Cease holding references to listeners

The `@listeners` variable is global state that holds a reference to each
`Listener` instance and can only be cleared by calling `Listen.stop`.
This prevents individual listeners from being garbage collected if they
are abandoned before `Listen.stop` is called.

This commit replaces `@listeners` with a comparable call to
`ObjectSpace.each_object`.

Partially addresses #476.

lib/listen.rb

@@ -32,21 +32,16 @@ class << self
     # @return [Listen::Listener] the listener
     #
     def to(*args, &block)
-      @listeners ||= []
-      Listener.new(*args, &block).tap do |listener|
-        @listeners << listener
-      end
+      Listener.new(*args, &block)
     end
 
     # This is used by the `listen` binary to handle Ctrl-C
     #
     def stop
       Internals::ThreadPool.stop
-      @listeners ||= []
 
       # TODO: should use a mutex for this
-      @listeners.each(&:stop)
-      @listeners = nil
+      ObjectSpace.each_object(Listener, &:stop)
     end
   end
 end

spec/lib/listen/listener_spec.rb

@@ -79,6 +79,12 @@
     allow(block).to receive(:call)
   end
 
+  after do
+    allow(backend).to receive(:stop)
+    allow(processor).to receive(:teardown)
+    subject.stop
+  end
+
   describe 'initialize' do
     it { should_not be_paused }
 

spec/lib/listen_spec.rb

@@ -1,23 +1,15 @@
 RSpec.describe Listen do
-  let(:listener) { instance_double(Listen::Listener, stop: nil) }
-
-  after do
-    Listen.stop
-  end
-
   describe '.to' do
-    it 'initalizes listener' do
-      expect(Listen::Listener).to receive(:new).with('/path') { listener }
+    it 'initializes listener' do
+      expect(Listen::Listener).to receive(:new).with('/path')
       described_class.to('/path')
     end
   end
 
   describe '.stop' do
     it 'stops all listeners' do
-      allow(Listen::Listener).to receive(:new).with('/path') { listener }
-      expect(listener).to receive(:stop)
-      described_class.to('/path')
-      Listen.stop
+      expect(Listen::Listener.allocate).to receive(:stop)
+      described_class.stop
     end
   end
 end