Jetpack Compose Academy · 课时

返回 Flow 的响应式查询

在 Compose 中观察数据库。

返回 Flow 的响应式查询 是 CoddyKit 上的免费 Jetpack Compose Academy 课时。这是第 2 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。

Static Reads Go Stale

A one-time query gives you data once, but the table keeps changing. You want the UI to refresh whenever rows are added or removed.

Return a Flow Instead

Change a DAO query to return a Flow. Room now emits a fresh list every single time the underlying table changes. 🔄

@Query("SELECT * FROM Note")
fun observeAll(): Flow<List<Note>>

No suspend on Flow Queries

A Flow query is not marked suspend. It returns the stream immediately, and emissions arrive as the data updates over time.

Room Watches the Tables

Behind the scenes Room tracks which tables a query touches. When any of them changes, it re-runs the query and emits a new value.

Collect the Flow Safely

In your ViewModel, collect the Flow inside a coroutine. Use viewModelScope so collection stops when the ViewModel clears.

viewModelScope.launch {
    dao.observeAll().collect { notes -> /* update state */ }
}

Turn Flow Into State

Convert the stream into observable UI state with stateIn, giving an initial value so the screen has something to show at once.

val notes = dao.observeAll().stateIn(viewModelScope, SharingStarted.Eagerly, emptyList())

Observe It in Compose

In the Composable read the StateFlow with collectAsStateWithLifecycle. The list recomposes automatically on every change.

val notes by viewModel.notes.collectAsStateWithLifecycle()

Insert and Watch It Appear

Insert a row anywhere and the Flow emits again, so the new item shows up instantly without you refreshing manually.

Map Inside the Flow

You can transform results with map before they reach the UI, like sorting or filtering, all reactively in the stream.

dao.observeAll().map { it.sortedBy(Note::text) }

Flows Beat LiveData Here

Flow integrates cleanly with coroutines and Compose, making it the modern choice over LiveData for reactive Room queries.

One Source, Always Current

With reactive queries the database becomes your single source of truth, and the UI is just a reflection of it at every moment.

Quick Check

What happens when a Room DAO query returns a Flow?

Recap: Reactive Reads

You made queries return Flow, collected them into state, and watched Compose redraw on every change. Next you will organize this behind a repository.

常见问题解答

「返回 Flow 的响应式查询」课时是免费的吗？

是的 — 「返回 Flow 的响应式查询」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。

「返回 Flow 的响应式查询」这节课中我会学到什么？

在 Compose 中观察数据库。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 2 节课，共 4 节。

「返回 Flow 的响应式查询」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。

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