# Hydra Typegen

## Motivation

Hydra Typegen is a code generation tool for creating Typescript types for substrate events and extrinsics. Its primary use-case is to provide type-safe interfaces for Hydra mappings. For example, once a typescript class for the `Balances.Transfer` event is generated, a mapping can look like

```typescript
export async function balancesTransfer(
  db: DatabaseManager,
  _event: SubstrateEvent
) {
  const event = new Balances.TransferEvent(_event);
  const transfer = new Transfer()
  transfer.from = Buffer.from(event.data.accountIds[0].toHex())
  transfer.to = Buffer.from(event.data.accountIds[1].toHex())
  transfer.value = event.data.balance.toBn()
  transfer.block = event.ctx.blockNumber
  transfer.comment = `Transferred ${transfer.value} from ${transfer.from} to ${transfer.to}`

  console.log(`Saving ${JSON.stringify(transfer, null, 2)}`)

  await db.save<Transfer>(transfer)
}
```

## Commands

* [`hydra-typegen help [COMMAND]`](/hydra/hydra-docs/hydra-typegen.md#hydra-typegen-help-command)
* [`hydra-typegen typegen [CONFIG]`](/hydra/hydra-docs/hydra-typegen.md#hydra-typegen-typegen-config)

## `hydra-typegen help [COMMAND]`

display help for hydra-typegen

```
USAGE
  $ hydra-typegen help [COMMAND]

ARGUMENTS
  COMMAND  command to show help for

OPTIONS
  --all  see all commands in CLI
```

*See code:* [*@oclif/plugin-help*](https://github.com/oclif/plugin-help/blob/v3.2.2/src/commands/help.ts)

## `hydra-typegen typegen [CONFIG]`

Generate Typescript classes for the Substrate events

```
USAGE
  $ hydra-typegen typegen [CONFIG]

ARGUMENTS
  CONFIG  Path to YML config file. Overrides the flag options

OPTIONS
  -c, --calls=calls          Comma-separated list of substrate calls in the format <module>.<name>
  -d, --debug                Output debug info
  -e, --events=events        Comma-separated list of substrate events in the formation <module>.<name>

  -h, --blockHash=blockHash  Hash of the block from which the metadata will be fetched. Only applied if metadata is
                             pulled via an RPC call

  -i, --typelib=typelib      A JavaScript module from which the custom types should be imported, e.g.
                             '@joystream/types/augment'

  -m, --metadata=metadata    [default: metadata.json] Chain metadata source. If starts with ws:// or wss:// the metadata
                             is pulled by an RPC call to the provided endpoint. Otherwise a relative path to a json file
                             matching the RPC call response is expected

  -o, --outDir=outDir        [default: generated/types] A relative path the root folder where the generated files will
                             be generated

  -s, --[no-]strict          Strict mode. If on, the generated code throws an error if the input event argument types
                             don't much the metadata definiton

  -t, --typedefs=typedefs    A relative path to a file with JSON definitions for custom types used by the chain
```

*See code:* [*src/commands/typegen/index.ts*](https://github.com/Joystream/hydra/blob/v3.0.0-beta.8/src/commands/typegen/index.ts)&#x20;

A full sample Hydra project can be found [here](https://github.com/Joystream/hydra/tree/master/packages/sample)

## Quickstart

A minimal example for generating classes for the `Balances.Transfer` and `Treasury.Deposit` events in Kusama:

```bash
hydra-typegen typegen --metadata wss://kusama-rpc.polkadot.io Balances.Transfer,Treasury.Deposit
```

It is also possible to run `hydra-typegen` against a manifest file:

```bash
hydra-typegen typegen typegen.yml --debug
```

The config file `typegen.yml` can look like this:

```
# Typegen will pull the metadata from Kusama at block with the given hash
metadata:
  source: wss://kusama-rpc.polkadot.io
  blockHash: '0x45eb7ddd324361adadd4f8cfafadbfb7e0a26393a70a70e5bee6204fc46af62e'
# events and calls for which the typescript types will be generated
events:
  - Balances.Transfer
calls:
  - Balances.transfer
outDir: ./generated
```

Hydra-typegen also supports config files like the one below:

```
metadata:
  source: wss://kusama-rpc.polkadot.io
  blockHash: '0x45eb7ddd324361adadd4f8cfafadbfb7e0a26393a70a70e5bee6204fc46af62e'
events:
  - Balances.Transfer
calls:
  - Balances.transfer
outDir: ./generated
```

Simply run `hydra-typegen <path-to-config-file>`

## Custom types

Hydra Typegen supports custom substrate types via the `--typedefs` flag. The provided `.json` file should include type definitions for the arguments and parameters of the events and extrinsics to be generated. The type definitions file is copied to the generated sources.

In the config file, place the definition into the `customTypes` section. It assumes that all the custom runtime types are already available for import from a library, so that e.g. the generated import statement

```
import { MyCustomRuntimeClass } from 'my/types/definitions
```

is correctly resolved.

```
...
customTypes: 
    lib: <from where the custom types are imported>,
    typedefs: <path to a json file with custom runtime types definitions>,
```


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