Jetpack Compose is Google’s declarative user interface toolkit for Android, in which screens are built from Kotlin functions that describe the UI for the current state. In mobile app development, it replaces the traditional approach of inflating XML layouts and mutating View objects by hand: when data changes, Compose automatically re-executes the affected functions and updates only what changed.
Compose reached its first stable release in 2021 and is now the recommended way to build native Android interfaces. It is part of the Jetpack library suite, works with existing View-based code and underlies Compose Multiplatform, which extends the same programming model to desktop, web and iOS.
How Jetpack Compose builds the interface
The basic unit is a composable function, a Kotlin function marked with the @Composable annotation. It takes data as parameters and emits UI elements by calling other composables such as Text, Button or Column. The function does not return a widget object; it describes what should appear, and the runtime builds and maintains an internal tree from those descriptions.
A Kotlin compiler plugin transforms composable functions so that the runtime can track which parts of the tree depend on which data. When a piece of observable state changes, Compose schedules recomposition: it re-runs only the composables that read that state and skips those whose inputs are unchanged.
| Phase | What happens | Triggered by |
|---|---|---|
| Composition | Composables run and build or update the UI tree | State reads inside composable functions |
| Layout | Elements are measured and placed in a single pass | Size or position changes |
| Drawing | Elements are rendered to the canvas | Visual changes such as color or alpha |
Reading state in a later phase is an important optimization. An animation that reads its value only in the drawing phase avoids recomposition entirely, which keeps frames within the budget of about 16 ms at 60 Hz or about 8 ms at 120 Hz.
Core concepts: state, modifiers and side effects
Compose code is organized around a small set of core building blocks. Mastering them is what separates working code from efficient, maintainable code.
| Concept | Purpose | Typical API |
|---|---|---|
| State | Values whose changes trigger recomposition | mutableStateOf, StateFlow |
| Remember | Keeps values across recompositions | remember, rememberSaveable |
| State hoisting | Moves state to the caller for reuse and testing | Value and callback parameters |
| Modifiers | Chainable size, padding, gestures and drawing | Modifier.padding().clickable() |
| Lazy layouts | Compose only visible list items | LazyColumn, LazyVerticalGrid |
| Side effects | Run non-UI work safely from composables | LaunchedEffect, DisposableEffect |
The recommended architecture is unidirectional data flow: a ViewModel exposes state, composables render it, and user events travel back up as callbacks. State hoisting keeps most composables stateless, which makes them reusable, previewable in Android Studio and easy to test.
Jetpack Compose compared with other UI approaches
Compose belongs to the generation of declarative UI frameworks that also includes SwiftUI, Flutter and React Native. The main differences lie in language, rendering and platform reach.
| Framework | Language | Rendering | Primary platforms |
|---|---|---|---|
| Android Views (XML) | Kotlin or Java with XML | Native Android widgets | Android |
| Jetpack Compose | Kotlin | Own rendering on Android’s graphics stack | Android |
| Compose Multiplatform | Kotlin | Skia-based rendering | Android, iOS, desktop, web |
| SwiftUI | Swift | Apple’s native frameworks | iOS, macOS and other Apple platforms |
| Flutter | Dart | Own rendering engine | Android, iOS, web, desktop |
| React Native | JavaScript or TypeScript | Native platform widgets | Android, iOS |
Compared with the View system, Compose removes XML layouts, view binding and manual view updates, and typically needs noticeably less code for the same screen. Compared with Flutter and React Native, it offers direct access to Android APIs without a bridge, at the cost of being Android-first unless the team adopts Compose Multiplatform or Kotlin Multiplatform.
Adopting Compose in real projects
Compose is designed for incremental adoption. ComposeView places Compose content inside an existing View hierarchy, and AndroidView embeds classic Views, such as a map or a web view, inside a composable.
| Situation | Recommended approach | Reason |
|---|---|---|
| New Android app | Compose from the start | Current tooling, libraries and guidance target Compose |
| Large View-based app | New screens in Compose, migrate gradually | Avoids a risky full rewrite |
| Heavy custom Views | Wrap them with AndroidView | Reuses proven code |
| Shared code with iOS | Evaluate Compose Multiplatform | One UI codebase across platforms |
Teams usually start with Material 3 components and a central theme that defines colors, typography and shapes once for the whole app. Navigation, ViewModels, paging and dependency injection libraries all provide Compose integrations, and UI tests use semantic matchers rather than view IDs, so the testing approach also changes during migration.
Performance pitfalls and common mistakes
Most Compose performance problems come from unnecessary recomposition or work done in the wrong phase, not from the framework itself.
- Unstable parameters: mutable collections or classes the compiler cannot prove stable prevent skipping; immutable data classes solve this.
- Side effects in composition: network calls or logging directly in a composable run on every recomposition; use effect handlers instead.
- Missing keys in lazy lists: without stable
keyvalues, reordering items forces needless recomposition and loses item state. - Judging speed in debug builds: debug builds are much slower; measure release builds with baseline profiles enabled.
- Expensive calculations without remember: sorting or formatting inside a composable repeats on every recomposition.
The Layout Inspector in Android Studio shows recomposition and skip counts per composable, which is the most direct way to find hot spots before optimizing.
Explained without jargon
Jetpack Compose is a modern way to build the screens of Android apps. Instead of drawing each button and label and then changing them one by one, developers write a description such as “show this list, and a button if the user is signed in”.
When the information changes, Compose updates the screen by itself, a bit like a spreadsheet recalculating cells when a number changes. The result is less code, fewer display bugs and faster development of new screens.
A typical use case
A team maintaining a banking app with about 60 XML-based screens needs to build a new spending analytics feature with charts, filters and animated transitions. Rather than rewriting the app, it builds the new feature entirely in Compose and embeds it in the existing navigation through ComposeView.
The analytics screens use a ViewModel that exposes a single state object, stateless composables for each chart, and LazyColumn with stable keys for transaction lists. The feature ships with noticeably less UI code than a comparable older screen, previews let designers review states without running the app, and a baseline profile keeps scrolling smooth on mid-range phones. Encouraged by the result, the team adopts a gradual migration plan in which every redesigned screen moves to Compose.