[Ethereum] To get event args when I using ethers.js

ethers.jsevents

I'm a three months old programmer.
So if what I'm saying is ambiguous, please let me know and help me being a better programmer.

Now I'm coding a test code as using hardhat and ether.js.

My solidity contract is about orderbook token exchange.
When people make an order, the contract emit specific event named "Make".

event Make(
    bytes32 indexed orderId, 
    address indexed maker,
    address makerToken, 
    address takerToken,
    uint256 amountSell,
    uint256 amountBuy
);

orderId is made by hashing some data.

And I want to check if event is emitted correctly.
While coding test codes, I've spent quite a long time to know how to get emitted events.

finally I can get events using code below.
but what I really want is

get 1st arg of event and put it a var. (in case, bytes32 hash)
looks like, const hash = await events.args[0];
compare 2nd arg of event and compare it with value. (in case, address maker)
looks like, expect(await events.args[1]).to.equal(add1.address);

actually it's not a two question. just one.

here's my test code.

const { expect } = require("chai");
const { ethers } = require("hardhat");

describe("EX contract", function () {
    let EX, ex, token, Token, add0, add1, add2, add3;

    before(async function () {
      EX = await ethers.getContractFactory("EX");
      Token = await ethers.getContractFactory("Token");
      ex = await EX.deploy(1000);
      token = await Token.deploy();
      [add0, add1, add2, add3] = await ethers.getSigners();
    });
  
    beforeEach(async function () {
      await token.mint(add1.address, 200000);
    });

    describe("Make / Take order", function () {
        await ex.connect(add1).makeOrder(token.address, 100, {value : 100});
        await ex.connect(add1).makeOrder(token.address, 100, {value : 200});
        
        let eventFilter = ex.filters.Make();
        let events = await ex.queryFilter(eventFilter, "latest");

        console.log(events);            //"events" contains too many information.
    });
});

Help me please, mates!

Best Answer

I suggest to try OpenZeppelin test helpers. They have a check expectEvent that reads events from the transaction's receipt.

In your case it should look like

it("Make / Take order", async function () {
  const receipt = await ex.connect(add1).makeOrder(token.address, 100, {value : 100});

  expectEvent(receipt, 'Make', {
    orderId: '<EXPECTED_ORDER_ID>',
    maker: '<EXPECTED_MAKER>',
  });
});

Related Solutions

[Ethereum] Web3j how to get event args when parsing logs

For the following event:

event MyEvent(address indexed _arg1, bytes32 indexed _arg2, uint8 _arg3);

You can extract the event arguments from the log like this

// Event definition
public static final Event MY_EVENT = new Event("MyEvent", Arrays.<TypeReference<?>>asList(new TypeReference<Address>(true) {}, new TypeReference<Bytes32>(true) {}, new TypeReference<Uint8>(false) {}));

// Event definition hash
private static final String MY_EVENT_HASH = EventEncoder.encode(MY_EVENT);

// Filter
EthFilter filter = new EthFilter(DefaultBlockParameterName.EARLIEST, DefaultBlockParameterName.LATEST, <contract-address>);

// Pull all the events for this contract
web3j.ethLogFlowable(filter).subscribe(log -> {
    String eventHash = log.getTopics().get(0); // Index 0 is the event definition hash

    if(eventHash.equals(MY_EVENT_HASH)) { // Only MyEvent. You can also use filter.addSingleTopic(MY_EVENT_HASH) 
        // address indexed _arg1
        Address arg1 = (Address) FunctionReturnDecoder.decodeIndexedValue(log.getTopics().get(1), new TypeReference<Address>() {});
        // bytes32 indexed _arg2
        Bytes32 arg2 = (Bytes32) FunctionReturnDecoder.decodeIndexedValue(log.getTopics().get(2), new TypeReference<Bytes32>() {});
        // uint8 _arg3
        Uint8 arg3 = (Uint8) FunctionReturnDecoder.decodeIndexedValue(log.getTopics().get(3), new TypeReference<Uint8>() {});
    }

});

[Ethereum] How to listen to contract events using ethers.js

This answer assumes that you understand how to connect to a contract using Ethers.

Your question specifies listening for an event to be emitted, and to do a task based on that.

Here is the link to the Ethers v5 documentation, which we'll expand on a bit below: https://docs.ethers.io/v5/api/providers/provider/#Provider--events

(Ethers v4 is here and here)

Basically, the on function is what you're looking for. Using a provider, you can set up an on to trigger something whenever a particular event is emitted. This will trigger the callback (the second argument in the on, a function) anytime that event is emitted on the blockchain. If you are looking for the on to be triggered only when the events of a particular contract are called, you'll need to use a filter - the provider will trigger the callback anytime this event is emitted from any contract.

If you look at the example in the link above to the v5 docs (at least right now 22/09/2020), towards the bottom there are examples of how to use filters (either a general filter or a topic filter - we'll discuss the first). Basically:

filter = {
    address: THE_ADDRESS_OF_YOUR_CONTRACT,
    topics: [
        // the name of the event, parnetheses containing the data type of each event, no spaces
        utils.id("Transfer(address,address,uint256)")
    ]
}
provider.on(filter, () => {
    // do whatever you want here
    // I'm pretty sure this returns a promise, so don't forget to resolve it
})

There are many more details about how and what you can filter in the links above, but this is the fundamental building block. I hope this helps!

Update: Based on a comment, I thought I should add that on can also be used with a contract object. If you have a variable representing a contract (which we'll call contract), you can also hook an on onto it. Here's a link to a section in the v4 docs which has an example (which at least as of now isn't in the v5 docs yet) - scroll down to the "listening to an Event" code example. It has the same format where you pass in an event name or filter for the first arg, and then can write a function callback as the second. This will only return events from contract.

Best Answer

Related Solutions

[Ethereum] Web3j how to get event args when parsing logs

[Ethereum] How to listen to contract events using ethers.js

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