事件与存储
Gas 方面的权衡
事件与存储 是 CoddyKit 上的免费 Web3 & DApp Development Fundamentals 课时。 这是第 4 节课，共 4 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 Web3 & DApp Development Fundamentals 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Web3 & DApp Development Fundamentals 课程共包含 4 节课。
本课时的部分内容尚未翻译，以英文显示。
Two Ways to Record Data
When something happens in a contract you can record it in storage or in an event log. Choosing wisely is a core gas-optimization skill.
This lesson compares the trade-offs so you know when to use each.
Storage Recap
Storage is the contract's permanent state. It is the only data the contract itself can read back later, but writing to it is very expensive.
<code>mapping(address => uint256) public balances; // contract can read this</code>Events Recap
Events write to the transaction log. They are far cheaper, but the contract cannot read them back on-chain - only off-chain clients can.
<code>event BalanceChanged(address indexed who, uint256 newBalance);
// off-chain apps read this; the contract cannot</code>Gas Cost Difference
Writing a fresh storage slot (an SSTORE from zero) costs around 20,000 gas. Emitting a log topic or word costs only a few hundred gas. Events can be an order of magnitude cheaper.
The Golden Rule
A simple guideline:
- Store data the contract needs to read or use in future logic
- Emit data only the outside world needs (history, UI, analytics)
Do not store data purely for display - emit it instead.
What Must Be Storage
Balances, ownership, allowances, and any value used in require checks or future calculations must live in storage, because the contract reads them.
<code>function transfer(address to, uint256 amt) public {
require(balances[msg.sender] >= amt); // needs storage
balances[msg.sender] -= amt;
balances[to] += amt;
}</code>What Can Be Events Only
A transaction history, audit trail, or notification feed is perfect for events. The contract never needs to revisit those records, so storing them would waste gas.
<code>event Logged(address indexed user, string action, uint256 time);
function act(string memory a) public {
emit Logged(msg.sender, a, block.timestamp); // no storage
}</code>Use Both Together
The most common pattern is to store the new state and emit an event describing the change. The storage value drives logic; the event keeps the UI in sync.
<code>function deposit() public payable {
balances[msg.sender] += msg.value; // storage for logic
emit BalanceChanged(msg.sender, balances[msg.sender]); // event for UI
}</code>Logs Cannot Be Queried On-Chain
A critical limitation: another contract cannot read your emitted logs. If contract B needs a value, contract A must expose it via storage or a return value, never via an event.
Avoid Storing for History
A common beginner mistake is pushing every action into a storage array to build a history. This grows unbounded and becomes hugely expensive. Emit events instead and let an off-chain indexer rebuild the history.
<code>// Anti-pattern: unbounded, costly
// historyArray.push(record);
// Better:
emit ActionRecorded(msg.sender, block.timestamp);</code>Decision Checklist
Ask: Does any on-chain code read this value later?
- Yes - storage (and optionally emit too)
- No, it is only for display or audit - event only
This single question resolves most design choices.
Quick Check
Test your understanding of events versus storage.
Recap
You learned the storage vs events trade-off:
- Storage is readable on-chain but expensive (~20k gas per fresh slot)
- Events are cheap but only off-chain clients can read them
- Store what the contract needs; emit what only the outside world needs
- The common pattern: update storage AND emit an event
Never store data purely for history or display.
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常见问题解答
「事件与存储」课时是免费的吗？
是的 — 「事件与存储」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Web3 & DApp Development Fundamentals 课程的其余内容，请升级到 CoddyKit PRO。 Web3 & DApp Development Fundamentals 课程共包含 4 节课。
「事件与存储」这节课中我会学到什么？
Gas 方面的权衡 你通过在浏览器中直接运行的动手代码来练习 Web3 & DApp Development Fundamentals，全天候 AI 导师会在你学习这节课的过程中回答你的问题。
学习 Web3 & DApp Development Fundamentals 需要有经验吗？
无需任何先前经验。CoddyKit 上的 Web3 & DApp Development Fundamentals 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 4 节课，共 4 节。
「事件与存储」课时需要多长时间？
大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。
我能在这节 Web3 & DApp Development Fundamentals 课中编写并运行代码吗？
能。每节 Web3 & DApp Development Fundamentals 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。