Class.html

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title: Class
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            <span>Ruby on Rails 8.0.0</span><br />
        
        <div class="type">Class</div>
        <h1>
            Class
            
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                    <a href="Object.html">Object</a>
                    
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        <ul class="files">
            
            <li><a href="../files/activesupport/lib/active_support/core_ext/class/attribute_rb.html">activesupport/lib/active_support/core_ext/class/attribute.rb</a></li>
            
            <li><a href="../files/activesupport/lib/active_support/core_ext/class/subclasses_rb.html">activesupport/lib/active_support/core_ext/class/subclasses.rb</a></li>
            
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    <h2 id="methods">Methods</h2>
    <ul>
      
        <li>
          <a href="#method-i-class_attribute">class_attribute</a>
        </li>
      
        <li>
          <a href="#method-i-descendants">descendants</a>
        </li>
      
    </ul>
  

  

  
    

    

    

    

    <!-- Methods -->
    
    
      <h2 id="instance-public-methods">Instance Public methods</h2>
      
        <div class="method">
          <h3 id="method-i-class_attribute">
            
              class_attribute(*attrs, instance_accessor: true, instance_reader: instance_accessor, instance_writer: instance_accessor, instance_predicate: true, default: nil)
            
          </h3>

          
            <div class="description">
              <p>Declare a class-level attribute whose value is inheritable by subclasses. Subclasses can change their own value and it will not impact parent class.</p>

<h4 id="method-i-class_attribute-label-Options">Options</h4>
<ul><li>
<p><code>:instance_reader</code> - Sets the instance reader method (defaults to true).</p>
</li><li>
<p><code>:instance_writer</code> - Sets the instance writer method (defaults to true).</p>
</li><li>
<p><code>:instance_accessor</code> - Sets both instance methods (defaults to true).</p>
</li><li>
<p><code>:instance_predicate</code> - Sets a predicate method (defaults to true).</p>
</li><li>
<p><code>:default</code> - Sets a default value for the attribute (defaults to nil).</p>
</li></ul>

<h4 id="method-i-class_attribute-label-Examples">Examples</h4>

<pre><code>class Base
  class_attribute :setting
end

class Subclass &lt; Base
end

Base.setting = true
Subclass.setting            # =&gt; true
Subclass.setting = false
Subclass.setting            # =&gt; false
Base.setting                # =&gt; true
</code></pre>

<p>In the above case as long as Subclass does not assign a value to setting by performing <code>Subclass.setting = <em>something</em></code>, <code>Subclass.setting</code> would read value assigned to parent class. Once Subclass assigns a value then the value assigned by Subclass would be returned.</p>

<p>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 <code>class_attribute</code> with mutable structures as <code>Array</code> or <code>Hash</code>. In such cases, you don’t want to do changes in place. Instead use setters:</p>

<pre><code>Base.setting = []
Base.setting                # =&gt; []
Subclass.setting            # =&gt; []

# Appending in child changes both parent and child because it is the same object:
Subclass.setting &lt;&lt; :foo
Base.setting               # =&gt; [:foo]
Subclass.setting           # =&gt; [:foo]

# Use setters to not propagate changes:
Base.setting = []
Subclass.setting += [:foo]
Base.setting               # =&gt; []
Subclass.setting           # =&gt; [:foo]
</code></pre>

<p>For convenience, an instance predicate method is defined as well. To skip it, pass <code>instance_predicate: false</code>.</p>

<pre><code>Subclass.setting?       # =&gt; false
</code></pre>

<p>Instances may overwrite the class value in the same way:</p>

<pre><code>Base.setting = true
object = Base.new
object.setting          # =&gt; true
object.setting = false
object.setting          # =&gt; false
Base.setting            # =&gt; true
</code></pre>

<p>To opt out of the instance reader method, pass <code>instance_reader: false</code>.</p>

<pre><code>object.setting          # =&gt; NoMethodError
object.setting?         # =&gt; NoMethodError
</code></pre>

<p>To opt out of the instance writer method, pass <code>instance_writer: false</code>.</p>

<pre><code>object.setting = false  # =&gt; NoMethodError
</code></pre>

<p>To opt out of both instance methods, pass <code>instance_accessor: false</code>.</p>

<p>To set a default value for the attribute, pass <code>default:</code>, like so:</p>

<pre><code>class_attribute :settings, default: {}
</code></pre>
            </div>
          

          

          

          
          
            <details class="method__source">
              <summary>
                <span class="label">📝 Source code</span>
              </summary>

              <pre><code class="ruby"># File activesupport/lib/active_support/core_ext/class/attribute.rb, line 86
  def class_attribute(*attrs, instance_accessor: true,
    instance_reader: instance_accessor, instance_writer: instance_accessor, instance_predicate: true, default: nil)

    class_methods, methods = [], []
    attrs.each do |name|
      unless name.is_a?(Symbol) || name.is_a?(String)
        raise TypeError, &quot;#{name.inspect} is not a symbol nor a string&quot;
      end

      name = name.to_sym
      ::ActiveSupport::ClassAttribute.redefine(self, name, default)

      unless singleton_class?
        methods &lt;&lt; &lt;&lt;~RUBY if instance_reader
          silence_redefinition_of_method def #{name}
            defined?(@#{name}) ? @#{name} : self.class.#{name}
          end
        RUBY
      end

      methods &lt;&lt; &lt;&lt;~RUBY if instance_writer
        silence_redefinition_of_method(:#{name}=)
        attr_writer :#{name}
      RUBY

      if instance_predicate
        class_methods &lt;&lt; &quot;silence_redefinition_of_method def #{name}?; !!self.#{name}; end&quot;
        if instance_reader
          methods &lt;&lt; &quot;silence_redefinition_of_method def #{name}?; !!self.#{name}; end&quot;
        end
      end
    end

    location = caller_locations(1, 1).first
    class_eval([&quot;class &lt;&lt; self&quot;, *class_methods, &quot;end&quot;, *methods].join(&quot;;&quot;).tr(&quot;\n&quot;, &quot;;&quot;), location.path, location.lineno)
  end</code></pre>
              
                <a href="https://github.com/rails/rails/blob/dd8f7185faeca6ee968a6e9367f6d8601a83b8db/activesupport/lib/active_support/core_ext/class/attribute.rb#L86" target="_blank" class="github_url">🔎 See on GitHub</a>
              
            </details>
          
        </div>
        
        <div class="method">
          <h3 id="method-i-descendants">
            
              descendants()
            
          </h3>

          
            <div class="description">
              <p>Returns an array with all classes that are &lt; than its receiver.</p>

<pre><code>class C; end
C.descendants # =&gt; []

class B &lt; C; end
C.descendants # =&gt; [B]

class A &lt; B; end
C.descendants # =&gt; [B, A]

class D &lt; C; end
C.descendants # =&gt; [B, A, D]
</code></pre>
            </div>
          

          

          

          
          
            <details class="method__source">
              <summary>
                <span class="label">📝 Source code</span>
              </summary>

              <pre><code class="ruby"># File activesupport/lib/active_support/core_ext/class/subclasses.rb, line 19
  def descendants
    subclasses.concat(subclasses.flat_map(&amp;:descendants))
  end</code></pre>
              
                <a href="https://github.com/rails/rails/blob/dd8f7185faeca6ee968a6e9367f6d8601a83b8db/activesupport/lib/active_support/core_ext/class/subclasses.rb#L19" target="_blank" class="github_url">🔎 See on GitHub</a>
              
            </details>
          
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