Add Avoid Dynamically Dispatching Private Methods as a Ruby TIL

ruby/avoid-dynamically-dispatching-private-methods.md

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+# Avoid Dynamically Dispatching Private Methods
+
+Ruby makes it easy to call any method on an object with the
+[`Object#send`](https://docs.ruby-lang.org/en/4.0/Object.html#method-i-send)
+method. It's powerful because I can dynamically call methods (e.g. based on a
+variable that is being passed in) and I can even dispatch private methods which
+is normally disallowed.
+
+```ruby
+obj.send(:initialize, new_args)
+```
+
+If I want the benefits of dynamic dispatch without allow private methods to be
+called, then I can instead reach for the
+[`Object#public_send`](https://docs.ruby-lang.org/en/4.0/Object.html#method-i-public_send)
+method.
+
+Here is a basic `Invoice` class to demonstrate:
+
+```ruby
+class Invoice
+  TAX_RATE = 0.08
+
+  def initialize(subtotal:)
+    @subtotal = subtotal
+  end
+
+  def total
+    (@subtotal + tax).round(2)
+  end
+
+  private
+
+  def tax
+    @subtotal * TAX_RATE
+  end
+end
+
+invoice1 = Invoice.new(subtotal: 250)
+
+puts "Total: #{invoice1.send(:total)}"
+puts "Tax: #{invoice1.send(:tax)}"
+
+# this will error
+invoice1.public_send(:tax)
+```
+
+When I run this file, the two `send` statements both work while the
+`public_send` on a private method raises:
+
+```bash
+❯ ruby object_send.rb
+Total: 270.0
+Tax: 20.0
+object_send.rb:25:in 'Kernel#public_send': private method 'tax' called for an instance of Invoice (NoMethodError)
+
+invoice1.public_send(:tax)
+        ^^^^^^^^^^^^
+Did you mean?  tap
+	from object_send.rb:25:in '<main>'
+```
+
+Where this behavior might be most useful is in a Rails _concern_ file where I am
+defining some shared behavior that needs to work dynamically across classes that
+are including it.
+
+```ruby
+module ValidatesAttachment
+  extend ActiveSupport::Concern
+
+  IMAGE_TYPES = %w[image/gif image/jpeg image/jpg image/png].freeze
+  MAX_SIZE = 5.megabytes
+
+  class_methods do
+    def validates_attachment(name, content_types: IMAGE_TYPES, max_size: MAX_SIZE, types_message: nil)
+      validate do
+        attachment = public_send(name)
+
+        # ... validation logic omitted
+      end
+    end
+  end
+end
+```