Jetpack Compose has revolutionized Android development by providing a modern, intuitive way to build UI with Kotlin. One of its key building blocks is the Composable Function, which allows developers to create reusable UI components that seamlessly adapt to different states. But what exactly goes on behind the scenes when you define and use a Composable function? In this blog, we’ll unlock the secrets of a Composable function in Jetpack Compose, taking a deep dive into its internal anatomy. By the end, you’ll have a clear understanding of how Composables work under the hood and how to harness their full potential to build efficient, scalable UIs in your Android applications.

What is a Composable Function?

A composable function is a special function used to define UI components in Jetpack Compose. You create one by adding the @Composable annotation to a function:

@Composable
fun MyComposableFunction() {
    // Define your UI here
}

Now, the first question that comes to mind is:

What is @Composable?

The @Composable annotation is a special annotation in Jetpack Compose that marks functions or expressions as “composable.” When a function is marked with @Composable, it means:

• The function can be used to create UI in a Compose-based app.
• It must be called only from other @Composable functions. You can’t call a @Composable function from a regular function.

Understanding the @Composable Annotation in Detail

Basically, in Jetpack Compose, the fundamental building block for creating a UI is called a composable function. Which is annoted with @Composable annotation, Let’s break down what it is and how it works.

First, if you see or find any @Composable annotation, right-click on it and select ‘Go To’ -> ‘Declarations & Usage,’ which will redirect you to the Composable.kt file.

There, you will find some commented documentation and code. We will focus on the code, but first, let’s analyze what the documentation says.

/**
 * [Composable] functions are the fundamental building blocks of an application built with Compose.
 *
 * [Composable] can be applied to a function or lambda to indicate that the function/lambda can be
 * used as part of a composition to describe a transformation from application data into a
 * tree or hierarchy.
 *
 * Annotating a function or expression with [Composable] changes the type of that function or
 * expression. For example, [Composable] functions can only ever be called from within another
 * [Composable] function. A useful mental model for [Composable] functions is that an implicit
 * "composable context" is passed into a [Composable] function, and is done so implicitly when it
 * is called from within another [Composable] function. This "context" can be used to store
 * information from previous executions of the function that happened at the same logical point of
 * the tree.
 */

It says the following about CF:

• Composable Functions:
  Functions marked with @Composable are the core components of a Compose UI.
• Transform Data to UI:
  These functions describe how data should be displayed in a UI hierarchy (tree of UI elements).
• Implicit Context:
  When you call a composable function, it gets an implicit “composable context.” This context helps Compose keep track of previous function calls and updates efficiently during recompositions.
• Restriction:
  @Composable functions can only be called from other @Composable functions. You can’t call them directly from non-composable functions.

Now, let’s see how @Composable is defined.

Annotation Declaration

@MustBeDocumented
@Retention(AnnotationRetention.BINARY)
@Target(
    AnnotationTarget.FUNCTION,
    AnnotationTarget.TYPE,
    AnnotationTarget.TYPE_PARAMETER,
    AnnotationTarget.PROPERTY_GETTER
)
annotation class Composable

Have you noticed that the @Composable annotation itself is defined by three other annotations? Let’s break this down further, and see them one by one.

• @MustBeDocumented: This annotation indicates that the annotated element should be included in the generated documentation (e.g., when using KDoc to generate documentation).
• @Retention(AnnotationRetention.BINARY): Specifies that the annotation is retained in the compiled bytecode (e.g., .class files) but is not available at runtime (not accessible via reflection).
• @Target: It defines the types of elements to which the annotation can be applied. For example, the @Composable annotation can be applied to functions and properties, indicating that these are composable functions in Jetpack Compose. In short, it specifies where the @Composable annotation can be applied. 

It can be used on:

• Functions (AnnotationTarget.FUNCTION):

@Composable fun MyComponent() { /*...*/ }


//Example

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

• Types (AnnotationTarget.TYPE):

fun <T : @Composable () -> Unit> someFunction() { /*...*/ }



// Example 

// A CF that takes no parameters and returns Unit
@Composable
fun Greeting(name: String) {
    // This is a composable that displays a greeting
    Text(text = "Hello, $name!")
}

// A generic function that takes a CF as a parameter
fun <T : @Composable () -> Unit> someFunction(composable: T) {
    // Here you can invoke the composable function
    composable()
}

@Preview
@Composable
fun PreviewSomeFunction() {
    // Passing the Greeting CF as a parameter to someFunction
    someFunction { Greeting("Compose") }
}

• Property Getters (AnnotationTarget.PROPERTY_GETTER):

val isEnabled: Boolean
  @Composable get() { return true }


//Another Example 

val greetingText: String
    @Composable get() = "Hello from a composable property!"

One remains. Why did I leave it for last? Because, theoretically, it exists, but practically, it hasn’t been implemented by the Compose team yet. Let’s see what it is in more detail.

• Type Parameters (AnnotationTarget.TYPE_PARAMETER):

fun <T : @Composable () -> Unit> someFunction() { /*...*/ }



// Example 

// A composable function that takes no parameters and returns Unit
@Composable
fun Greeting(name: String) {
    // This is a composable that displays a greeting
    Text(text = "Hello, $name!")
}

// A generic function that takes a composable function as a parameter
fun <T : @Composable () -> Unit> someFunction(composable: T) {
    // Here you can invoke the composable function
    composable()
}

@Preview
@Composable
fun PreviewSomeFunction() {
    // Passing the Greeting composable function as a parameter to someFunction
    someFunction { Greeting("Compose") }
}

While the annotation declares AnnotationTarget.TYPE_PARAMETER as a valid target, the Compose compiler does not actually support constraining type parameters with @Composable functions. This can lead to issues such as a ClassCastException.

fun <T : @Composable () -> Unit> someFunction(composable: T) {
    composable()
}

What’s wrong? Why does this fail?

• fun <T : @Composable () -> Unit> someFunction() is not supported by the Compose compiler. The @Composable annotation cannot be applied as a constraint to a type parameter in practice, even though AnnotationTarget.TYPE_PARAMETER exists in the annotation’s definition.
• Practical Workaround: Instead of using generics, define function parameters directly with @Composable () -> Unit.

// Instead of constraining a type parameter with @Composable, use a regular function parameter

@Composable
fun someFunction(content: @Composable () -> Unit) {
    content()
}

The theoretical declaration of AnnotationTarget.TYPE_PARAMETER for @Composable might indicate future or planned support, but as of now, it’s not usable due to the unique nature of composable functions and how the Compose compiler handles them.

Composable Function Restrictions and Rules

Can Only Call From Other @Composable Functions

A @Composable function can only be called from within another @Composable function or a composable context. This is necessary because @Composable functions require the Compose runtime to manage their lifecycle, track state, and handle recompositions properly.

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

fun regularFunction() {
    // This will cause an error!
    Greeting("Jetpack Compose")
}

It will suggest adding @Composable to the regular function, with the message: ‘@Composable invocations can only happen from the context of a @Composable function.’

Implicit Context

When a @Composable function is called, it operates within a composable context. This context allows the Compose runtime to:

• Track changes to state.
• Recompose parts of the UI efficiently when the state changes.
• Manage the lifecycle of composable functions.

This context is automatically provided when you’re within a composable function, making it possible for the Compose framework to determine how and when to re-render parts of the UI.

What is the “Implicit Context” in Jetpack Compose?

When a @Composable function is called, Compose internally maintains a composition context that tracks essential information about the composition. This context is implicitly managed by Compose and facilitates the following aspects:

Position in the UI Tree:

• Compose needs to know where the current @Composable function resides in the hierarchy of composables. This allows Compose to correctly place and update elements in the UI tree.

State Management:

• When a composable uses remember or rememberSaveable to store state across recompositions, the composition context tracks these values and ensures they persist between recompositions.
• This is how Compose can remember stateful values even as the composable function is called multiple times.

Recomposition Tracking:

• The composition context helps Compose determine when a @Composable function needs to be recomposed.
• When data or state changes, Compose uses the context to know which parts of the UI need to be redrawn and updates only the affected composables.

Slot Management:

• The context also manages “slots,” which represent placeholders for UI elements. This allows composables to efficiently update, insert, or remove UI elements during recomposition.

Parent-Child Relationships:

• It maintains relationships between parent and child composables, ensuring proper recomposition and state propagation.
• abc

Implicit Handling: 

• The composition context is indeed handled automatically. You don’t need to manually pass it when calling a @Composable function. It gets established and propagated by the Compose runtime.

In short,

When a @Composable function is called, Jetpack Compose internally manages a composition context. This context helps with:

• UI Tree Positioning: Tracking where the composable is in the UI hierarchy.
• State Management: Remembering values created with remember or rememberSaveable across recompositions.
• Recomposition Tracking: Identifying which composables need to be redrawn when data changes.
• Slot Management and Parent-Child Relationships: Efficiently managing dynamic UI elements and their relationships.

You don’t manually pass this context — Compose handles it automatically when you call @Composable functions.

Conclusion

Understanding the internal anatomy of a Composable function in Jetpack Compose is essential for mastering the framework and creating more efficient Android applications. By exploring how Composables are structured, recomposition is triggered, and state management works, you’ll be able to write cleaner, more maintainable code. Armed with this knowledge, you’ll unlock the true power of Jetpack Compose and elevate your Android development skills to new heights.

happy UI composing..!