Project: Rspec-mocks

Publisher: rspec-pritesh

Any Instance

rspec-mocks provides two methods, allow_any_instance_of and
expect_any_instance_of, that will allow you to stub or mock any instance of a class. They
are used in place of allow or expect:

allow_any_instance_of(Widget).to receive(:name).and_return("Wibble")
expect_any_instance_of(Widget).to receive(:name).and_return("Wobble")

These methods add the appropriate stub or expectation to all instances of Widget.

This feature is sometimes useful when working with legacy code, though in general we
discourage its use for a number of reasons:

The rspec-mocks API is designed for individual object instances, but this feature operates on entire classes of objects. As a result there are some sematically confusing edge cases. For example, in expect_any_instance_of(Widget).to receive(:name).twice it isn't clear whether each specific instance is expected to receive name twice, or if two receives total are expected. (It's the former.)
Using this feature is often a design smell. It may be that your test is trying to do too much or that the object under test is too complex.
It is the most complicated feature of rspec-mocks, and has historically received the most bug reports. (None of the core team actively use it, which doesn't help.)

Scenarios

Use `allow_any_instance_of` to stub a method
Use `allow_any_instance_of` to stub multiple methods
Stubbing any instance of a class with specific arguments
Block implementation is passed the receiver as first arg
Use `expect_any_instance_of` to set a message expectation on any instance

Use `allow_any_instance_of` to stub a method

Given

a file named "example_spec.rb" with:

RSpec.describe "allow_any_instance_of" do
  it "returns the specified value on any instance of the class" do
    allow_any_instance_of(Object).to receive(:foo).and_return(:return_value)

    o = Object.new
    expect(o.foo).to eq(:return_value)
  end
end

When

I run rspec example_spec.rb

Then

the examples should all pass

Use `allow_any_instance_of` to stub multiple methods

Given

a file named "example_spec.rb" with:

RSpec.describe "allow_any_instance_of" do
  context "with receive_messages" do
    it "stubs multiple methods" do
      allow_any_instance_of(Object).to receive_messages(:foo => 'foo', :bar => 'bar')

      o = Object.new
      expect(o.foo).to eq('foo')
      expect(o.bar).to eq('bar')
    end
  end
end

When

I run rspec example_spec.rb

Then

the examples should all pass

Stubbing any instance of a class with specific arguments

Given

a file named "example_spec.rb" with:

RSpec.describe "allow_any_instance_of" do
  context "with arguments" do
    it "returns the stubbed value when arguments match" do
      allow_any_instance_of(Object).to receive(:foo).with(:param_one, :param_two).and_return(:result_one)
      allow_any_instance_of(Object).to receive(:foo).with(:param_three, :param_four).and_return(:result_two)

      o = Object.new
      expect(o.foo(:param_one, :param_two)).to eq(:result_one)
      expect(o.foo(:param_three, :param_four)).to eq(:result_two)
    end
  end
end

When

I run rspec example_spec.rb

Then

the examples should all pass

Block implementation is passed the receiver as first arg

Given

a file named "example_spec.rb" with:

RSpec.describe "allow_any_instance_of" do
  it 'yields the receiver to the block implementation' do
    allow_any_instance_of(Array).to receive(:shuffle) do |array|
      array.unshift(array.pop)
    end

    expect([1, 2, 3].shuffle).to eq([3, 1, 2])
  end
end

When

I run rspec example_spec.rb

Then

the examples should all pass

Use `expect_any_instance_of` to set a message expectation on any instance

Given

a file named "example_spec.rb" with:

RSpec.describe "expect_any_instance_of" do
  before do
    expect_any_instance_of(Object).to receive(:foo)
  end

  it "passes when an instance receives the message" do
    Object.new.foo
  end

  it "fails when no instance receives the message" do
    Object.new.to_s
  end
end

When

I run rspec example_spec.rb

Then

it should fail with the following output:

2 examples, 1 failure

Exactly one instance should have received the following message(s) but didn't: foo

Last published over 7 years ago by Pritesh Jain.

Any Instance

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