gateway.rb

# 單純用來決定該有哪些 interface 和一些是在我們這邊處理的邏輯
class PaymentGateway
  attr_reader :email

  SUBSCRIPTION_AMOUNT = 10.to_money

  def initialize(user)
    @email = user.email
  end

  def charge_for_subscription
    raise 'overwrite me'
  end

  def create_as_customer
    raise 'overwrite me'
  end
end

class PaymentGatewayFactory
  def initialize(user)
    # 假設user payment_method 就是存 BrainTree, 就看user 是用哪一種 braintree or stripe etc...
    user.payment_method.constantize.new(user)
  end
end

class BrainTree < PaymentGateway
  def charge_for_subscription
    braintree_id = BraintreeGem.find_user(email).braintree_id
    BraintreeGem.charge(braintree_id, SUBSCRIPTION_AMOUNT)
  end

  def create_as_customer
    BraintreeGem.create_customer(email)
  end
end

class Strip < PaymentGateway
  def charge_for_subscription
    # StripGem charge
  end

  def create_as_customer
    # StripGem create customer
  end
end

class User
  def charge_for_subscription
    PaymentGatewayFactory.new(user).charge_for_subscription
  end

  def create_as_customer
    PaymentGatewayFactory.new(user).create_as_customer
  end
end