RSpec ships with a number of useful Expression Matchers. An Expression Matcher is any object that responds to the following methods: 

  matches?(actual)
  failure_message
  negative_failure_message #optional
  description #optional

See Spec::Expectations to learn how to use these as Expectation Matchers. See Spec::Mocks to learn how to use them as Mock Argument Constraints.

Predicates

In addition to those Expression Matchers that are defined explicitly, RSpec will create custom Matchers on the fly for any arbitrary predicate, giving your specs a much more natural language feel.

A Ruby predicate is a method that ends with a "?" and returns true or false. Common examples are empty?, nil?, and instance_of?.

All you need to do is write +should be_+ followed by the predicate without the question mark, and RSpec will figure it out from there. For example: 

  [].should be_empty => [].empty? #passes
  [].should_not be_empty => [].empty? #fails

In addtion to prefixing the predicate matchers with "be_", you can also use "be_a_" and "be_an_", making your specs read much more naturally: 

  "a string".should be_an_instance_of(String) =>"a string".instance_of?(String) #passes

  3.should be_a_kind_of(Fixnum) => 3.kind_of?(Numeric) #passes
  3.should be_a_kind_of(Numeric) => 3.kind_of?(Numeric) #passes
  3.should be_an_instance_of(Fixnum) => 3.instance_of?(Fixnum) #passes
  3.should_not be_instance_of(Numeric) => 3.instance_of?(Numeric) #fails

RSpec will also create custom matchers for predicates like has_key?. To use this feature, just state that the object should have_key(:key) and RSpec will call has_key?(:key) on the target. For example: 

  {:a => "A"}.should have_key(:a) => {:a => "A"}.has_key?(:a) #passes
  {:a => "A"}.should have_key(:b) => {:a => "A"}.has_key?(:b) #fails

You can use this feature to invoke any predicate that begins with "has_", whether it is part of the Ruby libraries (like +Hash#has_key?+) or a method you wrote on your own class.

Custom Expectation Matchers

When you find that none of the stock Expectation Matchers provide a natural feeling expectation, you can very easily write your own.

For example, imagine that you are writing a game in which players can be in various zones on a virtual board. To specify that bob should be in zone 4, you could say: 

  bob.current_zone.should eql(Zone.new("4"))

But you might find it more expressive to say: 

  bob.should be_in_zone("4")

and/or 

  bob.should_not be_in_zone("3")

To do this, you would need to write a class like this: 

  class BeInZone
    def initialize(expected)
      @expected = expected
    end
    def matches?(target)
      @target = target
      @target.current_zone.eql?(Zone.new(@expected))
    end
    def failure_message
      "expected #{@target.inspect} to be in Zone #{@expected}"
    end
    def negative_failure_message
      "expected #{@target.inspect} not to be in Zone #{@expected}"
    end
  end

… and a method like this: 

  def be_in_zone(expected)
    BeInZone.new(expected)
  end

And then expose the method to your specs. This is normally done by including the method and the class in a module, which is then included in your spec: 

  module CustomGameMatchers
    class BeInZone
      ...
    end

    def be_in_zone(expected)
      ...
    end
  end

  describe "Player behaviour" do
    include CustomGameMatchers
    ...
  end

or you can include in globally in a spec_helper.rb file required from your spec file(s): 

  Spec::Runner.configure do |config|
    config.include(CustomGameMatchers)
  end
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