Don't fallback to polling

If the user has specified `Spring.watch_method = :listen`, but listen is
not available, just let it throw a `LoadError`.

Gemfile

@@ -4,5 +4,3 @@ source 'https://rubygems.org'
 gemspec
 
 gem 'listen', :require => false
-gem 'rb-inotify', :require => false
-gem 'rb-fsevent', :require => false

lib/spring/watcher.rb

@@ -1,36 +1,30 @@
 require "spring/watcher/abstract"
-require "spring/watcher/listen"
-require "spring/watcher/polling"
 
 module Spring
   class << self
-    attr_accessor :watch_interval, :watch_method
+    attr_accessor :watch_interval
     attr_writer :watcher
+    attr_reader :watch_method
+  end
+
+  def self.watch_method=(method)
+    case method
+    when :polling
+      require_relative "watcher/polling"
+      @watch_method = Watcher::Polling
+    when :listen
+      require_relative "watcher/listen"
+      @watch_method = Watcher::Listen
+    else
+      @watch_method = method
+    end
   end
 
   self.watch_interval = 0.2
   self.watch_method = :polling
 
   def self.watcher
-    @watcher ||= watcher_class.new(Spring.application_root_path, watch_interval)
-  end
-
-  def self.watcher_class
-    if watch_method.to_s == 'listen'
-      if Watcher::Listen.available?
-        Watcher::Listen
-      else
-        puts %Q{Listen gem was not found, please add this to your Gemfile. `gem 'listen', group: ['test','development']`
-Falling back to Wather::Polling}
-        Watcher::Polling
-      end
-    elsif watch_method.to_s == 'polling'
-      Watcher::Polling
-    elsif watch_method.kind_of?(Class)
-      watch_method
-    else
-      raise NotImplementedError
-    end
+    @watcher ||= watch_method.new(Spring.application_root_path, watch_interval)
   end
 
   def self.watch(*items)

lib/spring/watcher/listen.rb

@@ -1,16 +1,11 @@
+require "listen"
+require "listen/version"
+
 module Spring
   module Watcher
     class Listen < Abstract
       attr_reader :listener
 
-      def self.available?
-        require "listen"
-        require "listen/version"
-        true
-      rescue LoadError
-        false
-      end
-
       def start
         unless @listener
           @listener = ::Listen.to(*base_directories, relative_paths: false)

test/apps/rails-3-2/Gemfile

@@ -4,7 +4,4 @@ gem 'rails', '~> 3.2.0'
 gem 'sqlite3'
 gem 'rspec'
 
-# gem 'listen'
-gem 'rb-inotify', :require => false  # linux
-gem 'rb-fsevent', :require => false  # mac os x
-gem 'rb-kqueue',  :require => false  # bsd
+# gem 'listen', '~> 1.0'

test/unit/watcher_test.rb

@@ -3,8 +3,8 @@
 require "fileutils"
 require "active_support/core_ext/numeric/time"
 require "spring/watcher"
-
-raise "listen not loaded" unless Spring::Watcher::Listen.available?
+require "spring/watcher/polling"
+require "spring/watcher/listen"
 
 module WatcherTests
   LATENCY = 0.001
@@ -141,8 +141,7 @@ class ListenWatcherTest < ActiveSupport::TestCase
   include WatcherTests
 
   def watcher_class
-    Spring.watch_method = :listen
-    Spring.watcher_class
+    Spring::Watcher::Listen
   end
 
   test "root directories" do
@@ -168,16 +167,6 @@ class PollingWatcherTest < ActiveSupport::TestCase
   include WatcherTests
 
   def watcher_class
-    Spring.watch_method = :polling
-    Spring.watcher_class
-  end
-end
-
-class CustomWatcherTest < ActiveSupport::TestCase
-  class DummyListener; end
-
-  def test_watcher_class
-    Spring.watch_method = DummyListener
-    assert_equal Spring.watcher_class, DummyListener
+    Spring::Watcher::Polling
   end
 end