When stubbing Foo.call, in order for block that contains Bar.call to be called, we need to use and_yield. It can take arguments. See also https://www.rubydoc.info/gems/rspec-mocks/RSpec%2FMocks%2FMessageExpectation:and_yield

RSpec.describe Thing do
  before do
    allow(Foo).to receive(:call).and_yield
    allow(Bar).to receive(:call)
  end

  describe '.call' do
    it 'calls Foo' do
      described_class.call
      expect(Foo).to have_received(:call)
    end

    it 'calls Bar' do
      described_class.call
      expect(Bar).to have_received(:call)
    end
  end
end

We can expect the subject under test to yield control by passing a block to expect with an argument. The argument itself is a block, captured (&b) and passed to the method call. See also https://relishapp.com/rspec/rspec-expectations/v/3-11/docs/built-in-matchers/yield-matchers

it 'yields control to a block' do
  expect do |b|
    described_class.call(&b)
  end.to yield_control
end

This isn’t related to block testing, but putting this here for now.

Can be used to stub multiple calls to the same method, which could raise and retry see also https://www.rubydoc.info/github/rspec/rspec-mocks/RSpec%2FMocks%2FMessageExpectation:and_invoke