EventWatcher

Description

Plasma chain contracts emit various important Ethereum contract events that the client needs to be aware of. These events include things like notifications of deposits and exits. EventWatcher provides the necessary functionality to watch for these events.

Event Uniqueness

It’s often important that an event only be handled once. We might otherwise end up in a situation in which, for example, a client attempts to credit the same deposit twice. We also don’t want to increase client load by querying the same events multiple times.

EventWatcher is therefore slightly more complicated than a basic Ethereum event watcher. Each EventWatcher instance has access to a local database that it MUST use to store the hash of any event that’s already been seen. The hash of an event is computed as the keccak256 hash of the hash of the ABI encoded transaction that emitted event prepended to the index of the event.

EventWatcher will also store the current block up to which it has checked for a given event on a given contract. EventWatcher MUST store a different block for each tuple of (contract, event name, event filter).

API

Methods

on

function on(
  contractAddress: string,
  contractAbi: any,
  eventName: string,
  eventFilter: any,
  callback: (eventName: string, eventData: any) => void
): void
Description

Starts watching an event for a given contract.

Parameters
  1. contractAddress - string: Address of the contract to watch.
  2. contractAbi - any: JSON ABI of the contract to watch.
  3. eventName - string: Name of the event to watch.
  4. eventFilter - any: An event filter for the event.
  5. callback - (eventName: string, eventData: any) => void: Callback to be triggered whenever a matching event is detected in the smart contract.

off

function off(
  contractAddress: string,
  contractAbi: any,
  eventName: string,
  eventFilter: any,
  callback: (eventName: string, eventData: any) => void
): void
Description

Stops watching an event for a given contract. Will only remove the listener that corresponds to the specific event name, filter, and callback provided.

Parameters
  1. contractAddress - string: Address of the contract to watch.
  2. contractAbi - any: JSON ABI of the contract to watch.
  3. eventName - string: Name of the event to watch.
  4. eventFilter - any: An event filter for the event.
  5. callback - (eventName: string, eventData: any) => void: Callback to be triggered whenever a matching event is detected in the smart contract.