Understanding Jetpack Compose Recomposition

Jetpack Compose is Android’s modern UI toolkit designed to simplify UI development. One of its core concepts is recomposition, which is fundamental to understanding how Compose efficiently updates the UI. In this blog post, we’ll delve deep into what recomposition is, how it works, and best practices to ensure optimal performance.

What is Recomposition in Jetpack Compose?

Recomposition is the process by which Jetpack Compose re-executes the composable functions in your UI to update the UI tree based on new inputs or state changes. It is Compose’s way of efficiently updating only the parts of the UI that need to change, rather than redrawing the entire screen.

How Does Recomposition Work?

1. Initial Composition:

   When the app initially runs, Compose executes the composable functions to create a UI tree. This is called the initial composition.

2. State Changes:

   Composable functions can depend on state variables. When these state variables change, Compose schedules recomposition.

3. Re-execution:

   During recomposition, Compose re-executes the composable functions that read the changed state variables. Compose is smart; it only re-executes functions that might have changed, skipping the rest.

4. UI Update:

   Compose compares the new UI tree with the previous one. If there are differences, it updates the actual UI elements to reflect these changes efficiently.

Key Principles of Recomposition

• Composable functions should be idempotent:

  Idempotent means that running a composable function multiple times with the same inputs should produce the same result and have no side effects. This ensures predictable behavior during recomposition.

• Composable functions can be skipped:

  Compose can skip re-executing composable functions if the input parameters haven’t changed, improving performance. Therefore, ensure that your composables can handle being skipped without causing issues.

• Recomposition is optimistic:

  Compose might run recomposition multiple times for the same state change. However, it ensures that only the final state is reflected in the UI.

• Recomposition prioritizes frequency over precision:

  Compose might re-execute functions more often than necessary, but it aims to get the UI updates as fast as possible.

Example: A Simple Counter

Let’s look at a basic example of a counter implemented in Jetpack Compose to illustrate recomposition:

import androidx.compose.runtime.*
import androidx.compose.material.*
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.sp
import androidx.compose.foundation.layout.*
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.unit.dp

@Composable
fun CounterApp() {
    var count by remember { mutableStateOf(0) }

    Column(
        modifier = Modifier.fillMaxSize(),
        verticalArrangement = Arrangement.Center,
        horizontalAlignment = Alignment.CenterHorizontally
    ) {
        Text(text = "Count: $count", fontSize = 24.sp)
        Spacer(modifier = Modifier.height(16.dp))
        Button(onClick = { count++ }) {
            Text(text = "Increment")
        }
    }
}

@Preview(showBackground = true)
@Composable
fun PreviewCounterApp() {
    CounterApp()
}

In this example:

• count is a state variable managed by remember { mutableStateOf(0) }. When the button is clicked, count is incremented.
• When count changes, Compose recomposes the CounterApp composable, specifically the Text composable that displays the count value.
• Compose intelligently updates only the Text composable with the new count, leaving other parts of the UI untouched.

Best Practices for Efficient Recomposition

To ensure that recomposition is efficient and your Compose UI performs well, consider these best practices:

1. Minimize State Changes

Reduce the frequency of state changes to prevent unnecessary recompositions. Debounce or throttle rapid updates when possible.

2. Use remember Wisely

Use remember to avoid recreating objects or performing expensive operations on every recomposition. Store only the state that’s required to persist across recompositions.

val expensiveObject = remember {
    // Initialize an expensive object
    ExpensiveClass()
}

3. Use Immutable Data Structures

Leverage immutable data structures to make state changes easily detectable by Compose. When data is immutable, Compose can efficiently compare old and new states.

data class ImmutableData(val value: Int)

@Composable
fun MyComposable(data: ImmutableData) {
    Text(text = "Value: ${data.value}")
}

4. Optimize UI Structure

Avoid deeply nested composables. Complex UI hierarchies can make recomposition slower. Break down your UI into smaller, more manageable composables.

5. Key Your Lists

When using lists, provide keys to help Compose identify and reuse composables. Use the key parameter in the items block to provide a unique identifier for each item.

import androidx.compose.foundation.lazy.LazyColumn
import androidx.compose.foundation.lazy.items

data class Item(val id: Int, val text: String)

@Composable
fun MyList(items: List) {
    LazyColumn {
        items(items, key = { item -> item.id }) { item ->
            Text(text = item.text)
        }
    }
}

6. Use derivedStateOf for Complex Transformations

Use derivedStateOf to create a derived state from one or more state variables. This can optimize recomposition by ensuring that updates only occur when the derived state changes.

import androidx.compose.runtime.derivedStateOf

@Composable
fun MyComposable(name: String) {
    val isNameLong = remember {
        derivedStateOf { name.length > 5 }
    }

    if (isNameLong.value) {
        Text(text = "Long Name")
    } else {
        Text(text = "Short Name")
    }
}

Conclusion

Understanding recomposition is crucial for building performant and efficient UIs in Jetpack Compose. By adhering to the principles of recomposition and following best practices, you can ensure that your Compose applications deliver a smooth and responsive user experience. Properly managing state and optimizing your composable functions will lead to better performance and maintainability.