Android Interview Series 2024 — Part 7 (Jetpack Compose)

4. What is a Composable function, and how do you define one?

A Composable function is a fundamental building block in Jetpack Compose. It’s a special function that defines a piece of UI in a declarative way. By marking a function with @Composable, you make it possible for Jetpack Compose to track and manage the UI it represents, automatically handling updates whenever the underlying data or state changes.

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

5. Explain the Jetpack Compose Lifecycle.

The Jetpack Compose Lifecycle is similar to the traditional Android lifecycle but has unique characteristics due to its declarative, reactive nature.

• Composition is the initial phase where the UI elements are created for the first time.
• This phase only occurs once for a given part of the UI (unless it needs to be recreated). Once composed, the UI stays on the screen until it is either updated (via recomposition) or removed (via disposal).
• When a composable function is called for the first time, Jetpack Compose: builds the UI tree by evaluating the composable functions and creating UI elements and adds the resulting UI elements to the screen, establishing the initial view structure.
• Recomposition is the phase where Jetpack Compose updates the UI in response to state changes.
• During recomposition, only the functions affected by the state changes are re-evaluated.
• If no state changes occur, Compose will not recompose.
• Recomposition happens whenever a value that a composable depends on changes.
• Disposal is the phase where Compose removes composable functions that are no longer needed from the UI.
• This typically happens when a composable: goes out of scope due to a change in the UI tree (e.g., navigating away from a screen or conditionally removing a component) or is replaced by another composable.
• During the disposal phase, Compose: cleans up resources associated with the composable, such as cancelling coroutines, releasing listeners, or disposing of state objects and executes DisposableEffect cleanup code if defined, ensuring no resources are left hanging.

6. What is a Modifier in Jetpack Compose?

A Modifier in Jetpack Compose is a powerful and flexible tool used to modify the appearance, behavior, and layout of composable functions. Modifiers are essential for adding properties like padding, size, background, click actions, and layout adjustments to composable elements without altering the composable function itself.

• Modifiers are stateless. They do not hold or manage state.
• Modifiers are chainable. They can be chained to apply multiple properties sequentially, creating a flexible way to build complex UI behaviors.
• Modifiers are reusable. They are designed to be highly reusable, allowing you to define them once and apply them to multiple composables.

7. What are the different types of Modifier?

Layout modifiers: Layout modifiers control the size, padding, alignment, and general layout behavior of a composable.

• padding: Adds padding around a composable.
• fillMaxSize / fillMaxWidth / fillMaxHeight: Makes the composable fill the available space.
• size: Sets an explicit width and height.
• wrapContentSize: Wraps the composable’s size to its content and positions it within the available space.
• align(): modifier specifies the alignment of a composable within its parent layout.
• weight(): is used in Row or Column layouts to distribute space among children based on their weight.

Appearance modifiers: Appearance modifiers help you modify the look of composables by adding background colors, borders, and opacity.

• background: Sets a background color.
• border: Adds a border around the composable.
• alpha: Adjusts the transparency of a composable.
• clip: clips the composable to a specified shape.
• shadow: adds a shadow effect to a composable.

Behaviour modifiers: Behavior modifiers allow you to add interactivity, such as click handling, scroll behavior, and gestures.

• clickable: Makes the composable respond to click events.
• scrollable: Adds scroll behavior (e.g., for custom scrollable components).
• toggleable: adds toggle behavior, useful for creating switch-like components.
• draggable: allows dragging gestures on the composable.

Animation modifiers: add animations and transitions to composables.

• animateContentSize: Automatically animates size changes.
• graphicsLayer: applies transformations such as scaling, rotation, and translation.

Custom modifiers: You can create custom modifiers by defining extension functions on Modifier. This is useful for applying a specific combination of modifiers that you might use frequently.

8. How to create Responsive Layouts with Jetpack Compose?

• Use Modifier with Adaptive Sizing: Using Modifier functions like fillMaxWidth(), fillMaxSize(), weight(), and wrapContentSize() allows your composables to adapt to the available screen space.
• Responsive Layouts with ConstraintLayout: ConstraintLayout allows you to create more complex responsive layouts by defining constraints between elements, similar to XML-based ConstraintLayout in Android.
• BoxWithConstraints allows you to access the constraints of the available space, enabling you to create conditional layouts based on the screen size or orientation.

produceState(
// The initial value of the state before any data is produced.
initialValue: T, 
// he dependency keys that determine when produceState should restart the coroutine.
// If any of the keys change, the coroutine will be re-launched.
vararg keys: Any?,
// A lambda that contains the suspendable code to produce the state.
producer: suspend ProduceScope<T>.() -> Unit
): State<T>