How does the testEnteringRaffleEmitsEvent function work?

[nabil-ziani]

Hi everyone,

Here is the function I have questions about:

function testEnteringRaffleEmitsEvent() public funded {
    // Arrange
    vm.prank(PLAYER);

    // Act
    vm.expectEmit(true, false, false, false, address(raffle));
    emit RaffleEntered(PLAYER);

    // Assert
    raffle.enterRaffle{value: entranceFee}();
}

I do not understand why we need to emit the event within the test, because then we are not testing raffle.enterRaffle() I would think?

I was thinking it should be like this, because raffle.enterRaffle() should emit the event and we want to test that it does (but it fails):

function testEnteringRaffleEmitsEvent() public funded {
    // Arrange
    vm.prank(PLAYER);

    // Act
    vm.expectEmit(true, false, false, false, address(raffle));

    // Assert
    raffle.enterRaffle{value: entranceFee}();
}

Can someone clarify it for me?

[chainflash]

Bro

vm.expectEmit(true, false, false, false, address(raffle));

We write this part above because the expectEmit function is defined in this way:

function expectEmit(
  bool checkTopic1,
  bool checkTopic2,
  bool checkTopic3,
  bool checkData,
  address emitter
) external;

In fact, we are saying that we expect an event with these parameters to be emitted, but something is still missing.
We haven't specified the name and value of the event, so we need to add the following line as well :

emit RaffleEntered(PLAYER);

Here, we manually (mock) emit the event that we expect and event name and value are specified.
Think of it this way: it’s like saying, “Wait, an event is going to be emitted. Okkkkkk so check if it matches the fake event we emitted.”

raffle.enterRaffle{value: entranceFee}();

Then by calling the function, the actual event is emitted and compared with our expectations.

[EngrPips]

Phenomenal!

[chainflash]

I appreciate👍

[nabil-ziani]

Awesome, thank you!

[chainflash]

You're welcome.