# Events in Solidity

By [DeFi Simon](https://paragraph.com/@defi-simon) · 2024-03-21

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This blog introduces `event` in Solidity, using transfer events in ERC20 tokens as an example .

**Events**[**​**](https://www.wtf.academy/en/docs/solidity-101/Event/#events)
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The event in `solidity` are the transaction logs stored on the `EVM` (Ethereum Virtual Machine). They can be emitted during function calls and are accessible with the contract address. Events have two characteristics：

*   Responsive: Applications (e.g. [ether.js](https://learnblockchain.cn/docs/ethers.js/api-contract.html#id18)) can subscribe and listen to these events through `RPC` interface and respond at frontend.
    
*   Economical: It is cheap to store data in events, costing about 2,000 `gas` each. In comparison, store a new variable on-chain takes at least 20,000 `gas`.
    

### **Declare events**[**​**](https://www.wtf.academy/en/docs/solidity-101/Event/#declare-events)

The events are declared with the `event` keyword, followed by event name, then the type and name of each parameter to be recorded. Let's take the `Transfer` event from the `ERC20` token contract as an example：

    event Transfer(address indexed from, address indexed to, uint256 value);
    

`Transfer` event records three parameters: `from`，`to`, and `value`，which correspond to the address where the tokens are sent, the receiving address, and the number of tokens being transferred. Parameter `from` and `to` are marked with `indexed` keywords, which will be stored at a special data structure known as `topics` and easily queried by programs.

### **Emit events**[**​**](https://www.wtf.academy/en/docs/solidity-101/Event/#emit-events)

We can emit events in functions. In the following example, each time the `_transfer()` function is called, `Transfer` events will be emitted and corresponding parameters will be recorded.

        // define _transfer function，execute transfer logic
        function _transfer(
            address from,
            address to,
            uint256 amount
        ) external {
    
            _balances[from] = 10000000; // give some initial tokens to transfer address
    
            _balances[from] -=  amount; // "from" address minus the number of transfer
            _balances[to] += amount; // "to" address adds the number of transfer
    
            // emit event
            emit Transfer(from, to, amount);
        }
    

**EVM Log**[**​**](https://www.wtf.academy/en/docs/solidity-101/Event/#evm-log)
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EVM uses Log to store Solidity events. Each log contains two parts: `topics` and `data`.

![](https://storage.googleapis.com/papyrus_images/14e9402081fcf6ce1f4332320a93cd9ad165332ede7a6db63b88dac400bc3d46.png)

### `Topics`[**​**](https://www.wtf.academy/en/docs/solidity-101/Event/#topics)

`Topics` is used to describe events. Each event contains a maximum of 4 `topics`. Typically, the first `topic` is the event hash: the hash of the event signature. The event hash of `Transfer` event is calculated as follows:

    keccak256("Transfer(addrses,address,uint256)")
    
    //0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef
    

Besides event hash, `topics` can include 3 `indexed` parameters, such as the `from` and `to` parameters in `Transfer` event. The anonymous event is special: it does not have a event name and can have 4 `indexed` parameters at maximum.

`indexed` parameters can be understood as the indexed "key" for events, which can be easily queried by programs. The size of each `indexed` parameter is 32 bytes. For the parameter is larger than 32 bytes, such as `array` and `string`, the hash of the underlying data is stored.

### `Data`[**​**](https://www.wtf.academy/en/docs/solidity-101/Event/#data)

Non-indexed parameters will be stored in the `data` section of the log. They can be interpreted as "value" of the event and can't be retrieved directly. But they can store data with larger size. Therefore, `data` section can be used to store complex data structures, such as `array` and `string`. Moreovrer, `data` consumes less gas compared to `topic`.

### **Query event on etherscan**[**​**](https://www.wtf.academy/en/docs/solidity-101/Event/#query-event-on-etherscan)

Etherscan is a block explorer that lets you view public data on transactions, smart contracts, and more on the Ethereum blockchain. First, I deployed the contract to an ethereum testnet (Rinkeby or Goerli). Second, I called the `_transfer` function to transfer 100 tokens. After that, you can check the transaction details on `etherscan`：[**URL**](https://rinkeby.etherscan.io/tx/0x8cf87215b23055896d93004112bbd8ab754f081b4491cb48c37592ca8f8a36c7)

Click `Logs` button to check the details of the event：

![](https://storage.googleapis.com/papyrus_images/1d52ca900f203d4f24b5b59fd983753332de36c840fa976034c345d15cf04350.png)

There are 3 elements in `Topics`: `[0]` is hash of the event, `[1]` and `[2]` are the `indexed` parameters defined in `Transfer` event (`from` and `to`). The element in `Data` is the non-indexed parameter `amount`.

**Summary**[**​**](https://www.wtf.academy/en/docs/solidity-101/Event/#summary)
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In this article, we introduced how to use and query events in `solidity`. Many on-chain analysis tools are based on solidity events, such as `Dune Analytics`.

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*Originally published on [DeFi Simon](https://paragraph.com/@defi-simon/events-in-solidity)*