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Project: RSpec Mocks 3.2

Publisher: RSpec

Matching arguments

Use with to specify the expected arguments. A message expectation constrained by with
will only be satisfied when called with matching arguments. A canned response for an
allowed message will only be used when the arguments match.

To match... ...use an expression like: ...which matches calls like:
Literal arguments with(1, true) foo(1, true)
Anything that supports case equality (===) with(/bar/) foo("barn")
Any list of args with(any_args) foo()
foo(1)
foo(:bar, 2)
Any sublist of args (like an arg splat) with(1, any_args) foo(1)
foo(1, :bar, :bazz)
An empty list of args with(no_args) foo()
Anything for a given positional arg with(3, anything) foo(3, nil)
foo(3, :bar)
Against an interface with(duck_type(:each)) foo([])
A boolean with(3, boolean) foo(3, true)
foo(3, false)
A subset of a hash with(hash_including(:a => 1)) foo(:a => 1, :b => 2)
An excluded subset of a hash with(hash_excluding(:a => 1)) foo(:b => 2)
A subset of an array with(array_including(:a, :b)) foo([:a, :b, :c])
An instance of a specific class with(instance_of(Fixnum)) foo(3)
An object with a given module in its ancestors list with(kind_of(Numeric)) foo(3)
Any RSpec matcher with(<matcher>) foo(<object that matches>)
Scenarios
  1. Basic example
  2. Using a custom matcher
  3. Responding differently based on the arguments
Basic example
Given
a file named "basic_example_spec.rb" with: 
RSpec.describe "Constraining a message expectation using with" do
  let(:dbl) { double }
  before { expect(dbl).to receive(:foo).with(1, anything, /bar/) }

  it "passes when the args match" do
    dbl.foo(1, nil, "barn")
  end

  it "fails when the args do not match" do
    dbl.foo(1, nil, "other")
  end
end
When
I run rspec basic_example_spec.rb
Then
it should fail with the following output:
2 examples, 1 failure
Failure/Error: dbl.foo(1, nil, "other")
Double received :foo with unexpected arguments
expected: (1, anything, /bar/)
got: (1, nil, "other")
Using a custom matcher
Given
a file named "custom_matcher_spec.rb" with: 
RSpec::Matchers.define :a_multiple_of do |x|
  match { |actual| (actual % x).zero? }
end

RSpec.describe "Using a custom matcher" do
  let(:dbl) { double }
  before { expect(dbl).to receive(:foo).with(a_multiple_of(3)) }

  it "passes when the args match" do
    dbl.foo(12)
  end

  it "fails when the args do not match" do
    dbl.foo(13)
  end
end
When
I run rspec custom_matcher_spec.rb
Then
it should fail with the following output:
2 examples, 1 failure
Failure/Error: dbl.foo(13)
Double received :foo with unexpected arguments
expected: (a multiple of 3)
got: (13)
Responding differently based on the arguments
Given
a file named "responding_differently_spec.rb" with: 
RSpec.describe "Using #with to constrain responses" do
  specify "its response depends on the arguments" do
    dbl = double

    # Set a default for any unmatched args
    allow(dbl).to receive(:foo).and_return(:default)

    allow(dbl).to receive(:foo).with(1).and_return(1)
    allow(dbl).to receive(:foo).with(2).and_return(2)

    expect(dbl.foo(0)).to eq(:default)
    expect(dbl.foo(1)).to eq(1)
    expect(dbl.foo(2)).to eq(2)
  end
end
When
I run rspec responding_differently_spec.rb
Then
the examples should all pass

Last published about 4 years ago by myronmarston.