100 | +But what does `app_class.instance` really do? |
| 101 | + |
| 102 | +`instance` is just a method name. What we really need is `app_class.new`. |
| 103 | + |
| 104 | +When I was reading these code |
| 105 | +```ruby |
| 106 | +# ../gems/railties/lib/rails/application.rb |
| 107 | +module Rails |
| 108 | + class Application < Engine |
| 109 | + def instance |
| 110 | + super.run_load_hooks! # This line confused me. |
| 111 | + end |
| 112 | + end |
| 113 | +end |
| 114 | +``` |
| 115 | +After a deep research, I realized that this code is equal to |
| 116 | +```ruby |
| 117 | +def instance |
| 118 | + a_returned_value = super # Keyword 'super' will call the ancestor's same name method: 'instance'. |
| 119 | + a_returned_value.run_load_hooks! |
| 120 | +end |
| 121 | +``` |
| 122 | + |
| 123 | +```ruby |
| 124 | +# ../gems/railties/lib/rails/railtie.rb |
| 125 | +module Rails |
| 126 | + class Railtie |
| 127 | + def instance |
| 128 | + @instance ||= new # 'Rails::Railtie' is the top ancestor. Now 'app_class.instance' is 'YourProject::Application.new'. |
| 129 | + end |
| 130 | + end |
| 131 | +end |
| 132 | +``` |
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