elgalu · May 9, 2019 08:39 · Mar 3, 2013 · Mar 3, 2013
diff --git a/write_expectation.rb b/write_expectation.rb
@@ -1,6 +1,15 @@
+require 'rspec'
 require 'stringio'
 
-# ref: http://greyblake.com/blog/2012/12/14/custom-expectations-with-rspec/
+# Custom matcher to test text written to standard output and standard error
+#
+# @example
+#   expect { $stderr.puts "Some random error" }.to write(/Some.* error/).to(:stderr)
+#
+# @example
+#   expect { $stderr.puts "Some specific error" }.to write('Some specific error').to(:stderr)
+#
+# @note http://greyblake.com/blog/2012/12/14/custom-expectations-with-rspec/
 RSpec::Matchers.define :write do |message|
   chain(:to) do |io|
     @io = io

diff --git a/write_expectation.rb b/write_expectation.rb
@@ -0,0 +1,64 @@
+require 'stringio'
+
+# ref: http://greyblake.com/blog/2012/12/14/custom-expectations-with-rspec/
+RSpec::Matchers.define :write do |message|
+  chain(:to) do |io|
+    @io = io
+  end
+
+  match do |block|
+    output =
+      case io
+      when :stdout then fake_stdout(&block)
+      when :stderr  then fake_stderr(&block)
+      else fail("Allowed values for `to` are :stdout and :stderr, got `#{io.inspect}`")
+      end
+
+    case message
+    when String then output.include? message
+    when Regexp then output.match message
+    else fail("Allowed types for write `message` are String or Regexp, got `#{message.class}`")
+    end
+  end
+
+  description do
+    %Q[write #{message.inspect} to #{@io}]
+  end
+
+  def failure_message(to = 'to')
+    %Q[expected #{to} #{description} but got #{@buffer.inspect}]
+  end
+
+  failure_message_for_should do
+    failure_message 'to'
+  end
+
+  failure_message_for_should_not do
+    failure_message 'not to'
+  end
+
+  # Fake STDERR and return a string written to it.
+  def fake_stderr
+    original_stderr = $stderr
+    $stderr = StringIO.new
+    yield
+    @buffer = $stderr.string
+  ensure
+    $stderr = original_stderr
+  end
+
+  # Fake STDOUT and return a string written to it.
+  def fake_stdout
+    original_stdout = $stdout
+    $stdout = StringIO.new
+    yield
+    @buffer = $stdout.string
+  ensure
+    $stdout = original_stdout
+  end
+
+  # default IO is standard output
+  def io
+    @io ||= :stdout
+  end
+end
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