Clear DefinitionsSmart Contract Event
1 min read
Pronunciation
[smart kon-trakt ih-vent]
Analogy
Imagine a vending machine (smart contract) that, in addition to dispensing a snack, also prints a little receipt (emits an event) every time a purchase is made. This receipt logs what was bought, when, and for how much. External systems (like an inventory management app) can collect these 'receipts' to keep track of sales without having to constantly open up the vending machine to check its internal state.
Definition
A mechanism in smart contracts that allows them to emit signals or log information to the blockchain when certain actions occur during function execution. Off-chain applications can listen for these events to track contract activity, update user interfaces, or trigger other processes.
Key Points Intro
Events provide a way for smart contracts to communicate occurrences to the outside world.
Key Points
A way for smart contracts to log significant occurrences during their execution.
Events are stored on the blockchain as part of transaction logs.
Off-chain applications (dApp frontends, indexers, analytics tools) can subscribe to and listen for specific events.
Cheaper than storing large amounts of data directly in contract storage.
Can include indexed parameters, allowing for efficient searching and filtering of past events.
Example
An ERC-20 token contract emits a `Transfer` event whenever tokens are moved, logging the sender, receiver, and amount. A decentralized exchange might emit a `Swap` event when a trade occurs, detailing the tokens exchanged and the price.
Technical Deep Dive
In Ethereum (Solidity), events are declared using the `event` keyword. They can have parameters, some of which can be marked as `indexed`. Indexed parameters are stored in a way that allows for faster searching of logs. When an event is emitted (using the `emit` keyword), its signature and data are included in the transaction receipt's log section. Off-chain clients use JSON-RPC methods like `eth_getLogs` or WebSocket subscriptions to listen for and retrieve these events.
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