From Data to UI in Jetpack Compose – Understanding the Compose Phases

Part of the series "Android Development Series by Mircea Ioan Soit"

Jetpack Compose is reactive by design. When you update the underlying data, the UI automatically reflects the changes. But have you ever wondered how Compose goes from data to UI under the hood?

Understanding the internal phases of Compose can help you write more efficient, bug-free UIs and reason clearly about recomposition, performance, and state management.

Let’s break it down.

The Three Key Phases in Jetpack Compose

Jetpack Compose goes through three major phases to render the UI:

1. Composition
2. Layout
3. Drawing

Each phase has a specific role and understanding their order helps you grasp how your UI updates in response to data changes.

1. Composition: Building the UI Tree

What happens here:

• This is where your Composable functions are executed.
• Compose builds a UI tree based on the current state.
• It creates a structure (called the "composition") that represents the UI.

Triggered by:

• Initial rendering
• Changes in state variables (remember, mutableStateOf, StateFlow, etc.)

Best practices:

• Keep Composables pure (no side effects).
• Use stable data types and structures to avoid unnecessary recomposition.

2. Layout: Measuring and Positioning

What happens here:

• Compose measures each Composable and decides how much space it should take.
• It positions each element on the screen, respecting constraints from parent to child.

Why it matters:

• This phase defines how and where your Composables appear.
• Inefficient layouts can degrade performance, especially in complex UI trees.

Tools to debug:

• Layout Inspector in Android Studio
• Custom Layout composables for fine control

3. Drawing: Painting the Pixels

What happens here:

• Compose translates the measured and positioned elements into actual pixels.
• This includes drawing text, images, colors, shapes, and any custom graphics.

Optimizations:

• Drawing is skipped if nothing has changed visually.
• You can use DrawModifier or Canvas to hook into this phase manually.

How State Triggers the Pipeline

Jetpack Compose uses snapshot state to monitor changes. When a value changes:

1. Only affected Composables are recomposed.
2. Layout and drawing are re-run only if needed.

This makes Compose highly efficient. You don’t have to manually invalidate views like in the old View system.

var count by remember { mutableStateOf(0) }

Text("Clicked $count times", modifier = Modifier.clickable { count++ })        

In this example:

• count++ updates state.
• Only the Text Composable is recomposed.
• Layout and drawing run again only for that Text.

Avoiding Pitfalls

• Too much recomposition can hurt performance. Use derivedStateOf, remember, or LaunchedEffect to limit it.
• Unstable parameters can trigger unnecessary recomposition. Make sure your Composables accept stable types or use @Stable.
• State hoisting keeps recomposition scope smaller. Push state up the tree when possible.

Conclusion

Jetpack Compose simplifies building UIs, but understanding the underlying phases helps you write better code. From Composition to Layout to Drawing, each step ensures your UI reflects the current state efficiently and predictably.

Key Takeaways:

• Jetpack Compose uses a 3-phase model: Composition → Layout → Drawing.
• Only updated Composables are recomposed; the rest are skipped.
• Performance depends on smart use of state and careful modifier usage.