# EVM Event Emitted

EVM Event Emitted is used to trigger a midpoint whenever an on-chain event is emitted. Unlike startpoint sources, EVM Event Emitted sources can be used for existing production contracts - without contract modification. 

When configuring an EVM Event Emitted source, specify the event signature through `arguments` along with any filters to apply to indexed topics. EVM Event Emitted can be usd when creating alerts, notifications, and listeners.

**Definition**

```graphql
chainId: string # required
contractAddress: string # required
eventName: string # required
arguments: [
    {
        name: string
        datatype: string
        indexed: boolean
        value: string
    }
]
extracts: [
    {
        name: string
        from: string
    }
]
excludeLogs: [string]
```

**`chainId`: String.** The Chain ID of the chain to listen for an event from. For a complete list of Chain Ids that we currently support, take a look at [Chain IDs](/midpoint-documentation/more-reading/chain-ids.md).

**`contractAddress`: String.** The address of the contract that the event is emitted from.

**`eventName`: String.** This is name of the event that you are listening for. 

**`arguments`:** This specifies the signature of the event, and the data that is picked out from your event to be used in later tasks.

**`arguments.name`: String.** This is the name of an argument that the event emits. This is a human-readable name that you set: for clarity, it might be best to set this name to the name of the parameter you are calling. 

**`arguments.datatype`: String.** This defines the solidity datatype of the value that is emitted from the on-chain event. This datatype must match the datatype of your on-chain event exactly, in the same order as they are defined in the on-chain event.

**`arguments.indexed`: Boolean.** This determines whether or not your particular argument is indexed, or not. An indexed parameter can be further filtered by specifying the value below.

**`arguments.value`: String** This is the value of the indexed parameter. This is not necessary to define if the particular argument is or is not indexed. You only need to define a value for this argument if you only want to listen to events that emit a particular indexed value, for example, a contract address. 

**`extracts`: See** [**extracts**](/midpoint-documentation/core-concepts/midpoints/source-and-task-definitions/extracts.md)**.** Legal values:

* **`Timestamp`** Unixtime of execution.

**`excludeLogs`: See** [**exclude logs**](/midpoint-documentation/core-concepts/midpoints/source-and-task-definitions/exclude-logs.md)**.** Legal values:

* **`TaskDefinition`** The complete definition of this task at runtime.
* **`TaskOutputData`** The complete set of returned values from this task

## Returned Values

The EVM Event Emitted source returns each of the arguments listed under the `arguments` field for use in future tasks - in addition to each field specified in `extracts`.

## **Examples**

Let's say you want to listen to a particular event.

```solidity
// Sample Event
event ReturnValue(address indexed _from, int256 _value);
```

A correctly configured Evm Event Emitted source would be:

```json
{
    "evmEventEmittedSource": {
        "chainId": "1",
        "contractAddress": "0x5B39558897Bb0047803F9E498D238fB5811E6050",
        "eventName": "ReturnValue",
        "arguments": [
            {
                "name": "_from",
                "datatype": "address",
                "indexed": true,
                "value": "0xC05f817a17bC0c505E48AC9CC52b5cb556F7B296"
            },
            {
                "name": "_value",
                "datatype": "int256",
                "indexed": false
            }
        ]
    }
}
```

The above EVM Event Emitted Source would listen for events emitted from the contract `0x5B39558897Bb0047803F9E498D238fB5811E6050` on Ethereum mainnet. It will only pick out events that emit a value of `0xC05f817a17bC0c505E48AC9CC52b5cb556F7B296` for the \_from parameter.

## Tips on Using EVM Event Emitted

This section is incomplete.


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