Add exercise 3 from chapter 14

Author: Bernard Pietraga

chapter_14/working_with_multiple_processes_3.exs

@@ -0,0 +1,47 @@
+defmodule Linker do
+  import :timer, only: [sleep: 1]
+
+  def notify(sender) do
+    send sender, "Hello Kermit!"
+    exit(:boom)
+  end
+
+  def read_message do
+    receive do
+      msg ->
+        IO.puts "MESSAGE RECIEVED #{inspect msg}"
+        read_message()
+    after 0 ->
+        IO.puts "No more messages in mailbox"
+    end
+  end
+
+  def run do
+    Process.flag(:trap_exit, true)
+    spawn_link(Linker, :notify, [self()])
+    :timer.sleep(500)
+    read_message()
+  end
+end
+
+
+ExUnit.start
+
+defmodule LinkerTest do
+  use ExUnit.Case
+  import ExUnit.CaptureIO
+
+  test "run returns proper messages" do
+    messages_string = """
+    MESSAGE RECIEVED \"Hello Kermit!\"
+    MESSAGE RECIEVED {:EXIT, #PID<0.95.0>, :boom}
+    No more messages in mailbox
+    """
+
+    expected = capture_io fn ->
+      Linker.run
+    end
+
+    assert expected == messages_string
+  end
+end