Events on Caduceus are a way to raise awareness of, and track all historical transactions as they are persisted in blocks.
The eth subscribe method
This method of subscribing to events is like a “catch-all” method. It captures all event logs and therefore is all inclusive and exhaustive. Event logs are stored in a special area on EVM.
The subscribe method will return a Subscription instance. The subscription instance has these properties and methods:
- id – This is the subscription id
- unsubscribe(callback) – This method can be used to unsubscribe from the subscription
- subscribe(callback) – This method can be used to re-subscribe with the same parameters as the initial subscription had
- arguments – The subscription arguments that are used when re-subscribing.
You can filter what data you want to be returned. Since the Transfer event returns from, to, and value we can filter on these properties. Blockchain data return is not instantenous, and can take time to return results.
Contract instance event method
The second way to subscribe to events from a contract is to use the event methods that have been automatically generated by web3.js on your contract instance. This requires an ABI. This method is also temporary in nature, and will delete events when the user session ends.
Each events method will return an EventEmitter. The events that you can listen to are the ones we are listening to in the example code above. Where:
- data – Will fire each time an event of the type you are listening for has been emitted
- changed – Will fire for each event of the type you are listening for that has been removed from the blockchain.
- error – Will fire if an error in the event subscription occurs.
- connected – Will fire when the subscription has successfully established a connection. It will return a subscription id. This event only fires once.