Lesson 9: Events and arguments

[doz-8108]

From the video, the host said that the later emitted event will be at the front of the events array from Ethers.js transaction receipt.

In this case, the event RequestRaffleWinner will be placed in txReceipt.events[0] and the event emitted by i_vrfCoordinatorV2.requestRandomWords(...) will be txReceipt.events[1], am I right?

 

The other thing is why events[0] does not have the args property that represents the request Id, but events[1] has one since I see they both have an argument requestId.

[krakxn]

@doz-8108 The difference is because we are not using all the parameters in requestRandomWords. To give you some context, the following event is emitted by requestRandomWords in VRFCoordinatorV2:

emit RandomWordsRequested(
      keyHash,
      requestId,
      preSeed,
      subId,
      requestConfirmations,
      callbackGasLimit,
      numWords,
      msg.sender
    );

These are all derived from the function signature (of requestRandomWords) and since we are not using everything, the latter has more "events" emitted.

[othaime-en]

Hey @doz-8108 your assessment is partly true. Yes there are two events being emitted, but not in the order you put them. If you look at the code, we call the function i_vrfCoordinatorV2.requestRandomWords(...) first then we emit RequestRaffleWinner(requestId) So technically, the event at index 0 is the one emitted by i_vrfCoordinatorV2.requestRandomWords(...) then the event at index 1 is the one emitted by RequestRaffleWinner(requestId) That's why the first event (event at index 0) doesn't have args while the second has one arg (requestId) the one we put in the event declaration of RequestRaffleWinner

Concerning the absence of args in the first event being emmitted, as @krakxn has explained above, we didn't declare any event with args to sort of "catch" the arguments we want. If you can remember we didn't declare anything in the events section of our raffle contract to sort of catch the event being emitted by i_vrfCoordinatorV2.requestRandomWords(...)
However, If you want to see these args being emitted by the first event, add this code in your raffle contract

  event RandomWordsRequested(
    bytes32 indexed keyHash,
    uint256 requestId,
    uint256 preSeed,
    uint64 indexed subId,
    uint16 minimumRequestConfirmations,
    uint32 callbackGasLimit,
    uint32 numWords,
    address indexed sender
  );

[othaime-en]

If you wanna experiment, you could change the variable requestId in the RequestRaffleWinner(requestId) event to requestedId or whatever variable name you choose and see it being reflected as the arg in the second event. This could help you differentiate between the two requestIds from the first and second events.

Remember that the variable being emitted here emit RequestRaffleWinner(requestId) is the same variable from uint256 requestedId = i_vrfCoordinator.requestRandomWords(...) So be sure to change it here too. T

[krakxn]

The caveat is that it will be redundant, as so

(uint256 requestId, uint256 preSeed) = computeRequestId(keyHash, msg.sender, subId, nonce);

and because there is nonce too,

uint64 currentNonce = s_consumers[msg.sender][subId];

we will instead have to compute (below) it which is already in requestRandomWords.

uint64 currentNonce = s_consumers[msg.sender][subId];
    if (currentNonce == 0) {
      revert InvalidConsumer(subId, msg.sender);
    }
uint64 nonce = currentNonce + 1;

Therefore, it will inevitably be a waste of time (redundancy).

[othaime-en]

Yeah definitely. It will be redundant. you don't need both events, just one.
If you choose to add the code i've shared above (the RandomWordsRequested event), you could just do away with the RequestRaffleWinner event. Because they basically do the same thing.

[doz-8108]

Thanks, just saw those args after declaring the event in the Raffle contract!