What is Jetpack Compose?

Question

Accepted Answer

**Jetpack Compose** is Android's modern **declarative UI toolkit** — building UIs with composable Kotlin functions instead of XML layouts. It's Google's recommended approach for new Android UI, offering a reactive, concise, and more productive way to build interfaces.

## What Compose is

```text
JETPACK COMPOSE = a declarative UI toolkit (Kotlin code, no XML):
  → describe the UI as COMPOSABLE FUNCTIONS (annotated @Composable)
  → DECLARATIVE + REACTIVE — describe UI for a given state; it updates when state changes
  → Google's RECOMMENDED modern approach (similar philosophy to Flutter, React, SwiftUI)
```

## A composable function

```kotlin
@Composable
fun Greeting(name: String) {
  Column {                            // layout composable
    Text("Hello, $name")              // text composable
    Button(onClick = { /* ... */ }) {
      Text("Click me")
    }
  }
}
// → compose UI by CALLING composable functions (composition); no XML, no findViewById
```

## State and recomposition

```kotlin
@Composable
fun Counter() {
  var count by remember { mutableStateOf(0) }   // STATE
  Button(onClick = { count++ }) {                // change state
    Text("Count: $count")                         // UI reads state
  }
  // → when state changes, Compose RECOMPOSES (re-runs affected composables) → UI updates
}
```

State changes trigger **recomposition** (Compose re-runs the affected composable functions to update the UI) — the reactive model.

## Compose vs the traditional View system

```text
TRADITIONAL → XML layouts + imperative view manipulation (findViewById, setText, etc.)
COMPOSE → declarative Kotlin; less boilerplate; reactive state; more productive;
  the modern, recommended direction (new apps; interop with existing Views possible)
```

## Why it matters

Understanding Jetpack Compose is important because it's **Google's recommended modern approach to Android UI** and the direction of Android UI development, so it's increasingly essential knowledge.

Compose is a **declarative UI toolkit** that builds UIs with composable Kotlin functions instead of XML layouts — representing a fundamental shift in how Android UIs are built, aligned with the modern declarative philosophy shared by Flutter, React, and SwiftUI.

Understanding the **declarative, reactive model** — describing the UI as composable functions for a given state, with the UI automatically updating (via **recomposition**) when state changes — is the core concept, contrasting with the traditional imperative approach (manually manipulating views with findViewById and setters).

Understanding **composables** (building UIs by composing functions, with no XML or findViewById) and **state with recomposition** (state changes triggering re-runs of affected composables to update the UI) is necessary for building Compose UIs.

Understanding the **contrast with the traditional View system** (XML + imperative manipulation vs declarative Kotlin with less boilerplate, reactive state, and greater productivity) clarifies why Compose is preferred and how it differs.

Since Compose is Google's recommended approach for new Android UI (with new apps increasingly adopting it and it being the clear direction of Android UI development), and since understanding its declarative, reactive model and how it differs from the traditional system is increasingly essential for modern Android work, understanding Jetpack Compose is important, increasingly-essential Android knowledge — the modern, recommended way to build Android UIs, representing the direction of the platform and increasingly expected knowledge for Android developers building new apps, a significant evolution from the traditional XML-based view system.