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