Introduce Module#delegate_missing_to

[gsamokovarov]

When building decorators, a common pattern may emerge:

class Partition
  def initialize(first_event)
    @events = [ first_event ]
  end

  def people
    if @events.first.detail.people.any?
      @events.collect { |e| Array(e.detail.people) }.flatten.uniq
    else
      @events.collect(&:creator).uniq
    end
  end

  private
    def respond_to_missing?(name, include_private = false)
      @events.respond_to?(name, include_private)
    end

    def method_missing(method, *args, &block)
      @events.send(method, *args, &block)
    end
end

With Module#delegate_missing_to, the above is condensed to:

class Partition
  delegate_missing_to :@events

  def initialize(first_event)
    @events = [ first_event ]
  end

  def people
    if @events.first.detail.people.any?
      @events.collect { |e| Array(e.detail.people) }.flatten.uniq
    else
      @events.collect(&:creator).uniq
    end
  end
end

David suggested it in #23824.

[rails-bot]

r? @schneems

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[gsamokovarov]

r? @dhh

[connorjacobsen]

Is it worth considering using public_send instead of send here?

[rafaelfranca]

I don't think so. The delegation should be fully. Even private methods should be delegated.

[matthewd]

@rafaelfranca that has some pretty questionable results -- e.g., external calls to the private Kernel methods will now work on this object. I guess you want local methods to be able to call private instance methods... but IMO, it seems fair to force those to be more explicit: we're composing around the object's public API, not injecting methods inside it. Arguably, it'd be weirder if we could call private methods, yet have a separate set of ivars.

[rafaelfranca]

oh, yeah, that is what I wanted. I agree we should use public_send here.

[maclover7]

#25133

[jeremy]

Comparing with stdlib Delegator, it sends to the target if it responds to the method (effectively a publis_send) and supers if not (so base method_missing deals with an unimplemented method rather than raising NoMethodError on our public_send).

[jeremy]

Seeing 3ac9956 switches to a respond_to? + public_send and supers.

[jeremy]

This explains what it does, but isn't enough guidance to decide whether to use delegate …, to: @events (explicitly delegate specific methods to a composed object), delegate_missing_to :@events (blanket-delegate public methods to a composed object, but don't act as a transparent proxy), or Ruby stdlib SimpleDelegator (full-class delegation to act as a transparent proxy) or Forwardable (single-method delegation like delegate …, to: …).

[gsamokovarov]

👍 Will get on it later on tonight, unless someone beats me to it.