Jetpack Compose Deep Dive (Part 4): Understanding Composables, Reusability, and Performance

In this article, we’re taking a deeper look into the core foundation of Jetpack Compose—Composables—and exploring strategies for enhancing reusability and performance. By the end of this post, you’ll have a clear understanding of:

• What Composables are.
• How to make your UI code modular and reusable.
• Techniques for ensuring performance in a reactive UI framework.

1. What are Composables?

Composables are the building blocks of Jetpack Compose. At their core, they are functions annotated with @Composable that tell Jetpack Compose how to draw and structure UI elements. Unlike XML layouts, where UI is defined in static files, Composables allow developers to create dynamic UIs through a declarative programming model.

Key Features of Composables:

• Declarative UI: Instead of mutating UI elements (like findViewById in the traditional approach), Composables describe what the UI should look like at any given state, and Compose automatically handles rendering.
• Composable Functions: Any function in Jetpack Compose that returns UI elements must be marked with the @Composable annotation.

Basic Example:

@Composable
fun Greeting(name: String) {
    Text(text = "Hello, $name!")
}        

Here, Greeting() is a composable function that takes a name as an argument and displays a greeting message.

2. Reusability in Composables

One of the most significant advantages of Jetpack Compose is the ability to reuse UI components. Composables are designed to be highly modular, which encourages the creation of small, self-contained functions that can be reused across your app.

Why Reusability Matters:

• Modularity: Breaking down UI into small, reusable components makes code more organized and maintainable.
• Testability: Reusable Composables are easier to test because they can be treated as isolated UI elements.
• Consistency: Reusing the same components across your app ensures that your UI remains consistent, both visually and functionally.

Example: Reusable Button Composable:

@Composable
fun MyButton(label: String, onClick: () -> Unit) {
    Button(onClick = onClick) {
        Text(text = label)
    }
}        

This MyButton() function encapsulates the logic for a button. Instead of repeating the button code throughout the app, you can simply pass different label values and onClick actions to reuse this button everywhere.

Best Practices for Reusability:

1. Focus on Single Responsibility: Each Composable should focus on a single piece of UI or logic. For example, a button should only handle the button UI, not business logic.
2. Keep Composables Stateless: Try to avoid managing state within the Composable itself unless absolutely necessary. Stateless Composables are easier to reuse and compose together.
3. Parameterize for Flexibility: Use parameters to make your Composables flexible. For example, passing color, text, or callback functions as parameters allows a Composable to adapt to different use cases.

3. Performance Optimization: Recomposition and State Management

Jetpack Compose automatically updates the UI when the underlying state changes. This mechanism is known as recomposition. However, if not handled correctly, recomposition can lead to performance issues such as unnecessary redrawing of UI components.

How Recomposition Works:

• Whenever a state that a Composable relies on changes, Jetpack Compose recomposes that Composable, meaning it redraws only the parts of the UI that depend on the changed state.
• Recomposition is designed to be efficient, but if not managed properly, excessive recompositions can degrade performance.

Avoiding Unnecessary Recomposition:

1. Use remember for Caching Values: The remember function is used to store values across recompositions. This prevents recalculating values unless necessary. Example:

@Composable
fun Counter() {
    val count = remember { mutableStateOf(0) }
    Button(onClick = { count.value++ }) {
        Text(text = "Count: ${count.value}")
    }
}        

In this example, remember ensures that the count value persists across recompositions, so the count doesn’t reset on each UI update.

2. Optimize State Handling: Not all parts of your UI need to recompose every time state changes. For better performance, you should carefully manage where state is handled and how it is passed to child Composables.

State Hoisting:

• State hoisting is a pattern where state is moved up to a parent composable, allowing multiple child Composables to share and react to the same state.

Example:

@Composable
fun CounterScreen() {
    var count by remember { mutableStateOf(0) }
    CounterDisplay(count = count)
    IncrementButton { count++ }
}

@Composable
fun CounterDisplay(count: Int) {
    Text(text = "Count: $count")
}

@Composable
fun IncrementButton(onClick: () -> Unit) {
    Button(onClick = onClick) {
        Text("Increment")
    }
}        

In this example, the CounterScreen() hoists the count state and passes it down to CounterDisplay() and IncrementButton(), separating the display logic from the state management.

4. Composable Scoping and Stability

To ensure good performance, you also need to be aware of stability in Jetpack Compose. Composables are automatically scoped to the lifecycle of the UI, but you should make sure that unnecessary recompositions don't occur due to unstable parameters.

What is Stability?

A stable class is one where the data it represents doesn’t change unexpectedly or too frequently. Jetpack Compose tries to recompose only stable parts of your UI. If a class is unstable, it can trigger more recompositions than necessary.

Ensuring Stability:

• Use immutable data where possible.
• Avoid passing complex objects that may trigger recomposition.

5. Advanced Performance Optimization Techniques

a. Use key() for Dynamic Lists:

When dealing with dynamic lists of data, such as lists in a LazyColumn, it’s important to ensure that Compose knows how to identify items uniquely. The key() function allows you to provide a stable identifier for each item, reducing unnecessary recompositions.

Example:

LazyColumn {
   itemsIndexed(items = productList, key = { index, item -> item.id }) { index, item ->
       ProductItem(item)
   }
}        

b. DerivedStateOf for Expensive Calculations:

For cases where you need to perform expensive calculations based on the state, you can use derivedStateOf() to calculate derived values only when the state changes.

Example:

val totalPrice by derivedStateOf {
   cartItems.sumOf { it.price * it.quantity }
}        

Conclusion

Jetpack Compose brings a new paradigm to Android UI development with its declarative approach, making the UI code cleaner and more efficient. By mastering the use of Composables, you can create highly reusable, maintainable, and modular UI components. However, understanding state management and performance optimization is key to ensuring that your apps remain performant, even as they grow more complex.

In the next post, we’ll dive into the exciting world of animations in Jetpack Compose, where we’ll explore how to add dynamic, interactive elements to your UI.