构建简单交换
AMM 机制
构建简单交换 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。
Goal: A Minimal AMM Swap
In this lesson we sketch a minimal constant-product swap contract in Solidity to see how the formula becomes code.
We keep it simple to focus on mechanics, not production safety.
Pool State
Our contract tracks two token reserves and the LP token supply. These three values fully describe the pool.
contract SimpleSwap {
IERC20 public tokenA;
IERC20 public tokenB;
uint public reserveA;
uint public reserveB;
uint public totalShares;
}Adding Liquidity
When a provider adds liquidity, we pull both tokens, update reserves, and mint shares proportional to the deposit.
function addLiquidity(uint amtA, uint amtB) external {
tokenA.transferFrom(msg.sender, address(this), amtA);
tokenB.transferFrom(msg.sender, address(this), amtB);
reserveA += amtA;
reserveB += amtB;
// mint shares to msg.sender
}The Swap Function Signature
A swap takes an input amount of one token and returns the maximum possible output of the other, governed by x * y = k.
function swapAforB(uint amountAIn)
external
returns (uint amountBOut)
{
// ... apply constant product
}Applying the Fee
Before computing output, we deduct a 0.30% fee by scaling the input to 99.7% of its value.
uint amountInWithFee = amountAIn * 997 / 1000;
// 0.30% of the input stays in the poolComputing the Output Amount
Using constant product, the output is derived so the product of new reserves still equals k.
amountBOut =
(reserveB * amountInWithFee) /
(reserveA + amountInWithFee);Updating Reserves
After computing the output we update both reserves and transfer the output token to the trader.
reserveA += amountAIn;
reserveB -= amountBOut;
tokenB.transfer(msg.sender, amountBOut);Putting the Swap Together
Here is the full swap path: pull input, apply fee, compute output, update reserves, send output.
function swapAforB(uint amountAIn) external returns (uint out) {
tokenA.transferFrom(msg.sender, address(this), amountAIn);
uint inWithFee = amountAIn * 997 / 1000;
out = (reserveB * inWithFee) / (reserveA + inWithFee);
reserveA += amountAIn;
reserveB -= out;
tokenB.transfer(msg.sender, out);
}Protecting Against Slippage
Real swaps accept a minAmountOut parameter and revert if the output is worse than expected.
This shields traders from price changes (or front-running) between submitting and executing.
require(out >= minAmountOut, "slippage");What We Left Out
A production AMM adds much more:
- Reentrancy guards
- Fee accounting for LPs
- Flash-loan and price-manipulation defenses
- Events for indexers
Never deploy a teaching contract to mainnet.
Putting It Together
A swap is just the constant-product formula expressed in code: take input minus fee, compute the output that keeps k, update reserves, and transfer.
Everything else (LP accounting, safety) wraps around this core idea.
Quick Check
Test your swap implementation understanding.
Recap: Building a Simple Swap
You learned that:
- A pool tracks reserves and shares
- The swap applies a fee then the constant product formula
- Output =
reserveB * inWithFee / (reserveA + inWithFee) - minAmountOut guards against slippage
- Production needs guards, events, and audits
Course complete! Next course: DAO Development.
常见问题解答
「构建简单交换」课时是免费的吗？
是的 — 「构建简单交换」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。
「构建简单交换」这节课中我会学到什么？
AMM 机制 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。
「构建简单交换」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。