Yield matchers

README.md

@@ -112,6 +112,21 @@ expect { ... }.to throw_symbol(:symbol)
 expect { ... }.to throw_symbol(:symbol, 'value')
 ```
 
+### Yielding
+
+```ruby
+expect { |b| 5.tap(&b) }.to yield_control # passes regardless of yielded args
+
+expect { |b| yield_if_true(true, &b) }.to yield_with_no_args # passes only if no args are yielded
+
+expect { |b| 5.tap(&b) }.to yield_with_args(5)
+expect { |b| 5.tap(&b) }.to yield_with_args(Fixnum)
+expect { |b| "a string".tap(&b) }.to yield_with_args(/str/)
+
+expect { |b| [1, 2, 3].each(&b) }.to yield_successive_args(1, 2, 3)
+expect { |b| { :a => 1, :b => 2 }.each(&b) }.to yield_successive_args([:a, 1], [:b, 2])
+```
+
 ### Predicate matchers
 
 ```ruby

features/.nav

@@ -19,6 +19,7 @@
   - satisfy.feature
   - throw_symbol.feature
   - types.feature
+  - yield.feature
 - custom_matchers:
   - define_matcher.feature
   - define_diffable_matcher.feature

features/built_in_matchers/yield.feature

@@ -0,0 +1,146 @@
+Feature: yield matchers
+
+  There are four related matchers that allow you to specify whether
+  or not a method yields, how many times it yields, whether or not
+  it yields with arguments, and what those arguments are.
+
+    * `yield_control` matches if the method-under-test yields, regardless
+      of whether or not arguments are yielded.
+    * `yield_with_args` matches if the method-under-test yields with
+      arguments. If arguments are provided to this matcher, it will
+      only pass if the actual yielded arguments match the expected ones
+      using `===` or `==`.
+    * `yield_with_no_args` matches if the method-under-test yields with
+      no arguments.
+    * `yield_successive_args` is designed for iterators, and will match
+      if the method-under-test yields the same number of times as arguments
+      passed to this matcher, and all actual yielded arguments match the
+      expected ones using `===` or `==`.
+
+  Note: your expect block _must_ accept an argument that is then passed on to
+  the method-under-test as a block. This acts as a "probe" that allows the matcher
+  to detect whether or not your method yields, and, if so, how many times and what
+  the yielded arguments are.
+
+  Background:
+    Given a file named "my_class.rb" with:
+      """
+      class MyClass
+        def self.yield_once_with(*args)
+          yield *args
+        end
+
+        def self.raw_yield
+          yield
+        end
+
+        def self.dont_yield
+        end
+      end
+      """
+
+  Scenario: yield_control matcher
+    Given a file named "yield_control_spec.rb" with:
+      """
+      require './my_class'
+
+      describe "yield_control matcher" do
+        specify { expect { |b| MyClass.yield_once_with(1, &b) }.to yield_control }
+        specify { expect { |b| MyClass.dont_yield(&b) }.not_to yield_control }
+
+        # deliberate failures
+        specify { expect { |b| MyClass.yield_once_with(1, &b) }.not_to yield_control }
+        specify { expect { |b| MyClass.dont_yield(&b) }.to yield_control }
+      end
+      """
+    When I run `rspec yield_control_spec.rb`
+    Then the output should contain all of these:
+      | 4 examples, 2 failures                      |
+      | expected given block to yield control       |
+      | expected given block not to yield control   |
+
+  Scenario: yield_with_args matcher
+    Given a file named "yield_with_args_spec.rb" with:
+      """
+      require './my_class'
+
+      describe "yield_with_args matcher" do
+        specify { expect { |b| MyClass.yield_once_with("foo", &b) }.to yield_with_args }
+        specify { expect { |b| MyClass.yield_once_with("foo", &b) }.to yield_with_args("foo") }
+        specify { expect { |b| MyClass.yield_once_with("foo", &b) }.to yield_with_args(String) }
+        specify { expect { |b| MyClass.yield_once_with("foo", &b) }.to yield_with_args(/oo/) }
+
+        specify { expect { |b| MyClass.yield_once_with("foo", "bar", &b) }.to yield_with_args("foo", "bar") }
+        specify { expect { |b| MyClass.yield_once_with("foo", "bar", &b) }.to yield_with_args(String, String) }
+        specify { expect { |b| MyClass.yield_once_with("foo", "bar", &b) }.to yield_with_args(/fo/, /ar/) }
+
+        specify { expect { |b| MyClass.yield_once_with("foo", "bar", &b) }.not_to yield_with_args(17, "baz") }
+
+        # deliberate failures
+        specify { expect { |b| MyClass.yield_once_with("foo", &b) }.not_to yield_with_args }
+        specify { expect { |b| MyClass.yield_once_with("foo", &b) }.not_to yield_with_args("foo") }
+        specify { expect { |b| MyClass.yield_once_with("foo", &b) }.not_to yield_with_args(String) }
+        specify { expect { |b| MyClass.yield_once_with("foo", &b) }.not_to yield_with_args(/oo/) }
+        specify { expect { |b| MyClass.yield_once_with("foo", "bar", &b) }.not_to yield_with_args("foo", "bar") }
+        specify { expect { |b| MyClass.yield_once_with("foo", "bar", &b) }.to yield_with_args(17, "baz") }
+      end
+      """
+    When I run `rspec yield_with_args_spec.rb`
+    Then the output should contain all of these:
+      | 14 examples, 6 failures                                                               |
+      | expected given block not to yield with arguments, but did                             |
+      | expected given block not to yield with arguments, but yielded with expected arguments |
+      | expected given block to yield with arguments, but yielded with unexpected arguments   |
+
+  Scenario: yield_with_no_args matcher
+    Given a file named "yield_with_no_args_spec.rb" with:
+      """
+      require './my_class'
+
+      describe "yield_with_no_args matcher" do
+        specify { expect { |b| MyClass.raw_yield(&b) }.to yield_with_no_args }
+        specify { expect { |b| MyClass.dont_yield(&b) }.not_to yield_with_no_args }
+        specify { expect { |b| MyClass.yield_once_with("a", &b) }.not_to yield_with_no_args }
+
+        # deliberate failures
+        specify { expect { |b| MyClass.raw_yield(&b) }.not_to yield_with_no_args }
+        specify { expect { |b| MyClass.dont_yield(&b) }.to yield_with_no_args }
+        specify { expect { |b| MyClass.yield_once_with("a", &b) }.to yield_with_no_args }
+      end
+      """
+    When I run `rspec yield_with_no_args_spec.rb`
+    Then the output should contain all of these:
+      | 6 examples, 3 failures                                                             |
+      | expected given block not to yield with no arguments, but did                       |
+      | expected given block to yield with no arguments, but did not yield                 |
+      | expected given block to yield with no arguments, but yielded with arguments: ["a"] |
+
+  Scenario: yield_successive_args matcher
+    Given a file named "yield_successive_args_spec.rb" with:
+      """
+      def array
+        [1, 2, 3]
+      end
+
+      def array_of_tuples
+        [[:a, :b], [:c, :d]]
+      end
+
+      describe "yield_successive_args matcher" do
+        specify { expect { |b| array.each(&b) }.to yield_successive_args(1, 2, 3) }
+        specify { expect { |b| array_of_tuples.each(&b) }.to yield_successive_args([:a, :b], [:c, :d]) }
+        specify { expect { |b| array.each(&b) }.to yield_successive_args(Fixnum, Fixnum, Fixnum) }
+        specify { expect { |b| array.each(&b) }.not_to yield_successive_args(1, 2) }
+
+        # deliberate failures
+        specify { expect { |b| array.each(&b) }.not_to yield_successive_args(1, 2, 3) }
+        specify { expect { |b| array_of_tuples.each(&b) }.not_to yield_successive_args([:a, :b], [:c, :d]) }
+        specify { expect { |b| array.each(&b) }.not_to yield_successive_args(Fixnum, Fixnum, Fixnum) }
+        specify { expect { |b| array.each(&b) }.to yield_successive_args(1, 2) }
+      end
+      """
+    When I run `rspec yield_successive_args_spec.rb`
+    Then the output should contain all of these:
+      | 8 examples, 4 failures                                                                             |
+      | expected given block not to yield successively with arguments, but yielded with expected arguments |
+      | expected given block to yield successively with arguments, but yielded with unexpected arguments   |

lib/rspec/matchers.rb

@@ -554,6 +554,88 @@ def throw_symbol(expected_symbol=nil, expected_arg=nil)
       BuiltIn::ThrowSymbol.new(expected_symbol, expected_arg)
     end
 
+    # Passes if the method called in the expect block yields, regardless
+    # of whether or not arguments are yielded.
+    #
+    # @example
+    #
+    #   expect { |b| 5.tap(&b) }.to yield_control
+    #   expect { |b| "a".to_sym(&b) }.not_to yield_control
+    #
+    # @note Your expect block must accept a parameter and pass it on to
+    #   the method-under-test as a block.
+    # @note This matcher is not designed for use with methods that yield
+    #   multiple times.
+    def yield_control
+      BuiltIn::YieldControl.new
+    end
+
+    # Passes if the method called in the expect block yields with
+    # no arguments. Fails if it does not yield, or yields with arguments.
+    #
+    # @example
+    #
+    #   expect { |b| User.transaction(&b) }.to yield_with_no_args
+    #   expect { |b| 5.tap(&b) }.not_to yield_with_no_args # because it yields with `5`
+    #   expect { |b| "a".to_sym(&b) }.not_to yield_with_no_args # because it does not yield
+    #
+    # @note Your expect block must accept a parameter and pass it on to
+    #   the method-under-test as a block.
+    # @note This matcher is not designed for use with methods that yield
+    #   multiple times.
+    def yield_with_no_args
+      BuiltIn::YieldWithNoArgs.new
+    end
+
+    # Given no arguments, matches if the method called in the expect
+    # block yields with arguments (regardless of what they are or how
+    # many there are).
+    #
+    # Given arguments, matches if the method called in the expect block
+    # yields with arguments that match the given arguments.
+    #
+    # Argument matching is done using `===` (the case match operator)
+    # and `==`. If the expected and actual arguments match with either
+    # operator, the matcher will pass.
+    #
+    # @example
+    #
+    #   expect { |b| 5.tap(&b) }.to yield_with_args # because #tap yields an arg
+    #   expect { |b| 5.tap(&b) }.to yield_with_args(5) # because 5 == 5
+    #   expect { |b| 5.tap(&b) }.to yield_with_args(Fixnum) # because Fixnum === 5
+    #   expect { |b| File.open("f.txt", &b) }.to yield_with_args(/txt/) # because /txt/ === "f.txt"
+    #
+    #   expect { |b| User.transaction(&b) }.not_to yield_with_args # because it yields no args
+    #   expect { |b| 5.tap(&b) }.not_to yield_with_args(1, 2, 3)
+    #
+    # @note Your expect block must accept a parameter and pass it on to
+    #   the method-under-test as a block.
+    # @note This matcher is not designed for use with methods that yield
+    #   multiple times.
+    def yield_with_args(*args)
+      BuiltIn::YieldWithArgs.new(*args)
+    end
+
+    # Designed for use with methods that repeatedly yield (such as
+    # iterators). Passes if the method called in the expect block yields
+    # multiple times with arguments matching those given.
+    #
+    # Argument matching is done using `===` (the case match operator)
+    # and `==`. If the expected and actual arguments match with either
+    # operator, the matcher will pass.
+    #
+    # @example
+    #
+    #   expect { |b| [1, 2, 3].each(&b) }.to yield_successive_args(1, 2, 3)
+    #   expect { |b| { :a => 1, :b => 2 }.each(&b) }.to yield_successive_args([:a, 1], [:b, 2])
+    #   expect { |b| [1, 2, 3].each(&b) }.not_to yield_successive_args(1, 2)
+    #
+    # @note Your expect block must accept a parameter and pass it on to
+    #   the method-under-test as a block.
+    def yield_successive_args(*args)
+      BuiltIn::YieldSuccessiveArgs.new(*args)
+    end
+
     # Passes if actual contains all of the expected regardless of order. 
     # This works for collections. Pass in multiple args  and it will only 
     # pass if all args are found in collection.

lib/rspec/matchers/built_in.rb

@@ -26,6 +26,10 @@ module BuiltIn
       autoload :RespondTo,      'rspec/matchers/built_in/respond_to'
       autoload :Satisfy,        'rspec/matchers/built_in/satisfy'
       autoload :ThrowSymbol,    'rspec/matchers/built_in/throw_symbol'
+      autoload :YieldControl,   'rspec/matchers/built_in/yield'
+      autoload :YieldWithArgs,  'rspec/matchers/built_in/yield'
+      autoload :YieldWithNoArgs, 'rspec/matchers/built_in/yield'
+      autoload :YieldSuccessiveArgs, 'rspec/matchers/built_in/yield'
     end
   end
 end