[Ethereum] Dealing with events from other contracts

eventssoliditytruffleweb3js

I have 2 contracts. One contract is the one which I'm interacting directly with.

The event I want to listen is not triggered by the main contract, but the one it is called internally.

In the following example, if I invoke "testing" method, how can I listen for "MyEvent" triggered by the other contract?

contract OtherContract {

event MyEvent(uint8);
function doSomething() {
         MyEvent(1);
    }
}

contract Test {

    OtherContract constant otherContract  = OtherContract(0x0b258ee7bf483bb49a5956407702ca5b08197b4c);

    function testing() {
         otherContract.doSomething();        
    }
}

Best Answer

You could handle this with the Web3 JavaScript API. You would just need to reference the instance of the contract that you would like to watch events for.

var event = myContractInstance.MyEvent({parameters} [Filters])

// watch for changes
event.watch(function(error, result){
  if (!error)
    console.log(result);
});

See here for info about filters.

Related Solutions

Web3JS – Handling Contract Events with Truffle

Sorted out in the comments above, moving to an answer for future readers.

The code in the question is using truffle. When sending a transaction in truffle, the result you get back has a logs field, which is assembled via this code: https://github.com/trufflesuite/truffle-contract/blob/develop/contract.js#L44.

That code discards logs with topics that don't match a known event signature in the ABI of the contract you're calling. I consider that behavior to be a design bug.

Truffle – Testing Events via JavaScript Effectively

While Aquila's answer can work as a workaround, the way truffleAssert.eventEmitted() works is by applying a filter function on the event's arguments. The drawback of this is that you can't "assert" each argument individually, but this will allow you to run both assertions in this fashion.

I saw from your other question that you're using Truffle v5 with Solidity v0.5.0, so I updated your tests to use Web3 v1.0.

I used the following solidity code to test:

event Message(address id, bytes16 message);

function messages() public {
    emit Message(address(0), "Almost there");
    emit Message(address(msg.sender), "Congratulations");
}

And changed the Javascript tests to:

it("should check that the correct events are returned", async () => {
    let result = await casino.messages();

    truffleAssert.eventEmitted(result, 'Message', (ev) => {
      return ev.id === '0x0000000000000000000000000000000000000000' && web3.utils.hexToUtf8(ev.message) === 'Almost there';
    }, 'Contract should return the correct message.');

    truffleAssert.eventEmitted(result, 'Message', (ev) => {
      return ev.id === accounts[0] && web3.utils.hexToUtf8(ev.message) === 'Congratulations';
    }, 'Contract should return the correct message.');
  })

Both of these tests pass with Truffle v5, Solidity v0.5.0, truffle-assertions v0.7.1.

Best Answer

Related Solutions

Web3JS – Handling Contract Events with Truffle

Truffle – Testing Events via JavaScript Effectively

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