Web3 & DApp Development Fundamentals · レッスン

イベントのリッスン

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「イベントのリッスン」はCoddyKit上の無料Web3 & DApp Development Fundamentalsレッスンです。これはレッスン4/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはWeb3 & DApp Development Fundamentals学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Web3 & DApp Development Fundamentalsコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

What Are Events

Smart contracts emit events to log that something happened — a transfer, an approval, a vote. Events are written to the transaction's logs.

They are cheaper than storage.
Off-chain apps subscribe to them to stay in sync.

Events in the ABI

To decode events, ethers needs their definitions in the ABI:

const abi = [
  "event Transfer(address indexed from, address indexed to, uint256 value)",
];
const contract = new ethers.Contract(address, abi, provider);

The indexed keyword marks fields you can filter on.

const abi = [
  "event Transfer(address indexed from, address indexed to, uint256 value)",
];
const contract = new ethers.Contract(address, abi, provider);

Listening Live

Subscribe to new events with on. The callback fires whenever the event is emitted:

contract.on("Transfer", (from, to, value, event) => {
  console.log(from, "->", to, value);
});

This needs a provider that supports subscriptions, such as a WebSocket connection.

contract.on("Transfer", (from, to, value, event) => {
  console.log(from, "->", to, value);
});

WebSocket Providers

Live subscriptions work best over a persistent connection. Use a WebSocket provider:

const provider = new ethers.WebSocketProvider(
  "wss://mainnet.example-rpc.io/KEY"
);

HTTP providers can poll for events but a WebSocket pushes them in real time.

const provider = new ethers.WebSocketProvider(
  "wss://mainnet.example-rpc.io/KEY"
);

Listening Once

If you only care about the first occurrence, use once:

contract.once("Approval", (owner, spender, value) => {
  console.log("First approval seen");
});

The listener automatically removes itself after firing a single time.

contract.once("Approval", (owner, spender, value) => {
  console.log("First approval seen");
});

Querying Past Events

To read historical events, use queryFilter over a block range:

const logs = await contract.queryFilter(
  "Transfer",
  19000000,
  19000100
);
for (const log of logs) {
  console.log(log.args.from, log.args.value);
}

Each log exposes decoded args.

const logs = await contract.queryFilter(
  "Transfer",
  19000000,
  19000100
);
for (const log of logs) {
  console.log(log.args.from, log.args.value);
}

Filtering by Indexed Fields

Build a filter to match specific indexed values — for example transfers to one address:

const filter = contract.filters.Transfer(null, myAddress);
const logs = await contract.queryFilter(filter);

Passing null means match any value for that position.

const filter = contract.filters.Transfer(null, myAddress);
const logs = await contract.queryFilter(filter);

Live Filtered Listening

You can also pass a filter to on to react only to relevant events:

const filter = contract.filters.Transfer(null, myAddress);
contract.on(filter, (from, to, value) => {
  console.log("Received:", value);
});

This avoids processing events you do not care about.

const filter = contract.filters.Transfer(null, myAddress);
contract.on(filter, (from, to, value) => {
  console.log("Received:", value);
});

Removing Listeners

Long-running apps should clean up listeners to avoid leaks:

contract.off("Transfer", myHandler);
// or remove all listeners for an event
contract.removeAllListeners("Transfer");

Always remove listeners when a component unmounts or a job finishes.

contract.off("Transfer", myHandler);
// or remove all listeners for an event
contract.removeAllListeners("Transfer");

Handling Reorgs

Recent events can be reverted if the chain reorganizes. For reliable indexing:

Wait several confirmations before treating an event as final.
Track the block number and re-process if a reorg is detected.

This keeps your off-chain database consistent with the chain.

Provider-Level Log Filters

You can also subscribe at the provider level without a contract instance, using a raw log filter by topic:

provider.on({
  address: tokenAddress,
  topics: [ethers.id("Transfer(address,address,uint256)")],
}, (log) => {
  console.log("Raw log:", log);
});

This is lower-level but works across many contracts at once.

provider.on({
  address: tokenAddress,
  topics: [ethers.id("Transfer(address,address,uint256)")],
}, (log) => {
  console.log("Raw log:", log);
});

Quick Check

Test your understanding of event handling.

Recap

You learned to work with contract events.

Events log on-chain activity to transaction logs.
on subscribes live (best over a WebSocket provider); once fires a single time.
queryFilter reads historical events over a block range.
Build filters from indexed fields; pass null to match any.
Remove listeners to avoid leaks and wait for confirmations against reorgs.

よくある質問

「イベントのリッスン」レッスンは無料ですか？

はい。「イベントのリッスン」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Web3 & DApp Development Fundamentalsコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Web3 & DApp Development Fundamentalsコースには全4レッスンが含まれています。

「イベントのリッスン」で何を学びますか？

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Web3 & DApp Development Fundamentalsを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのWeb3 & DApp Development Fundamentalsは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン4/4です。

「イベントのリッスン」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

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