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Blockchain Smart Contracts with Solidity · レッスン

イベントとログデータ

イベントを使ってブロックチェーン上に情報を記録し、外部アプリケーションによる効率的な監視とインデックス作成を可能にします。

「イベントとログデータ」はCoddyKit上の無料Blockchain Smart Contracts with Solidityレッスンです。 これはレッスン3/4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Blockchain Smart Contracts with Solidity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。 Blockchain Smart Contracts with Solidity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 Solidity Events?

Smart contracts often need to communicate with the outside world. This is where events come in!

Events are a special feature in Solidity that allow your smart contracts to publish information to the blockchain log.

Think of them as notifications that external applications can listen for.

The Power of Event Logs

Events are crucial because directly reading contract state from off-chain applications can be inefficient and costly.

  • Cost-Effective: Storing data in events is cheaper than storing it in contract state.
  • Indexable History: They provide a historical record of actions, easily searchable by off-chain tools.
  • Monitoring: Wallets, explorers, and dApps can "subscribe" to events to react to changes.

Defining Your First Event

Declaring an event is similar to defining a function, but you use the event keyword.

You specify a name for the event and a list of parameters it will carry, just like function arguments.

These parameters define the data that will be logged when the event is emitted.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract EventExample {
    // Declares an event named 'LogMessage'
    // It takes a string and a number as parameters
    event LogMessage(string message, uint256 timestamp);
}

Triggering an Event

Once an event is declared, you can "trigger" it using the emit keyword within any function of your contract.

When emit is called, the event's data is recorded in the transaction's log, but not directly in the contract's storage.

This makes events a lightweight way to signal information.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract EventExample {
    event LogMessage(string message, uint256 timestamp);

    function doSomethingAndLog(string memory _msg) public {
        // Emits the LogMessage event
        // current timestamp is block.timestamp
        emit LogMessage(_msg, block.timestamp);
    }
}

Making Events Searchable

You can add the indexed keyword to up to three event parameters. This makes them searchable and filterable by off-chain applications.

  • indexed parameters: Stored in a special part of the log called "topics."
  • Non-indexed parameters: Stored in the "data" part of the log.
  • Benefit: Off-chain tools can quickly find transactions related to specific indexed values.

Event with Indexed Parameters

Let's see how to declare and emit an event with an indexed parameter.

Notice how `sender` is marked `indexed`, making it easy to filter for all events from a specific address.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract IndexedEventExample {
    event Transfer(address indexed from, address indexed to, uint256 amount);

    function transferTokens(address _to, uint256 _amount) public {
        // Imagine some transfer logic here
        // Then emit the event
        emit Transfer(msg.sender, _to, _amount);
    }
}

Logs are Not State

It's important to understand that event logs are not stored within the contract's state.

  • State: Data stored directly in the contract, readable by other contracts and functions. Expensive.
  • Logs: Part of the blockchain's transaction receipts. They are immutable and persistent, but not directly accessible by other contracts. Cheaper.

Events are primarily for off-chain consumption.

Practical Event Applications

Events are used everywhere in smart contracts. Here are some common applications:

  • Token Transfers: ERC-20 and ERC-721 standards heavily rely on `Transfer` and `Approval` events.
  • Ownership Changes: Notifying when a contract's owner is updated.
  • Data Updates: Signalling when important contract variables have changed.
  • User Interface Feedback: DApps listen for events to update UI in real-time.

Off-Chain Interaction

While Solidity contracts can't directly read events, off-chain applications (like web dApps) use web3 libraries (e.g., Web3.js, Ethers.js) to:

  • Filter Past Events: Query historical blockchain data for specific events.
  • Subscribe to New Events: Listen for events as they are emitted in real-time.

This enables powerful, responsive decentralized applications.

Event Concepts Check

Consider the following Solidity event declaration:

event UserRegistered(
    address indexed userAddress,
    string userName,
    uint256 indexed registrationTime
);

Which parameters are indexed and therefore easily searchable by off-chain tools?

Recap: Events & Logging

You've learned about the power of events in Solidity!

  • Events are emitted by contracts to log data on the blockchain.
  • They are crucial for off-chain monitoring, indexing, and dApp interaction.
  • Use the `event` keyword to declare and `emit` to trigger them.
  • `indexed` parameters make events easily searchable.

This mechanism is vital for building responsive and data-driven decentralized applications.

よくある質問

「イベントとログデータ」レッスンは無料ですか？

はい。「イベントとログデータ」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）、Blockchain Smart Contracts with Solidity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。 Blockchain Smart Contracts with Solidityコースには全4レッスンが含まれています。

「イベントとログデータ」で何を学びますか？

イベントを使ってブロックチェーン上に情報を記録し、外部アプリケーションによる効率的な監視とインデックス作成を可能にします。 ブラウザで直接実行するハンズオンコードでBlockchain Smart Contracts with Solidityを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Blockchain Smart Contracts with Solidity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Blockchain Smart Contracts with Solidity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3/4です。

「イベントとログデータ」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。

このBlockchain Smart Contracts with Solidityレッスンでコードを書いて実行できますか？

はい。すべてのBlockchain Smart Contracts with Solidityレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

このコースのすべてのレッスン

  1. 状態変数とストレージレイアウト
  2. マッピングと動的配列
  3. イベントとログデータ
  4. ストレージスロットとガス最適化
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