Defference between allow_any_instance_of and expect_any_instance_of

test_spec.rb

class SomeClass
  def greet
    "Hello."
  end
end

describe "allow_any_instance_of" do
  before { allow_any_instance_of(SomeClass).to receive(:greet).and_call_original }
  context "when calling Command#greet" do
    it "pass the test" do
      expect(SomeClass.new.greet).to eq "Hello."
    end
  end
  context "when not calling Command#greet" do
    it "pass the test" do
    end
  end
end

describe "expect_any_instance_of" do
  context "when calling Command#greet" do
    it "pass the test" do
      expect_any_instance_of(SomeClass).to receive(:greet).and_call_original
      SomeClass.new.greet
    end
  end

  context "when calling Command#greet" do
    it "fail the test" do
      expect_any_instance_of(SomeClass).to receive(:greet).and_call_original
    end
  end
end