Add new Feature class to toggle based on a closure

This enables toggling features as best fits us in each case. With this
new approach we can then toggle :customer_balance to an entire instance,
which is what we want in France.

config/initializers/feature_toggles.rb

@@ -1,5 +1,11 @@
 require 'open_food_network/feature_toggle'
 
-beta_testers = ENV['BETA_TESTERS']&.split(/[\s,]+/)
+beta_testers = ENV['BETA_TESTERS']&.split(/[\s,]+/) || []
 
-OpenFoodNetwork::FeatureToggle.enable(:customer_balance, beta_testers)
+OpenFoodNetwork::FeatureToggle.enable(:customer_balance) do |user|
+  if beta_testers == ['all']
+    true
+  else
+    beta_testers.include?(user.email)
+  end
+end

lib/open_food_network/feature_toggle.rb

@@ -30,11 +30,14 @@ module OpenFoodNetwork
       new.enabled?(feature_name, user)
     end
 
-    def self.enable(feature_name, user_emails)
-      return unless user_emails.present?
-
+    def self.enable(feature_name, user_emails = nil, &block)
       Thread.current[:features] ||= {}
-      Thread.current[:features][feature_name] = Feature.new(user_emails)
+
+      if user_emails.nil?
+        Thread.current[:features][feature_name] = BlockFeature.new(block)
+      else
+        Thread.current[:features][feature_name] = Feature.new(user_emails)
+      end
     end
 
     def initialize
@@ -86,4 +89,18 @@ module OpenFoodNetwork
       false
     end
   end
+
+  class BlockFeature
+    def initialize(block)
+      @block = block
+    end
+
+    def enabled?(user)
+      block.call(user)
+    end
+
+    private
+
+    attr_reader :block
+  end
 end

spec/initializers/feature_toggles_spec.rb

@@ -0,0 +1,47 @@
+require 'spec_helper'
+
+describe 'config/initializers/feature_toggles.rb' do
+  let(:user) { build(:user) }
+
+  around do |example|
+    original = ENV['BETA_TESTERS']
+    example.run
+    ENV['BETA_TESTERS'] = original
+  end
+
+  context 'when beta_testers is ["all"]' do
+    before { ENV['BETA_TESTERS'] = 'all' }
+
+    it 'returns true' do
+      require './config/initializers/feature_toggles' # execute the initializer's code block
+
+      enabled = OpenFoodNetwork::FeatureToggle.enabled?(:customer_balance, user)
+      expect(enabled).to eq(true)
+    end
+  end
+
+  context 'when beta_testers is a list of emails' do
+    context 'and the user is in the list' do
+      let(:other_user) { build(:user) }
+      before { ENV['BETA_TESTERS'] = "#{user.email}, #{other_user.email}" }
+
+      it 'enables the feature' do
+        require './config/initializers/feature_toggles' # execute the initializer's code block
+
+        enabled = OpenFoodNetwork::FeatureToggle.enabled?(:customer_balance, user)
+        expect(enabled).to eq(true)
+      end
+    end
+
+    context 'and the user is not in the list' do
+      before { ENV['BETA_TESTERS'] = '' }
+
+      it 'disables the feature' do
+        require './config/initializers/feature_toggles' # execute the initializer's code block
+
+        enabled = OpenFoodNetwork::FeatureToggle.enabled?(:customer_balance, user)
+        expect(enabled).to eq(false)
+      end
+    end
+  end
+end

spec/lib/open_food_network/feature_toggle_spec.rb

@@ -48,5 +48,31 @@ module OpenFoodNetwork
         end
       end
     end
+
+    context 'when passing in a block' do
+      let(:user) { build(:user) }
+
+      context 'and the block does not specify arguments' do
+        before do
+          FeatureToggle.enable(:foo) { 'return value' }
+        end
+
+        it "returns the block's return value" do
+          expect(FeatureToggle.enabled?(:foo, user)).to eq('return value')
+        end
+      end
+
+      context 'and the block specifies arguments' do
+        let(:users) { [user.email] }
+
+        before do
+          FeatureToggle.enable(:foo) { |user| users.include?(user.email) }
+        end
+
+        it "returns the block's return value" do
+          expect(FeatureToggle.enabled?(:foo, user)).to eq(true)
+        end
+      end
+    end
   end
 end