Raise argument error when passing a file path

The Listen gem doesn't support listening on changes of individual files,
but currently if we pass a file path to `Listen.on`, the listener is
successfully initialized, without actually listening on given files. To
improve clarity, we notify the user they need to pass a directory by
raising an argument error.

lib/listen/adapter/config.rb

@@ -16,6 +16,12 @@ def initialize(directories, queue, silencer, adapter_options)
           Pathname.new(directory.to_s).realpath
         end
 
+        @directories.each do |pathname|
+          unless pathname.directory?
+            fail ArgumentError, "must be a directory: #{pathname}"
+          end
+        end
+
         @silencer = silencer
         @queue = queue
         @adapter_options = adapter_options

spec/lib/listen/adapter/config_spec.rb

@@ -17,6 +17,7 @@
   # Here's what may be  passed to initializer
   let(:path1) { fake_path('/real/path1', realpath: real_path1) }
   let(:path2) { fake_path('/real/path2', realpath: real_path2) }
+  let(:path3) { fake_path('/real/path3', realpath: real_path3) }
 
   let(:current_path) do
     fake_path('/real/current_path', realpath: real_current_path)
@@ -29,6 +30,7 @@
   # something useful)
   let(:real_path1) { fake_path('/real/path1') }
   let(:real_path2) { fake_path('/real/path2') }
+  let(:real_path3) { fake_path('/real/path3', directory?: false) }
   let(:real_current_path) { fake_path('/real/current_path') }
 
   before do
@@ -38,6 +40,7 @@
 
     allow(Pathname).to receive(:new).with('/real/path1').and_return(path1)
     allow(Pathname).to receive(:new).with('/real/path2').and_return(path2)
+    allow(Pathname).to receive(:new).with('/real/path3').and_return(path3)
 
     allow(Pathname).to receive(:new).with(path1).and_return(path1)
     allow(Pathname).to receive(:new).with(path2).and_return(path2)
@@ -90,6 +93,13 @@
         expect(subject.directories).to eq([real_current_path])
       end
     end
+
+    context 'with file path' do
+      let(:directories) { ['/real/path3'] }
+      it 'raises argument error requesting a directory' do
+        expect { subject }.to raise_error(ArgumentError, /must be a directory/)
+      end
+    end
   end
 
   describe '#adapter_options' do

spec/spec_helper.rb

@@ -38,7 +38,7 @@ def ci?
 
 module SpecHelpers
   def fake_path(str, options = {})
-    instance_double(Pathname, str, { to_s: str }.merge(options))
+    instance_double(Pathname, str, { to_s: str, directory?: true }.merge(options))
   end
 end