# Transformers

### Structure

The structure of the transformers is quite similar to [The Graph](https://thegraph.com/) subgraphs in which we have functions named event handlers containing event objects as arguments.

Each transformer belongs to one contract.

```
type Transformer = {
 address?:string;
 getAddresses?:(chain:Chain) => Promise<Array<string>>;
 contract: Interface;
 eventHandlers: Record<Signature,EventHandler>;
 startBlock:number;
}
```

<table><thead><tr><th width="160.33333333333331">Property</th><th width="137">Type</th><th>Description</th></tr></thead><tbody><tr><td>address</td><td>optional</td><td>Contract address, if undefined will sync all the events of given signatures in event handlers</td></tr><tr><td>getAddresses</td><td>optional</td><td>If you have a mapping of contract addresses and want to sync events for all those addresses</td></tr><tr><td>contract</td><td>required</td><td>The interface of contract generated by typechain through contract ABI</td></tr><tr><td>eventHandlers</td><td>required</td><td>It's the key value object in which the key is the signature (keccak256 hash of event topic) of the event and the value is the callback function that will be executed when the event gets fired</td></tr><tr><td>startBlock</td><td>required</td><td>Block number from which the syncing of contract events will start</td></tr></tbody></table>

### Event Handler

The event handler is basically a function linked with its signature that is called when a specific event is fired.\
It takes event data as an argument and returns protocol value (contribution/extraction).

```
type EventHandler = (
  event: Event<any>
) => Promise<ProtocolValueReturnType | void | undefined>;
```

Event (passed as an argument in the event handler)

```
import {
  TransactionResponse,
  Block,
  TransactionReceipt,
} from '@ethersproject/providers';

interface Event<T> {
  address: string;
  blockNumber: number;
  params: T;
  signature: string;
  block: Promise<Block>;
  transaction: Promise<TransactionResponse>;
  transactionReceipt: Promise<TransactionReceipt>;
  transactionHash: string;
  transactionLogIndex: number;
  logIndex: number;
  chain: Chain;
}
```

Protocol Value (returned by event handler)

```
type ProtocolValueReturnType = {
  type: 'contribution' | 'extraction';
  label: string;
  value: number;
  user: string;
};
```


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