1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
|
# frozen_string_literal: true
require "active_support/core_ext/kernel/singleton_class"
require "active_support/core_ext/module/redefine_method"
require "active_support/core_ext/array/extract_options"
class Class
# Declare a class-level attribute whose value is inheritable by subclasses.
# Subclasses can change their own value and it will not impact parent class.
#
# ==== Options
#
# * <tt>:instance_reader</tt> - Sets the instance reader method (defaults to true).
# * <tt>:instance_writer</tt> - Sets the instance writer method (defaults to true).
# * <tt>:instance_accessor</tt> - Sets both instance methods (defaults to true).
# * <tt>:instance_predicate</tt> - Sets a predicate method (defaults to true).
# * <tt>:default</tt> - Sets a default value for the attribute (defaults to nil).
#
# ==== Examples
#
# class Base
# class_attribute :setting
# end
#
# class Subclass < Base
# end
#
# Base.setting = true
# Subclass.setting # => true
# Subclass.setting = false
# Subclass.setting # => false
# Base.setting # => true
#
# In the above case as long as Subclass does not assign a value to setting
# by performing <tt>Subclass.setting = _something_</tt>, <tt>Subclass.setting</tt>
# would read value assigned to parent class. Once Subclass assigns a value then
# the value assigned by Subclass would be returned.
#
# This matches normal Ruby method inheritance: think of writing an attribute
# on a subclass as overriding the reader method. However, you need to be aware
# when using +class_attribute+ with mutable structures as +Array+ or +Hash+.
# In such cases, you don't want to do changes in place. Instead use setters:
#
# Base.setting = []
# Base.setting # => []
# Subclass.setting # => []
#
# # Appending in child changes both parent and child because it is the same object:
# Subclass.setting << :foo
# Base.setting # => [:foo]
# Subclass.setting # => [:foo]
#
# # Use setters to not propagate changes:
# Base.setting = []
# Subclass.setting += [:foo]
# Base.setting # => []
# Subclass.setting # => [:foo]
#
# For convenience, an instance predicate method is defined as well.
# To skip it, pass <tt>instance_predicate: false</tt>.
#
# Subclass.setting? # => false
#
# Instances may overwrite the class value in the same way:
#
# Base.setting = true
# object = Base.new
# object.setting # => true
# object.setting = false
# object.setting # => false
# Base.setting # => true
#
# To opt out of the instance reader method, pass <tt>instance_reader: false</tt>.
#
# object.setting # => NoMethodError
# object.setting? # => NoMethodError
#
# To opt out of the instance writer method, pass <tt>instance_writer: false</tt>.
#
# object.setting = false # => NoMethodError
#
# To opt out of both instance methods, pass <tt>instance_accessor: false</tt>.
#
# To set a default value for the attribute, pass <tt>default:</tt>, like so:
#
# class_attribute :settings, default: {}
def class_attribute(*attrs)
options = attrs.extract_options!
instance_reader = options.fetch(:instance_accessor, true) && options.fetch(:instance_reader, true)
instance_writer = options.fetch(:instance_accessor, true) && options.fetch(:instance_writer, true)
instance_predicate = options.fetch(:instance_predicate, true)
default_value = options.fetch(:default, nil)
attrs.each do |name|
singleton_class.silence_redefinition_of_method(name)
define_singleton_method(name) { nil }
singleton_class.silence_redefinition_of_method("#{name}?")
define_singleton_method("#{name}?") { !!public_send(name) } if instance_predicate
ivar = "@#{name}"
singleton_class.silence_redefinition_of_method("#{name}=")
define_singleton_method("#{name}=") do |val|
singleton_class.class_eval do
redefine_method(name) { val }
end
if singleton_class?
class_eval do
redefine_method(name) do
if instance_variable_defined? ivar
instance_variable_get ivar
else
singleton_class.send name
end
end
end
end
val
end
if instance_reader
redefine_method(name) do
if instance_variable_defined?(ivar)
instance_variable_get ivar
else
self.class.public_send name
end
end
redefine_method("#{name}?") { !!public_send(name) } if instance_predicate
end
if instance_writer
redefine_method("#{name}=") do |val|
instance_variable_set ivar, val
end
end
unless default_value.nil?
self.send("#{name}=", default_value)
end
end
end
end
|