Web3 & DApp Development Fundamentals · Aula

Declaração de eventos

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Declaração de eventos é uma aula grátis de Web3 & DApp Development Fundamentals no CoddyKit. Esta é a aula 1 de 4. Você pode ler a aula completa abaixo gratuitamente — depois pratica ao vivo no navegador com um editor de código integrado e um tutor de IA 24/7. Faz parte do caminho de aprendizado de Web3 & DApp Development Fundamentals, e seu progresso é sincronizado entre a web e o app CoddyKit. O curso de Web3 & DApp Development Fundamentals inclui 4 aulas no total.

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What Are Events?

Events let a smart contract write entries to the blockchain's log. These logs are a cheap way to record that something happened and are the primary channel for contracts to communicate with off-chain applications.

Front-ends, indexers, and analytics tools all listen for events.

Declaring an Event

You declare an event with the event keyword and a list of typed parameters. The declaration defines the event's shape.

<code>event Transfer(address from, address to, uint256 amount);</code>

Emitting an Event

To fire an event you use the emit keyword followed by the event name and arguments. This appends a log entry to the transaction receipt.

<code>function send(address to, uint256 amount) public {
    // ... transfer logic
    emit Transfer(msg.sender, to, amount);
}</code>

Why Use Events?

Events solve two problems:

Cheap record-keeping - logs cost far less gas than storage
Off-chain communication - dApps cannot read storage in real time, but they can subscribe to events

Contracts themselves cannot read their own emitted events, however.

A Complete Example

Here is a small contract that emits an event whenever a value is updated, so any listening app knows the change immediately.

<code>contract Counter {
    uint256 public count;
    event Incremented(uint256 newCount);

    function increment() public {
        count += 1;
        emit Incremented(count);
    }
}</code>

Logs Are Not Storage

Event data is stored in a special log area of the transaction receipt, not in contract storage. This is why they are cheap, but it also means the contract cannot query past events on-chain.

Logs are accessible to off-chain clients and block explorers.

Naming and Conventions

By convention events are named with PascalCase and often describe a past action: Transfer, Approval, Deposit, OwnershipTransferred.

Standard interfaces like ERC-20 define required events you must emit.

<code>event Approval(address owner, address spender, uint256 value);
event Deposit(address account, uint256 amount);</code>

Multiple Parameters

Events can carry several parameters of any type, including string and bytes. The data is ABI-encoded into the log.

<code>event OrderPlaced(uint256 id, address buyer, string product, uint256 price);

function place(uint256 id, string memory p, uint256 price) public {
    emit OrderPlaced(id, msg.sender, p, price);
}</code>

Gas Cost of Events

Emitting an event costs gas, but much less than writing the same data to storage. The base cost plus a small per-byte fee makes events the preferred way to expose historical data.

Rule of thumb: store what the contract needs to read, emit what off-chain apps need to read.

Events in the ABI

Event declarations appear in the contract's ABI so that client libraries know how to decode the logs. The event signature is hashed to create a topic identifier.

<code>// ABI entry (simplified)
// { type: 'event', name: 'Transfer',
//   inputs: [from, to, amount] }</code>

Events for State Changes

A best practice is to emit an event for every meaningful state change. This gives applications a reliable, ordered history they can replay to rebuild state.

<code>address public owner;
event OwnerChanged(address previous, address next);

function setOwner(address next) public {
    emit OwnerChanged(owner, next);
    owner = next;
}</code>

Quick Check

Test your understanding of declaring events.

Recap

You learned how to declare and emit events:

Declare with event Name(types...)
Fire with emit Name(args...)
Logs are cheap, off-chain readable, and not queryable on-chain
Emit an event for every meaningful state change

Events are the bridge between your contract and the outside world.

Perguntas Frequentes

A aula “Declaração de eventos” é grátis?

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Nenhuma experiência prévia é necessária. Web3 & DApp Development Fundamentals no CoddyKit é estruturado para alunos iniciantes até avançados, então você pode começar aqui ou desde o início e aprender no seu ritmo. Esta é a aula 1 de 4.

Quanto tempo leva a aula “Declaração de eventos”?

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