延迟读取状态
延后读取状态，缩小重新组合的作用域。
延迟读取状态 是 CoddyKit 上的免费 Jetpack Compose Academy 课时。 这是第 3 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 Jetpack Compose Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Jetpack Compose 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。
Reading State Has a Cost
Every place you read a Compose state value becomes a subscriber. When that state changes, every reader recomposes.
Recomposition Scope
The nearest enclosing Composable that reads a state is its recomposition scope. Read high up, and a lot redraws together.
Read Low, Redraw Less
If you read state deeper, only that small Composable recomposes. Pushing the read down shrinks how much work happens.
The Lambda Trick
Pass a lambda that returns the value instead of the value itself. The read then happens later, inside a narrower scope.
Box(Modifier.offset { IntOffset(0, scroll.value) })Modifier.offset Example
The lambda offset reads scroll position during layout, not composition, so animating it skips recomposition entirely.
Defer with derivedStateOf
derivedStateOf recomputes only when its result truly changes, so a frequently changing source updates readers far less often.
val showButton by remember { derivedStateOf { scroll.value > 100 } }Cut the Noise
Without derivedStateOf, a button reading raw scroll redraws on every pixel. With it, the button recomposes only when the boolean flips.
Avoid Reading in the Parent
Reading state in a parent just to pass it down forces the parent and all siblings to recompose. Pass the state object instead.
Pass State, Not Value
Hand a child the whole State object and let it read inside itself. The parent stays stable while the child updates alone.
Lazy Lists Love Deferring
In a LazyColumn, deferred reads keep scroll updates from re-running item content, so long lists stay buttery smooth.
Read Late, Run Less
The core habit is simple: delay the moment you read state until the smallest possible scope needs it.
Quick Check
Confirm the deferring idea.
Recap: Defer the Read
Read state in the smallest scope, use lambda modifiers and derivedStateOf, and pass State objects down to keep redraws tiny. ✅
常见问题解答
「延迟读取状态」课时是免费的吗？
是的 — 「延迟读取状态」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 Jetpack Compose Academy 课程的其余内容，请升级到 CoddyKit PRO。 Jetpack Compose Academy 课程共包含 4 节课。
「延迟读取状态」这节课中我会学到什么？
延后读取状态，缩小重新组合的作用域。 你通过在浏览器中直接运行的动手代码来练习 Jetpack Compose 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 Jetpack Compose Academy 需要有经验吗？
无需任何先前经验。CoddyKit 上的 Jetpack Compose 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 3 节课，共 4 节。
「延迟读取状态」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 Jetpack Compose Academy 课中编写并运行代码吗？
能。每节 Jetpack Compose 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。