Kotlin Multiplatform Academy · 课时

共享的价格与税费计算器

将金额规则封装在一个跨平台位置

共享的价格与税费计算器 是 CoddyKit 上的免费 Kotlin Multiplatform Academy 课时。这是第 1 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 Kotlin Multiplatform Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Kotlin Multiplatform Academy 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。

Why Money Lives in Shared Code

Price and tax math should live in one shared place so Android and iOS can never disagree on a total. 💰

The Bug You Are Preventing

When each app rounds tax its own way, the same cart shows two prices. A single shared calculator kills that whole class of bug.

Where It Goes

This calculator belongs in commonMain, the source set both platforms compile, so there is exactly one source of truth.

Model the Price First

Start with a tiny data class that holds the amount in plain numbers. Keep it pure Kotlin so it works on every target.

data class Price(val amount: Double, val taxRate: Double)

The Tax Formula

Tax is just the amount times the rate. Expressing it as one function means both apps run the exact same line.

fun tax(amount: Double, rate: Double): Double = amount * rate

Add It to the Total

The total is the base amount plus the tax you just computed. Compose small functions instead of one giant one.

fun total(amount: Double, rate: Double): Double = amount + tax(amount, rate)

Wrap It in a Calculator

Group the rules in a small class so both apps get a clean, named entry point instead of loose functions.

class PriceCalculator {
  fun total(amount: Double, rate: Double) = amount + amount * rate
}

Watch the Rounding

Floating point can drift by a cent. Decide your rounding rule once in shared code so totals are identical everywhere.

fun roundMoney(value: Double): Double = kotlin.math.round(value * 100) / 100

Support Discounts Too

Real carts have discounts. Apply the discount before tax so the saving feels honest to the customer.

fun discounted(amount: Double, off: Double) = amount * (1 - off)

Default Values Help Callers

A sensible default tax rate keeps simple call sites short while still letting either app override it when needed.

fun total(amount: Double, rate: Double = 0.20) = amount + amount * rate

Both Apps Just Call It

Android and iOS each call the same total function. No copied math, no drift, one trusted answer for the price.

val due = PriceCalculator().total(amount = 49.99, rate = 0.18)

Quick Check

Where should the price and tax calculator live in a KMP project?

Recap: One Calculator to Rule Both

You modeled price, computed tax, totaled it, and handled rounding in commonMain. One calculator, zero price drift between apps. ✅

常见问题解答

「共享的价格与税费计算器」课时是免费的吗？

是的 — 「共享的价格与税费计算器」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 Kotlin Multiplatform Academy 课程的其余内容，请升级到 CoddyKit PRO。 Kotlin Multiplatform Academy 课程共包含 4 节课。

「共享的价格与税费计算器」这节课中我会学到什么？

将金额规则封装在一个跨平台位置你通过在浏览器中直接运行的动手代码来练习 Kotlin Multiplatform Academy，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 Kotlin Multiplatform Academy 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Kotlin Multiplatform Academy 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 1 节课，共 4 节。

「共享的价格与税费计算器」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 Kotlin Multiplatform Academy 课中编写并运行代码吗？

能。每节 Kotlin Multiplatform Academy 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。

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