ICE 3 – Events & Logs (Remix)

Goal: add events to a contract, emit on set(), then find and interpret logs in Remix (including indexed topics).

1) Why events?

Events are logs stored in the transaction receipt, cheap to write, easy to read off-chain.
dApps listen for events to update UIs without scanning full contract storage.
indexed parameters become searchable topics (filterable by frontends & APIs).

2) Contract with an event

Create SimpleStorageEvents.sol and paste:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract SimpleStorageEvents {
    uint256 private storedData;

    // Event with three indexed topics for fast filtering
    event ValueChanged(
        uint256 indexed oldValue,
        uint256 indexed newValue,
        address indexed setter
    );

    function set(uint256 x) public {
        uint256 oldVal = storedData;
        storedData = x;
        emit ValueChanged(oldVal, x, msg.sender);
    }

    function get() public view returns (uint256) {
        return storedData;
    }
}

3) Compile & Deploy

Open remix.ethereum.org.
Solidity Compiler → select 0.8.x → Compile SimpleStorageEvents.sol.
Deploy & Run → Environment:
- Remix VM (Prague) for quick demo, or
- Injected Provider – MetaMask to log on Ganache (from ICE 2).
Select SimpleStorageEvents → Deploy → confirm (if MetaMask).

4) Emit and read logs

In Deployed Contracts, expand the instance.
Call set(42) → confirm.
Open the terminal at the bottom; click the transaction entry.
In the tx details, find the logs/events section → you’ll see ValueChanged with topics and data.
- topics[0]: event signature hash
- topics[1..3]: indexed params → oldValue, newValue, setter
- data: non-indexed parameters (none here)
Call get() → returns 42.

On Ganache, you can also inspect the receipt in Ganache’s Transactions tab for the same event.

5) Frontend note (ethers.js)

// Listen for events (ethers v6)
import { BrowserProvider } from "ethers";
const provider = new BrowserProvider(window.ethereum);
const signer = await provider.getSigner();
const c = new ethers.Contract(addr, abi, signer);
c.on("ValueChanged", (oldV, newV, who, ev) => {
  console.log("ValueChanged", oldV.toString(), newV.toString(), who);
});

Events are the preferred way to notify UIs of changes; avoid scanning storage in loops.

Troubleshooting

No logs? Make sure you called set(), not just get().
Gas estimation failed: On Ganache, ensure the selected MetaMask account has test ETH (see ICE 2).
Wrong network: If using Injected Provider, MetaMask must be on Ganache (localhost) or a testnet.

Submission (what to turn in)

Screenshot: Remix tx details showing the ValueChanged event log.
Screenshot: get() returning your latest value.
3–5 sentences: What are indexed topics? Why are events cheaper than storage writes for UI updates?

Next: ICE 4 – Frontend (ethers.js): build a tiny web page that connects to MetaMask and calls set/get, listening for events.